The Philosophy of the Weather. And a Guide to Its Changes
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Title: The Philosophy of the Weather. And a Guide to Its Changes
Author: T. B. Butler
Release date: August 14, 2010 [eBook #33429]
Language: English
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*** START OF THE PROJECT GUTENBERG EBOOK THE PHILOSOPHY OF THE WEATHER. AND A GUIDE TO ITS CHANGES ***
Produced by Robin Monks and the Online Distributed
Proofreading Team at http://www.pgdp.net (This book was
produced from scanned images of public domain material
from the Google Print project and from The Internet Archive:
American Libraries.)
THE PHILOSOPHY OF THE WEATHER.
AND A GUIDE TO ITS CHANGES.
BY T. B. BUTLER.
NEW YORK:
D. APPLETON & COMPANY,
NOS. 346 & 348 BROADWAY.
1856.
Entered according to Act of Congress, in the year 1856, by
T. B. BUTLER,
In the Clerks Office of the District Court of the District of
Connecticut.
ELECTROTYPED BY
THOMAS B. SMITH,
82 & 84 Beekman Street.
PRINTED BY
J. F. TROW,
379 Broadway.
INTRODUCTION.
The atmospheric conditions and phenomena which constitute "The Weather"
are of surpassing interest. Now, we rejoice in the genial air and warm
rains of spring, which clothe the earth with verdure; in the alternating
heat and showers of summer, which insure the bountiful harvest; in the
milder, ripening sunshine of autumn; or the mantle of snow and the
invigorating air of a moderate winter's-day. Now, again, we suffer from
drenching rains and, devastating floods, or excessive and debilitating
heat and parching drought, or sudden and unseasonable frost, or extreme
cold. And now, death and destruction come upon us or our property, at any
season, in the gale, the hurricane, or the tornado; or a succession of
sudden or peculiar changes blight our expected crops, and plant in our
systems the seeds of epidemic disease and death. These, and other normal
conditions, and varied changes, and violent extremes, potent for good or
evil, are continually alternating above and around us. They affect our
health and personal comfort, and, through those with whom we are
connected, our social and domestic enjoyments. They influence our business
prosperity directly, or indirectly, through our near or remote dependence
upon others. They limit our pleasures and amusements--they control the
realities of to-day, and the anticipations of to-morrow. None can
prudently disregard them; few can withhold from them a constant attention.
Scientific men, and others, devote to them daily hours of careful
observation and registration. Devout Christians regard them as the
special agencies of an over-ruling Providence. The prudent, fear their
sudden, or silent and mysterious changes; the timid, their awful
manifestations of power; and they are, to each and all of us, ever present
objects of unfailing interest.
This _interest_ finds constant expression in our intercourse with each
other. A recent English writer has said: "The germ of meteorology is, as
it were, innate in the mind of every Englishman--the weather is his first
thought after every salutation." In the qualified sense in which this was
probably intended, it is, doubtless, equally true of us. Indeed, it is
often not only a "first thought" _after_ a salutation, but a part of the
salutation itself--an offspring of the same friendly feeling, or a part of
the same habit, which dictates the salutation--an expression of sympathy
in a subject of common and absorbing interest--a sorrowing or rejoicing
with those who sorrow or rejoice in the frowns and smiles of an
ever-changing, ever-influential atmosphere.
If consistent with our purpose, it would be exceedingly interesting to
trace the varied forms of expression in use among different classes and
callings, and see how indicative they are of character and employment.
The sailor deals mainly with the winds of the hour, and to him all the
other phases of the weather are comparatively indifferent. He speaks of
airs, and breezes, and squalls, and gales, and hurricanes; or of such
appearances of the sky as prognosticate them. The citizens, whose lives
are a succession of _days_, deal in such adjectives as characterize the
weather of _the day_, according to their class, or temperament, or
business; and it is pleasant, or fine, or _very_ pleasant or fine;
beautiful, delightful, splendid, or glorious; or unpleasant, rainy,
stormy, dismal, dreadful or horrible. The farmer deals with the weather
of considerable periods; with forward or backward _seasons_, with "cold
snaps" or "hot spells," and "wet spells" or "dry spells." And there are
many intermediate varieties. The acute observer will find much in them to
instruct and amuse him, and will probably be surprised to find how much
they have to do with his "first impressions" of others.
But I have a more important object in view. I propose to deal with "The
PHILOSOPHY _of the Weather_"--to examine the nature and operation of the
arrangements from which the phenomena result; to strip the subject, if
possible, of some of the complication and mystery in which traditionary
axioms and false theories continue to envelop it; to endeavor to grasp
_its principles_, and unfold them in a plain, concise, and systematic
manner, to the comprehension of "_the many_," who are equal partners with
the scientific in its practical, if not in its philosophic interest; and
to deduce a few general rules by which its changes may be understood, and,
ultimately, to a considerable extent, foreseen.
This is not an easy, perhaps not a prudent undertaking. Nor is my position
exactly that of a volunteer. A few words seem necessary, therefore, by way
of apology and explanation.
In the fall of 1853, in the evening of a fair autumnal day, I started for
Hartford, in the express train. Just above Meriden, an acquaintance
sitting beside me, who had been felicitating himself on the prospect of
fine weather for a journey to the north, called my attention to several
small patches of scud--clouds he called them--to the eastward of us,
between us and the full clear moon, which seemed to be enlarging and
traveling south--and asked what they meant.
"Ah!" said I, "they are scud, forming over the central and northern
portions of Connecticut, induced and attracted by the influence of a
storm which is passing from the westward to the eastward, over the
northern parts of New England, and are traveling toward it in a southerly
surface wind, which we have run into. They seem to go south, because we
are running north faster than they. You see them at the eastward because
they are forming successively as the storm and its influence passes in
that direction, and are most readily seen in the range of the moon; but
when we reach Hartford you will see them in every direction, more numerous
and dense, running north to underlie that storm."
I had seen such appearances too many times to be deceived. It was so. When
we arrived at Hartford they were visible in all directions, running to the
northward at the rate of twenty-five miles an hour. In the space of forty
minutes we had passed from a clear, calm atmosphere (and which still
remained so), into a cloudy, damp air, and brisk wind blowing in the same
direction we were traveling, and toward a heavy storm. My friend passed
on, and met the southern edge of the rain at Deerfield, and had a most
unpleasant journey during the forenoon of the next day. Taking the cars
soon afterwards, in the afternoon, for the south, I found him on his
return.
"Shall I have fair weather now till I get home?" said he.
"There are no indications of a storm here, or at present," I replied, "but
we may observe them elsewhere, and at nightfall."
He kept a sharp look-out, and, as we neared New Haven, discovered faint
lines of cirrus cloud low down in the west, extending in parallel bars,
contracting into threads, up from the western horizon, in an E. N. E.
direction toward the zenith.
"Now, what is that?" said he.
"The eastern outlying edge of a N. E. storm, approaching from the W. S. W.
It is now raining from 150 to 200 miles to the westward of the eastern
extremity of those bars of cirrus-condensation; perhaps more, perhaps
less; and under those bars of condensation the wind is attracted, and is
blowing from the N. E. toward the body of the storm, and where the
condensation is sufficiently dense to drop rain. That dense portion will
reach here, and it will rain from twelve to fifteen hours hence. As we
pass along the shore, and run under that out-lying advance
cirrus-condensation, we shall see that the vessels in the Sound have the
wind from the N. E., freshening, but we shall continue to have this light
and scarcely-perceptible air from the northward for a time--_the N. E.
wind always setting in toward an approaching storm, out on the Sound, much
sooner than upon the land_."
As we approached the storm, and the storm us, the evidence of denser
condensation at the west, and of wind from the east, blowing toward it,
became more apparent. The fore and aft vessels were running "up Sound"
with "sheet out and boom off," before a fresh N. E. breeze, and my friend
was astonished.
"I must understand this," said he; "how is it?"
"All very simple. The page of nature spread out above us is intelligible
to him who will attentively study it. The laws which produce the
impressions and changes upon that page, are few and comprehensible.
Although there is great variety, even upon the limited portion which is
bounded by our horizon, there is also substantial uniformity; and,
although the changes are always extensive, often covering an area of one
thousand miles or more, and our vision can not extend in any direction
more than from thirty to fifty, yet those changes are always, to a
considerable extent, intelligible, and may often be foreseen."
"Has meteorology made such progress?"
"By no means. It has, indeed, been raised to the dignity of a science, and
professorships endowed for its advancement. Some books have been written,
and many theories broached in relation to it; and innumerable observations
of the states of the barometer and thermometer, of the clouds, and the
quantity of fallen rain, and the direction and force of the wind--made and
recorded simultaneously in different countries--have been published and
compared; and a great many important facts established, and tables of
'_means_' constructed, and just inferences drawn, yet the _few and simple
arrangements_ upon which all the phenomena depend, and _their philosophy_,
have not yet been clearly elicited or understood."
"Have not the 'American Association for the Advancement of Science'
arrived at some definite and sound conclusion upon the subject?"
"No; it has been with them, for many years, an interesting subject for
papers and debate. Some very valuable articles, upon particular topics, or
branches of the subject, have been read and published. But the
_Cyclonologists_, as they term themselves, and who seem to think the great
question is, '_Are storms whirlwinds?_' appear with new editions and
phases of their favorite views as regularly as the annual meeting recurs;
and, though they have not convinced, they seem to have silenced their
opponents. The only conclusion, however, judging from their debates, to
which the Association appear to have come with any considerable unanimity,
is, that they are yet without sufficient _authentic observations_ and
well-established facts, to authorize the adoption of the Huttonian,
Daltonian, Gyratory, or Aspiratory, or any of the other numerous theories
which abound. And they are right. The subject is mystified by these
theories and speculations of the study, founded on barometrical and
thermometrical records, and the direction and force of the surface winds.
"The qualities of heat were among the earlier discoveries of science, and
all the phenomena of the weather were forthwith attributed to its
influence. Hastily-formed and erroneous views of its power, and the manner
of its action in particular localities, and under particular
circumstances, have retained the credence accorded to them when first
announced, although subsequent discoveries have shown their fallacy; some
new theory of _modification_ having been invented to reconcile the
discrepancies as soon as they appeared. Perhaps it is not too much to say
(however it may seem to one not thoroughly acquainted with the subject,
who does not know that the _primary_ and secondary modifying hypotheses
found in Kaemtz, may be counted by hundreds) that there is not remaining in
any other science, and possibly in all others, an equal amount of false
and absurd theory, and of forced and unnatural grouping of admitted facts
to sustain it, as in meteorology as at present taught and received.
Astronomy, as a science, is almost perfected--the nature, and size, and
orbits, of the distant worlds around us are known--while constant changes
and alternating atmospheric conditions, which all occur _within less than
six miles of us_, affecting all our important interests, and obvious to
our senses, although much talked off, and made the objects of many
theories, are but little understood."
"How, then, did you acquire the information you seem to possess?"
"By studying '_the countenance of the sky_,' for in no other way has such
information ever been, or can it ever be, acquired. By a long-continued,
daily, and sometimes hourly observation of the clouds and currents of the
atmosphere, in connection with such reports of the then state of the
weather elsewhere, as have fallen under my notice, and the effect of its
changes upon the animal creation--for very much can be learned from them.
Yonder flock of black ducks that sit on that inshore rock, above the
tide--the wildest and most suspicious of all their tribe--although the
air is calm about them, know well that a storm is at hand. They probably
both see and feel it. As twilight approaches they will fly away inland,
forty or fifty miles perhaps, and settle among the lilies or grass which
surround some fresh-water pond, certain of remaining while the storm
lasts, and for one day at least, out of danger, and undisturbed. Many a
time, in my boyhood, have I heard, in the stillness of evening, the
whistling of their wings, as they swept up the Connecticut valley, to
seek, on the borders of the coves, and in the creeks of the meadows, a
concealed and safe feeding-place during a coming storm. And many a time in
the autumn, after they had all passed down for the season, when the
indications of an approaching storm were clearly visible at nightfall,
have I waited for them to return, on the eastern margin of a bend in the
cove, on the eastern side of a creek, to shoot them, though invisible, by
shooting across the head of the wake, which they made upon the water in
alighting, and from which the few remaining rays of twilight that came
from the western sky were reflected.
"But I am far from being singular in this. That page is more extensively
read than is generally supposed. Many plain, unassuming men--farmers,
shipmasters, and others within the circle of my acquaintance--know more,
practically, of the weather than the most learned closet-theorist, or the
most indefatigable recorder of its changes. Every one, by studying the
page of nature above him, as he would the page of any other science, and
testing, by observation, the numerous theories invented to account for the
varied phenomena, may learn much, very much, that will be useful and
interesting to him, and which he can never learn from books, or
instruments, or theories alone."
"Well," said my friend, "I am too far advanced in life, as are many
others, to commence such observations, and you must publish."
I demurred, and he insisted.
"It is difficult to spare the time; and I can not neglect my profession,"
I urged.
"Where there is a will there is a way," he replied.
"It is difficult to make one's self understood without many
illustrations."
"Very well, they are easily obtained."
"But they cost money, and it is said 'science will not pay its way' like
fiction and humbug."
"That," said he, "is a libel--such science will. Every one is interested
in the weather--all talk about it--and thousands would carefully observe
it, if they could be correctly guided in their observations."
"I may get into unpleasant controversy."
"Suppose you do; you can yield your position if wrong, and maintain it if
right, and _magna est veritas_."
"But I may be mistaken in some of the views to which it will be necessary
to advert, if I attempt to systematize the subject."
"Be it so--your mistakes may lead others to the discovery of the truth.
Besides, the weather is _common property_, and every one has a right to
theorize about it, or to talk about it, as they please--even to call a
stormy day a pleasant one, or make any other mistaken remark concerning
it; and every other person is entitled to a like latitude of reply. And
further," said he, with some emphasis, "no important observation, in
relation to a subject of such interest, should be lost; and, if you have
observed one new fact, or drawn one new and just inference from those
which have been observed by others; and especially if, from observation
and reading, you can deduce from the phenomena an intelligible,
_observable, general system_, it is not only your right, but duty, to make
it known. Such a knowledge of the true system is greatly desired by every
considerate man."
To my friend's last argument I was compelled to yield. I could make no
reply consistent with the great principles of fraternity, which I shall
ever recognize. The promise was given. My friend went on his way, and I
went to the daguerreotypist to procure a copy of the then appearance of
the sky, as the first step toward its fulfillment. The fulfillment of that
promise, reader, you will find in the following work. It was commenced as
an article for a magazine, but it has grown on my hands to a volume.
Justice could not well be done to the subject in less space. It has been
written during occasional and distant intervals of relaxation from
professional avocations, or during convalescence from sickness, and it is,
for these reasons, somewhat imperfect in style and arrangement. But I have
no time to rewrite. There is much in it which will be old to those who
read journals of science, but new to those who do not. There is more which
will be new to all classes of readers, and may, perhaps, be deemed
heretical and revolutionary by conservative meteorologists; yet I feel
assured that the work is a step in the right direction--that it contains a
substantially accurate exposition of the Philosophy of the Weather, and
valuable suggestions for the practical observer.
I have inserted my name in the title-page, contrary to my original
intention, and at the suggestion of others; for I have no scientific
reputation which will aid the publisher to sell a copy. Nor do I desire to
acquire such reputation. It can never form any part of my "capital in
life." Nor has it influenced me at all in preparing the work. I have aimed
to fulfill a promise, too hastily given, perhaps--to put on record the
observations I have made, and the inferences I have drawn from those of
others--to induce and assist further observations, and, if possible, of a
_general_ and _connected character_--and to impress those who may read
what I have written with the belief, that _they will derive a degree of
pleasure from a daily familiarity with, and intelligent understanding of,
the "countenance of the sky," not exceeded by that which any other science
can afford them_.
I have examined, with entire freedom and fearlessness (but I trust in a
manner which will not be deemed censurable or in bad taste) the theories
and supposed erroneous views of others, for, in my judgment, the
advancement of the science requires it. Says Sir George Harvey, in his
able article on Meteorology, written for the Encyclopaedia Metropolitana:
"It is humiliating to those who have been most occupied in
cultivating the science of meteorology, to see an agriculturist or a
waterman, who has neither instruments nor theory, foretell the future
changes of the weather many days before they happen, with a precision
which the philosopher, aided by all the resources of science, would
be unable to attain."
The admissions contained in this paragraph, in relation to the comparative
uselessness of instruments and theories, and the value of practical
observation, are both in a good measure true. And the time has come, or
should speedily come, when "_pride of opinion_," and "_esprit du corps_,"
among theorists and philosophers, should neither be indulged in, nor
respected; and when their theories should be freely discussed, and rigidly
tested by the observations of practical men. Such measure, therefore, as I
have meted, I invite in return. Let whatever I have advanced, that is new,
or adopted that is old, be _as_ rigidly tested, and _as_ freely discussed.
Let the errors, if there be any--and doubtless there are--be detected and
exposed. Let the TRUTH be sought by all; and meteorology, as a PRACTICAL
SCIENCE, advance to that full measure of perfection and usefulness, of
which it is unquestionably susceptible.
TABLE OF CONTENTS.
PAGE
CHAPTER I.
Heat and moisture are indispensable to the fertillity of the
earth--Arrangements exist for their diffusion and distribution,
and all the phenomena of the weather result from their
operation--Heat furnished or produced mainly by the direct
action of the sun's rays--Manner in which it is diffused over
the earth--Other causes operate besides the sun's rays--The
earth intensely heated in its interior--Heat derived from the
great Oceanic currents, and the aerial currents which flow
from the tropics to the poles, and from magnetism and
electricity--Water distributed by an atmospheric machinery as
extensive as the globe--Evidences of this--Its distribution over
the continents of North America--Explanation of it--Source from
whence our supply of water is derived, and from which our rivers
return 1
CHAPTER II.
Our rivers return in the form of clouds, and in storms and
showers--Definition and character of storms--Differences in
the character of the clouds which constitute them--Nomenclature
of Howard--Its imperfections--New order of description--Low
fog--High fog--Storm fog--Storm scud--N. W. scud--Cumulus--
Stratus--Cirrus--Compounds of the two latter--recapitulation in
tabular form 24
CHAPTER III.
Our rivers do not return from the North Atlantic--All storms and
showers move from the westward to the eastward--Seeming clouds
seen moving from the eastward to the westward are scud--They are
incidents of the storm, and not a necessary part of it--The
storm clouds are above them, moving to the eastward--Occasions
when this may be seen--Admitted facts prove it--Investigations
prove it--May be known from analogy--From the fact that there is
an aerial current pursuing the same course in which the storms
originate--Character of this current--Its influence upon our
country--Importance of a knowledge of its origin, cause, and the
reciprocal action between it and the earth--To this end necessary
to go down "to the chambers of the South" 43
CHAPTER IV.
The trade wind region--Its extent and arrangements--Its belt of
daily rains and movable character--The trade winds--The extra
tropical belt of rains--Connection between them and their annual
movements--The counter-trades--Their origin and situation--One
of them constitutes our aerial current--It originates in the
South Atlantic as a surface-trade--Anomalies of the trade wind
region--Dry seasons--Humboldt's description of them--Exist where
the surface trades are situated--The rainless countries--
Concentrated counter-trade--Monsoons--Received theory in relation
to them a fallacy--Cause of the great central phenomena--
Calorific theory a fallacy--Land not hotter under the belt of
rains, nor sea materially so--Theory should be abandoned 52
CHAPTER V.
The agent, magnetism--Its character and currents--Oxygen
magnetic--Precipitation at the belt of rains occasioned by
depolarization--Storms originate in this central belt, and move
toward the poles 82
CHAPTER VI.
Course and functions of the counter-trade--Ours come from the
South Atlantic--Reason why it can not come from the Pacific--
Mistake of Mr. Redfield and Lieutenant Maury in regard to it--
All our storms originate in it--Proofs of this--State of the
weather, whether hot or cold affected by it--Proofs of this--All
our surface winds are incidents of it, and due to its conditions
and attractions--Proofs of this--Character of the different
winds--Anomalies of Mr. Blodgett accounted for--Received theory
in regard to sea and land breezes a mistaken one--Proofs of
this--Peculiar character of the N. W. wind--Identity with the
winter Mexican northers--Character of the West India hurricanes--
Of the thunder-gust--Of the tornado--Sundry particulars in
relation to the latter--Due to currents of electricity--
Proportions of winds in different localities--Examination of the
work of Professor Coffin upon that subject--Examination of
Lieutenant Maury's theory of the monsoons 92
CHAPTER VII.
Height of the counter-trade in different latitudes--Cause of the
Calms of Cancer--Influence of mountains upon the counter-trade--
Reports of Herndon and Gibbon--Focus of precipitation in the
extra-tropical belt north of its southern line--Evidences of
this--The elevation of the counter-trade above the earth varies
in the same latitude with the variations in the phenomena of the
weather--Temperature of the counter-trade--Rain dust, its origin
and indications--Volcanic ashes--How far they indicate its course
of progression--Question whether there is an eastern progression
of the body of the atmosphere above the machinery of distribution 179
CHAPTER VIII.
Important to understand the precise character of the reciprocal
action between the earth and the counter-trade--Connection
between the width and movements of the belt of inter-tropical
rains and the volume of the trades--Its peculiarities over
Africa, the Atlantic, and South America--The magnetic equator--
Character of the storms which originate in the inter-tropical
belt indicate local magnetic action--Supposed influence of
volcanic action--Gulf Stream changes its position--This the
result of magnetic action--Alternating contrasts of heat and
cold, and rain and drought--Dr. Webster's history of the
weather--Spots upon the sun--Their character and influence--Cold
or warm periods during the same decade, and during different
decades--Connection between the spots and magnetic disturbances
and variations--Influence of the moon upon the weather--No
decisive inference to be drawn from these facts, and a more
critical examination necessary 204
CHAPTER IX.
Examination of existing theories--Calorific theory the prevailing
one--Lateral overflow of Professor Dove--Absurdity of his views
in relation to them--His theory of hurricanes--Its absurdity--A
new theory by Mr. Dobson--Three theories advanced by
meteorologists of this country--Professor Espy's theory--Mr.
Bassnett's theory--Mr. Redfield's theory--Extended examination of
the latter--His theory in relation to the fall of the barometer
contradictory in its character--Philosophy of the barometric
change--No aid to be derived from these theories 232
CHAPTER X.
Further inquiry in relation to the reciprocal action between the
earth and the counter-trade--Terrestrial magnetism, and what we
know of it--Its elements, and their variations--Their connection
with the variations of atmospheric condition--Magnetism acts
through its connection with electricity--Character of the latter
and its variations--Their connection with atmospheric conditions--
Electricity as well as magnetism in excess over this country--
Effects of it upon our climate--Closer consideration of the
atmospheric phenomena--Their diurnal changes and connections
compared with those of magnetism and electricity--Grouping of all
the diurnal variations--Particular and separate examination of
them--Classification of storms--Examination in detail of the
several classes and the primary influence of the earth or
counter-trade in relation to each 285
CHAPTER XI.
Prognostics 340
THE PHILOSOPHY OF THE WEATHER.
CHAPTER I.
Heat and moisture are indispensable to the fertility of the earth. Without
suitable arrangements for their diffusion and distribution, and within the
limits of certain minima and maxima, it would not have been habitable, or
the design of its Creator perfected. These arrangements therefore exist,
and "while the earth remaineth seed time and harvest shall not cease." Few
and simple in their character, though necessarily somewhat complicated and
irregular in their operation, the ultimate result is always attained. A
beautiful system of compensations supplies the losses of every apparent
irregularity in one section or crop, by the abundance of others.
From the operation of these few, simple, connected, and intelligible
arrangements for the diffusion of heat and the distribution of moisture
over the earth, result all the phenomena which constitute the weather; and
by studying them, and their operation, we may acquire an accurate
knowledge of its "_Philosophy_."
The necessary heat is furnished, or produced, mainly by the direct action
of the sun's rays; and the most obvious feature in the arrangements for
its diffusion is that by which the sun is made to shine successively and
alternately upon different portions of the earth. Nothing animate or
organic could endure his burning rays, if they shone continuously or
vertically upon one point, or could exist without their occasional
presence. Hence the provision for a diurnal rotation, to prevent the
exposure of any portion of the globe to the action of those rays for
twenty-four consecutive hours, except for a limited period, and at a
considerable angle, in the polar regions. But the earth is spheroidal, and
a diurnal revolution would still leave that portion which lies under the
equator too much, and the other too little, exposed to the action of the
sun. This is obviated by an annual revolution of the earth around the sun,
and an obliquity of its axis, by reason of which the northern and southern
portions are alternately and, as far as the tropics vertically, exposed to
the sun; and it is made to travel (so to speak) from tropic to tropic,
producing summer and winter, and other important phenomena.
This obliquity and consequent change of exposure are in degree precisely
what the wants of the earth would seem to require. If it was greater, the
sun would travel further north and south, but the alternate winters would
be longer and more severe. If it was less, the end would not be as
perfectly attained.
The direct action of the sun's rays upon the earth, particularly those
portions which lie north and south of the tropics, is not the only source
from which the supply of heat is derived. Although there is a general
increase of heat in spring and summer when the sun travels north, and of
cold when he travels south in winter, yet there are frequent
irregularities attending both. Very sudden and great changes occur in each
of them. Frost sometimes, cool weather often, occurs in midsummer, and
considerable heat and tornadoes in midwinter. And ordinarily the maxima
and minima of each month and, indeed, of each week are widely apart. Even
in the polar regions, in midwinter, _where the sun does not shine at all_,
the same moderating changes with which we are conversant occur in degree.
An extract or two from the register found in Dr. Kane's narrative of the
"Grinnell Expedition" will illustrate this.
JANUARY 1851, (LATITUDE ABOUT 74 deg., LONGITUDE ABOUT 70 deg.).
Date. Wind. Force. Ther. Bar. Sky and Weather.
Jan. 3 calm -26.1 29.62 blue sky, m.
" 4 W. gent breeze -21.3 29.53 blue sky,
detached clouds, m.
" 5 W. by N. gent breeze -3.9 29.59 blue sky, m.,
clouded over.
" 6 W. by S. light breeze -0.8 29.67 clouded over, m.,
snow.
" 7 W. gent breeze -14.4 29.96 blue sky, detached
clouds, m.
" 8 W.S.W. light air -21.2 30.14 blue sky, m.
" 29 W.N.W. light air -18.9 30.19 blue sky.
" 30 NW. by W. light air -13.5 30.17 clouded over, m.
" 31 NW. by W. gent breeze -4.4 29.35 clouded over, snow.
Feb. 1 W. light breeze -11.7 29.27 cloudy, blue sky, m.
" 2 W. light air -25.1 29.62 blue sky, detached
clouds, m.
These extracts are instructive. It will be seen that on the 3d of
January, when the sun had been absent some weeks, it was calm, the
thermometer stood at 26 deg. below zero (the - or minus mark before the
figures indicates that), and the barometer at 29.62, with blue sky,
somewhat misty or hazy--(the letter "m." standing for misty or hazy)--a
state of the air which existed most of the time when it did not snow or
rain, and therefore is of no importance in this connection. The next day
the thermometer began to rise, and the barometer to fall. On the 5th it
clouded over, and the thermometer rose rapidly, and on the 6th it had
risen more than 25 deg., and snow fell. On the 7th it cleared off, the
thermometer fell rapidly, and the barometer rose. On the 8th the
thermometer had fallen to 21 deg. below zero, and the barometer had risen to
30.14. Another instance, in all respects similar, occurred the latter part
of the month. We shall see hereafter that these changes are precisely like
those which occur with us, and every where. That, as in the polar regions,
and whether the sun be present or absent, or obscured by clouds, and by
night as well as by day, the changes from warm to cold and from cold to
warm are sudden and great, and that the latter are connected with the fall
of rain and snow--that every where in winter it _moderates to storm_.
Many other instructive instances, especially in relation to the great
difference in the seasons in our own country, and upon the same parallels
elsewhere, might be cited if it were necessary. But they will more
appropriately appear in the sequel.
[Illustration: Fig. 1.
In the above cut the isothermal lines are Centigrade. The zero of the
Centigrade thermometer is the freezing point of water, or 32 deg. of
Fahrenheit. The boiling point of water is 100 deg. Centigrade, or 212 deg.
Fahrenheit. A degree of Centigrade is equal to one degree and four-fifths,
Fahrenheit. The 0 deg. line of the cut, therefore, is 32 deg. of Fahrenheit--the
line of 5 deg. above is 41 deg. Fahrenheit--the line of 5 deg. below is 23 deg.
Fahrenheit, and so on. The reader, who is not familiar with the difference
in the scale of the thermometer, is desired to remember this; for we shall
make occasional extracts in which the temperature is given in the
Centigrade scale.]
The cause of those irregularities, especially in the same seasons of
different years, and when very great, is often sought and supposed to be
found in the presence or absence of spots on the sun, ice floes and bergs
in the Atlantic, etc., etc. But neither the spots, nor ice, nor other
local causes produce them. The cause will be found in the character of the
arrangements we are considering, and the irregular action of the power
which controls them.
Nor is the temperature of the northern hemisphere, north of the tropics,
equal in the same latitudes. Very great diversities exist in the "annual
mean" as well as the "mean" of the different seasons. Accurate
observations at many points have enabled men of science to demonstrate
this by drawing isothermal lines (_i. e._, lines of equal average annual
heat) from point to point around the earth, which show at a glance these
differences. The annexed cut is a polar projection of the isothermal lines
of the northern hemisphere, as far down as the tropic, copied from
Kaemtz's Meteorology. The dotted lines show the parallels of latitude, the
dark lines the isothermal lines, or lines of equal annual average
temperature. The reader is desired to observe how rarely they correspond
with the parallels of latitude, and how they fall below in a few
instances, and in others with great uniformity rise almost to the pole.
Take, for example, the isothermal line of 0 or zero--that is, the line
where the mean or _average_ height of the thermometer _for the year_ is at
zero. At Behring's Straits this line is a little below the Arctic circle,
or the parallel of 66.30 north latitude. Passing east over North America,
it descends into Canada, almost to Lake Superior, and to about the 50th
parallel: that is to say, it is on an average during the year as cold on
our continent at the 50th parallel as it is near Behring's Straits at the
65th parallel. Passing east, the line of zero rises again over the
Atlantic Ocean until, in the meridian of Spitzbergen, it reaches, within
the Arctic circle, up almost to the 75th parallel. So, too, the isothermal
of 5 deg. below zero, which is below the 60th parallel in Siberia, rises in
the North Sea, above Behring's Straits, to the parallel of 75 deg., descending
on the continent in North America to the 55th parallel, and rising again
almost to the pole at Spitzbergen, to descend again in Siberia, while the
isothermals of 10 deg. and 15 deg. below zero, which in North America are but just
above the latitude of 60 deg. and 75 deg. respectively, ascend abruptly
_surrounding the magnetic pole_, and _falling short of the geographical
one_. Let this projection of the lines of equal temperature, and
particularly the situation of the magnetic poles, be studied well, for we
shall recur to it hereafter in illustration of many important portions of
our subject.
It is apparent from these facts, and were it necessary might be rendered
still more so by referring to others, that other causes operate in the
distribution of heat over the earth besides the direct action of the sun's
rays upon it. Doubtless very considerable allowance is to be made for the
difference of seasons, and difference during the same season upon the
land and upon the ocean; in mountainous countries and level ones. But
making every allowance for them, the fact that other causes have a
_controlling_ influence in producing the deviations still remains most
obvious. Neither the difference of temperature between the land and the
ocean, or land surfaces of unequal elevations, will account for the
elevation of the isothermal lines on different portions of the ocean, or
their extension around the magnetic poles.
Returning to a consideration of the arrangements for the diffusion of
heat, we observe: First, that the earth itself is intensely heated in its
interior. This is inferred, and justly, from the fact that the thermometer
is found to rise about one degree for every fifty-five feet of
descent--whether in boring artesian wells, exploring caves, or sinking
shafts in mines. It is demonstrated, also, by the existence of hot springs
and the action of volcanoes. Heat is supposed to be conducted from the
center toward the surface every where, but with difficulty and slowly. It
is also supposed to be conducted from the tropical regions toward the
poles. Such is the opinion of Humboldt. (Cosmos, vol. i. p. 167.)
Probably it reaches the surface and exerts an influence, also, upon the
weather through the ocean, and by heating it in its greatest depths.
Little attention has been paid, so far as I am informed, to the question
how far the ocean is thus heated in _tropical latitudes_. Doubtless a
portion of the warmth of the ocean there is derived from that source, and
it has its influence in changing the temperature of the deep-seated cold
polar currents of, the great oceans. Perhaps it may yet be found that the
icebergs are detached by it in the polar seas--the observations of Dr.
Kane point to such a result. (Grinnell Expedition, p. 113, and also chap.
48.)
Little need be said of the inconsiderable quantities of heat supposed to
be derived by radiation from the stars, the planets, and from space. If
any such are derived they are too inconsiderable to be of importance in
this inquiry.
Heat is also carried, and in quantities which exert very considerable
influence upon the weather, from the tropics to the poles by the great
oceanic currents which flow unceasingly from one to the other.
The most important of these with which we are acquainted is the Gulf
Stream of the Atlantic. Gathering in the South Atlantic, and passing north
through the Caribbean Sea and the Gulf of Mexico, it issues out through
the Bahama Channel, and flows north along the eastern coast of the United
States, but some distance from it, to Newfoundland, and from thence
continuing to the north-east and spreading out over the surface of the
ocean--a portion of it mingling with the waters of the North Atlantic in
passing--it flows up on the western coast of Europe, around the Faroe
Islands, and Spitzbergen, to the polar sea; passing around Greenland, and
perhaps through its Fiords, it descends again through the sounds and
channels of the Arctic regions into Baffin's Bay, and through Davis's
Straits, burdened with the icebergs and floes of the polar waters, to
return again to the South Atlantic. For reasons which will appear in the
sequel, it has comparatively little influence upon the weather of the
United States. Western Europe, however, Greenland, the islands which lie
in its course, and the polar seas, are most materially influenced.
Although not the only cause, it has very much to do with the remarkable
elevation of the isothermal lines over the Northern Atlantic, and upon
Western Europe, as seen upon the map.
A like oceanic current exists in the Pacific Ocean, the influence of which
may also be traced upon the map by the elevation of the isothermal lines
at the northern extremity of that ocean, and upon the north-west coast of
North America. A vast amount of heat is transported from the tropical to
the temperate and frozen regions of the earth by these great oceanic
currents.
Another supply is derived from aerial currents which flow from the tropics
toward the poles. These currents exist every where over the entire surface
of the earth, but in more concentrated volumes along the great "lines of
no variation," and greater magnetic intensity, on the western side of the
great oceans, over the eastern portions of the two continents of North
America and Asia. Not, as meteorological writers suppose, in the upper
portions of the atmosphere, having risen in the trade-wind region and run
off at the top toward the poles by force of gravity, but near, and
sometimes in contact with the earth. The influence of these aerial
currents upon the temperature of the atmosphere, and in producing the
phenomena we are to consider, is exceedingly important. We shall have
occasion to examine them with great care and minuteness under another
head, for upon them, more than any other portion of the arrangements,
depend not only the diffusion of heat, but also the distribution of
moisture.
Still another supply of heat, during the sudden changes, at least, is
produced by the action of terrestrial magnetism and electricity. Very
great progress has been made within a short period, in the investigation
of the nature of these agents. The identity, or at least intimate
association or connection of heat, light, electricity, and magnetism,
always suspected, has been in various ways, and by a variety of
experiments demonstrated. The influence of magnetism if distinct from
gravitation, is second only to that; and its agency in producing the
phenomena we are considering is primary and controlling. We will only, in
this connection, ask the reader to note the situation of the north
magnetic poles (for there are two of them); the manner in which the
isothermal lines _surround_ them; the fact that they are _poles of cold_,
_i. e._, that it is colder there than even to the north of them. We shall
recur to this part of the subject again.
Such, briefly considered, are the principal arrangements by which heat is
diffused over the earth.
Equally marked by infinite wisdom, and equally interesting and important,
are the arrangements by which moisture is distributed. Doubtless the
general belief is that this is a simple process; that water evaporates
and rises till it meets a colder stratum of atmosphere, and then condenses
and falls again; or that, according to the Huttonian theory, currents of
air of different temperatures mingle and equalize their heat, and the
aggregate mass when equalized in temperature is cooler, and therefore is
unable to hold as much moisture in solution as the most heated portion
had, and the excess falls in rain. But the process is by no means so
simple, nor is heat the sole or most powerful agent concerned in it.
Currents of air do not mingle, but stratify. Evaporation from the surface
of any given portion of the earth outside of the tropics does not alone
supply that portion with rain. _Vast and wonderful, coextensive with the
globe itself, and perfectly connected, is the machinery by which that
supply is furnished even to the most inconsiderable portion of its
surface._
Take your map of North America and note, in this respect, its
peculiarities. It extends from the Isthmus of Darien to the Arctic
regions, and from the 65th to the 160th meridian of west longitude from
Greenwich, and has upon its surface a type of every climate in the world.
For the purpose of simplifying and illustrating the matter in hand, let us
divide it into five sections. Let the first section embrace Central
America and Southern Mexico, south of 28 deg.; the second, Northern Mexico and
Southern New Mexico, California, etc., between the parallels of 28 deg. and
32 deg.; the third, Northern California, Utah, Southern Oregon, and Western
New Mexico, north of the parallel of 32 deg.; the fourth, the entire
continent north of 42 deg.; and the fifth, the eastern United States, east of
the meridian of 100 deg.. These divisions are not intended to be entirely
accurate in their separation, but substantially so for the purpose of
illustrating the differences which exist in each.
The accompanying diagram shows approximately, by dotted lines, the
divisions.
[Illustration: Fig. 2.]
Now let us see in what a diverse manner, and to what a different extent,
they are severally supplied with moisture.
Central America and Southern Mexico lie within the tropics--their rains
are tropical rains. The season is divided into wet and dry, as are the
seasons of all tropical countries which are not rainless. During the rainy
season it rains a portion of nearly every day, and during the dry season
the sky is clear, the air is pure, and rain seldom falls.
All around the earth within the tropics, over the land and over the sea,
there is a belt of almost daily rains, varying in width, north and south,
in different sections, but averaging about five hundred miles. This belt
of daily rains is formed at and by the meeting of N. E. and S. E. trades,
and travels north and south with them, as they do with the sun,
_encircling the globe_. By this narrow belt a portion of the earth's
surface, an average of some 35 deg. of latitude, is supplied with moisture.
Wherever it is situated at any given period, the tropical rainy season
exists; and when it is absent in its northern or southern transit, the dry
season prevails. Southern Mexico is within the range of this moving belt,
and in its course to the northward with the sun, in our summer from May to
October, it arrives over, and covers that country with a rainy season.
When the sun returns to the south, taking with it the trades and this belt
of tropical rains, that portion of Mexico is without rain, and dry, and so
continues until the rainy belt returns in the following year. While the
belt is over Southern Mexico it is nearly all _precipitation_, and there
is little _evaporation_; while that belt is _absent_ it is all
_evaporation_, with little or no _rain_. Surely this is not consistent
with the prevailing belief of simple evaporation, ascent to a colder
stratum, commingling, and condensation, and rain. Southern Mexico at least
is not supplied by mere evaporation from its surface, and must therefore
form an exception to that belief, and to the Huttonian theory.
But we shall recur again to the peculiarity of distribution within the
tropics.
Turn now for a brief space to Northern Mexico, Southern New Mexico, and
Southern California. In Northern Mexico, Southern New Mexico, Utah, and
California, between the parallels of 28 deg. and 32 deg., and particularly west of
the mountain ranges, we find an almost rainless region, sterile and
worthless, resembling that which is found upon nearly the same parallels
of north latitude in Northern Africa, Egypt, Arabia, Beloochistan,
Afghanistan, and North-western India; and in corresponding latitudes south
of the Equator, in Peru, a portion of Southern Africa, and the northern
and middle portions of New Holland. Why Northern Mexico and the other
countries named are thus sterile and comparatively rainless, we shall see
hereafter, when we examine critically the machinery of distribution as it
operates within the tropics. It is the fact that it is thus sterile and
rainless to which we desire to call attention in this place.
Mr. Bartlett thus describes it:
"On leaving the head waters of the Concho, nature assumes a new
aspect. Here shrubs and trees disappear, except the thorny chaparral
of the deserts; the water-courses all cease, nor does any stream
intervene until the Rio Grande is reached, three hundred and fifty
miles distant, except the muddy Pecos, which, rising in the Rocky
Mountains, near Santa Fe, crosses the great desert plain west of the
Llano Estacado, or Staked Plain.
"From the Rio Grande to the waters of the Pacific, pursuing a
westerly course along the 32d parallel, near El Paso Del Norte, there
is no stream of a higher grade than a small creek. I know of none but
the San Pedro and the Santa Cruz--the latter but a rivulet, losing
itself in the sands near the Gila--the other but a diminutive stream,
scarcely reaching that river. At the head-waters of the Concho,
therefore, begins that great desert region, which, with no
interruption save a limited valley or bottom-land along the Rio
Grande, and lesser ones near the small courses mentioned, extends
over a district embracing sixteen degrees of longitude, or about a
thousand miles, and is wholly unfit for agriculture. It is a
desolate, barren waste, which can never be rendered useful for man or
beast, save for a public highway."--_Bartlett's Personal Narrative_,
vol. i. p. 138.
Turning now to Central and Upper California, and Utah, and Southern
Oregon, we find still another peculiarity. Like Southern Mexico, they have
a rainy and dry season, but at a different period, and for a different
reason. The dry season of California, etc., is the summer of the northern
hemisphere, and her rainy season the winter. _California_ is, therefore,
_dry_ when Southern _Mexico_ is _wet_, and _vice versa_. The belt of rains
which supplies California with moisture during her rainy seasons is the
belt of _extra-tropical_ rains, which extends from the northern limit of
the north-east trades to the poles, encircling the earth. The southern
edge of this extra-tropical belt is _carried up_ on the western coast of
America, and in that portion of the continent in _summer_, when the sun
and trades, and the inter-tropical rainy belt travel to the north, and
uncover California, etc., leaving them without rain for a period of about
six months.
[Illustration: Fig. 3. IN SUMMER.]
As the sun, with the trades, travels south, the southern edge of the belt
of extra-tropical rain follows, and covers California, etc., again
extending gradually from the north to the south, and thus their wet
season returns. The annexed diagrams by the shading will show the
situation of the rainy belts which cover Mexico, Utah, New Mexico, and
California in summer and winter, and that the belts of rains are entirely
distinct and different in character.
[Illustration: Fig. 4. IN WINTER.]
Here again in this section of the continent, as in Mexico, evaporation is
going on for six months of the year, and were it not for the return of the
belt of rains from the north, in the fall, would go on for the entire year
without precipitation; and for the other six months precipitation is
vastly in excess. Nor can this be reconciled with, or explained by, the
Huttonian or any other received theory of rain. Here again it is obvious
that evaporation alone, however great or long continued, will not furnish
the evaporating section with rain.
The northern portion of the continent lies beneath the zone of
extra-tropical rains, and north of the northern limit of the N. E.
trades--is never uncovered from it, and has no distinct rainy or dry
season, although more rain falls at certain periods, and in certain
localities, than at others. The climate of that part of Oregon which lies
upon the Pacific, and the character of its rains, resemble those of
North-western Europe, and will be further explained hereafter.
Coming to the portion of the continent which we occupy, the 5th section,
we find it different still--a most favored region. Portions of it--Eastern
Texas, for instance--are upon the same parallels of latitude as the
rainless regions of Northern Mexico, etc. Eastern Texas, however, is not
rainless. Other portions are upon the same parallels as California, etc.,
yet have no distinct rainy and dry season. We repeat, this section is a
most favored region--without a parallel upon any portion of the earth's
surface, except, in degree, in China and some other portions of Eastern
Asia.
It is not only without a distinct rainy and dry season, but it is watered
by an average, annually, of more than forty inches of rain, while Europe,
although bounded on three sides by seas and oceans, and apparently much
more favorably situated, receives annually an average of only about
twenty-five--if we except Norway, and one or two other places, where the
fall is excessive. The distribution of this supply of moisture over the
United States is, in other respects, wonderful. Iowa, in the interior of
the continent, far away from the great oceans, on the east or west, or the
Gulf of Mexico on the south, receives fifty inches; some ten or fifteen
inches more than fall upon the slope east of the Alleghanies, and
contiguous to the great Atlantic (from which all our storms are,
erroneously, supposed to be derived), and the average over the entire
great interior valley is about forty-five inches, falling at all seasons
of the year.
Observe, then, by way of recapitulation: Southern Mexico has a rainy
season furnished by the belt of _inter_-tropical rains, which _travels up
over it from the south_ in summer. California has a rainy season, which is
furnished by the _extra_-tropical belt of rains, which travels _down from
the north_, and covers it in winter. Northern Mexico and the adjoining
regions west of the 100th meridian are between the limits of the two, and
neither travels far enough to reach them, except for brief and uncertain
periods; they are comparatively rainless; while the eastern portion of
the continent, _in all latitudes_, unlike the others, is without a
distinctly marked dry season, or a rainless region, and with the exception
of occasional droughts, is abundantly supplied with rain at all seasons of
the year.
And now, what is the explanation of all this? What produces the
extra-tropical belt of regular rains surrounding the earth, north of the
parallel of 30 deg. north, in some places, and 35 deg. in others, extending to the
pole, with its southern edge traveling up ten or more degrees in summer,
leaving large portions of the earth subject to a dry season; and back
again in the winter to give them a rainy one? What produces the narrow
belt of inter-tropical rains, encircling the earth; traveling up and down
every year over an average of 35 deg. of latitude, supplying every portion of
it alternately with rain? And what connects the two together over the
eastern portion of North America, so as to leave no distinctly marked wet
and dry season, and no rainless and sterile portion there? Are all these
the result of simple evaporation, ascent to a colder region, condensation,
and descent again? Demonstrably not. Of the forty inches which fall
annually upon the middle and eastern portions of the United States, an
average probably of one-half or twenty inches, runs off by the rivers to
the ocean, or is carried away eastward by the westerly and north-westerly
evaporating winds. The same is true, in degree, of the rain which falls
upon the other portions. Evaporation, therefore, could not keep up the
supply. From whence, then, does it come? this twenty inches, thus lost by
the rivers and winds, and with such wonderful regularity every year.
"All the rivers run into the sea, yet the sea is not full. _Note the place
whence the rivers come, hither they return again._"
But how is it that they thus return with such wonderful regularity, in a
narrow traveling belt of daily rains within the tropics, and a movable
belt of irregular rains without the tropics, extending to the poles,
leaving a space on each side of the equator encircling the earth in like
manner (except at two points, _viz._, Eastern Asia and Eastern North
America), from which they do not go, and to which they do not return, and
which is almost entirely unfurnished with rain? And all this without any
relation, whatever, to the contiguity of the oceans? Obviously this is not
the work of mere evaporation, or of the accidental or irregular
commingling of winds with different dew points, or quantities of moisture
in solution, or accidental, irregular changes of barometric pressure. _It
is one vast, wonderful, connected, and regular system--co-extensive with
the globe--necessary to the return of moisture from the oceans upon the
most inconsiderable portion of it, and to the condensation of the local
moisture of evaporation; and by it the waters are returned from the oceans
as regularly and bountifully upon the far interior of the great continents
in the same latitudes, as upon the "isles which rest in their bosoms."_
CHAPTER II.
Before proceeding to an examination of this connected atmospheric
machinery, and an investigation of the particular ocean from which our
rivers return, it may be well to look at the form in which they appear to
return, that we may have a clear understanding of terms.
They seem to return in the form of clouds, and in storms and showers,
although, in truth, they return in regular, uniform, ordinarily invisible
currents, and the storms and showers are but condensations in, and
discharges from portions of those currents, aided by the local moisture of
evaporation.
The term _storms_, seems to be used by European meteorologists to denote
what we term thunder showers or gusts, and tornados; while what we call
storms are denominated by them regular rains. As the terms are extensively
in use in this country, we must adhere to the meaning attached to them
_here_ rather than _there_.
Storms with us, then, are regular rains of from six to forty-eight or more
hours' continuance: generally without lightning, or thunder, or gusts, and
usually with wind of more or less force, from some easterly point. They
are called north-east storms, or south-east storms, according to the
point from which the surface winds blow. Practically we shall find that
this distinction is of some importance, for the north-east storms are the
longest, lasting generally twenty-four hours, or more, while the
south-east ones seldom, if ever, continue as long.
These storms extend over a considerable surface, rarely less than one
hundred miles in one direction or another, and sometimes fifteen hundred,
or more. Distinct showers cover but a small surface, sometimes not more
than forty to one hundred rods, as in the tornado, and rarely more than
ten miles. Belts of showers, each new one forming a little more to the
south, often, in summer, pass across the country, following each other in
succession; and these belts may be of considerable width, say thirty to
one hundred and fifty miles.
The clouds which constitute the storms and showers differ in appearance
and character, as well in the active as in the forming state. Clouds are
of distinct characters, alike, substantially, every where under like
circumstances; and a distinct nomenclature has been applied to them by Dr.
Howard, of London. He notes three kinds of primary clouds: _viz._, cirrus,
stratus, and cumulus; and inasmuch as the boundary line between them is
not very distinct, certain compounds of the three, _viz._: cirro-stratus,
cirro-cumulus, and cumulo-stratus. This nomenclature is every where
received, and portions of it are of great practical importance.
The three principal descriptions of cloud, _viz._: the cirrus, the
stratus, and the cumulus, we have very much as they have in Europe, and
doubtless as they exist every where outside of the tropics. The nimbus,
another cloud described by him, is not distinct from the cumulus or
stratus. An isolated, limited thunder-shower in a clear sky, presents the
appearance of a nimbus, as shown in the cuts, but the basis of it is a
cumulus, and it differs from an ordinary fair-weather cumulus merely in
the dark and fringe-like appearance of the rain as it is falling from its
lower surface, and sometimes in the existence of a stratus above and in
connection with it. A similar form is often assumed by the peculiar clouds
of the N. W. winds in March or November, when they assume the form of
_squalls_, and drop flurries of snow. The nimbus, therefore, is not a
distinct cloud, but an appearance which the cumulus, stratus, or
cirro-stratus has in a stormy or showery state, and does not deserve a
distinct name. It is but a cumulus, or a stratus, or cirro-stratus
dissolving in snow or rain. It is important that this term should be
abandoned. It tends to confuse and prevent a clear understanding of the
difference in the character of the clouds, and in relation to which
precision is both difficult and desirable.
The figures on pages 27 and 29, show the different kinds of clouds as
designated by Howard. They are copied from the engravings in the sixth
edition of Maury's "Sailing Directions."
[Illustration: Fig. 5.]
[Illustration: Fig. 6.]
Figure 5.
The cirrus is indicated by 1 bird.
The cirro-cumulus by 2 "
The cirro-stratus by 3 "
The cumulo-stratus by 4 "
Figure 6.
The cirrus by 1 "
The cumulus by 3 "
The stratus by 2 "
The nimbus by 4 "
How far these representations correspond with the actual appearance of the
different compound forms in England, I can not say. But although they
convey a _general_ idea, _they are not sufficiently accurate for practical
illustration or observation here_. Indeed Howard himself has omitted from
his last edition his plate of the clouds, assigning as a reason, "that the
real student will acquire his knowledge in a more solid manner by the
observation of nature, without the aid of drawings, and that the _more
superficial are liable to be led into error by them_." The collection of
forms in the cuts _does not contain some very important ones_, and
contains some which are not distinct forms; but they may aid us somewhat
in this inquiry, and, therefore, I have copied them. It is well, also, for
the reader to have the generally received description before him.
But for the purpose of _practical_ illustration hereafter, and greater
precision, I shall follow a somewhat different order in describing them,
and introduce two forms of _scud_ quite as important, practically, as any
other.
First, then, commencing at the earth, we have what may be properly termed
_fog_, or low fog. This forms, in still clear weather, in the valleys, and
over the surface of the rivers and other bodies of water, during the
night, and most frequently the latter part of it, and is at its acme at
sunrise, or soon after, limiting vision horizontally and perpendicularly,
and dissolving away during the forenoon. It is rarely more than from two
to four hundred feet in height at its upper surface, and often much less,
and is composed of vesicular condensed vapor, sometimes sufficiently dense
to fall in mist, and is doubtless in composition substantially what the
clouds are in the other strata of the atmosphere, as observed by us, or
passed through by aeronauts. I have never seen it carried up to any
considerable height into the other strata by any of the supposed ascending
currents, to form permanent clouds, and shall have occasion to allude to
the fact in another connection. It disappears usually before mid-day, and
has, when thus formed, no connection with any clouds which furnish rain.
To this Dr. Howard originally gave the name of stratus, and so it is
represented upon the cut; but the latter term may be with greater
propriety applied to the smooth uniform cloud in the superior strata from
which the rain or snow is known to fall, and I shall retain and so apply
it.
The next in order, ascending, is high fog. This is usually from one to
two thousand feet in height at its lower surface. It forms, like low fog,
during the night and in still weather; and is rarely, if ever, connected
with clouds which furnish rain. It breaks away and disappears between ten
and twelve in the forenoon, usually passing off to the eastward. This fog
is most commonly seen in summer and autumn, particularly the latter, and
unless distinguished from cloud will deceive the weather-watcher. It is
readily distinguishable. Although often very dense, obscuring the light of
the sun as perfectly as the clouds of a north-east storm, it differs from
them. It forms in still clear weather, is present only in the morning, is
perfectly uniform, and, before its dissolution commences, without breaks,
or light and shade, or apparent motion, and unaccompanied by scud or
surface wind. The storm clouds are never entirely uniform, or without
spots of light and shade, by which their nature can be discerned, and
rarely, when as dense as high fog, without scud running under them and
surface winds.
There is another fog still, connected with rain storms, but it does not
often precede them; occurring at all seasons, but most commonly in
connection with the warm S. E. thaws and rains of winter and spring; and
which usually comes on _after_ the rain has commenced and continued for
awhile, and the easterly wind has abated; occupying probably the entire
space from the earth to the inferior surface of the rain clouds or
stratus. Practically this does not require any further notice. It is an
_incident_ of the storm. When formed it remains while the storm clouds
remain, and passes off with them. It is sometimes exceedingly dense in
February and March, when it accompanies a thaw, and if there is a
considerable depth of snow, it has the credit of aiding essentially in its
dissolution.
Mingled with the smoke of London, it produced there the memorable _dark
day_ of the 24th of February, 1832, and at various other times has
produced others of like character. (See Howard's Climate of London, vol.
iii. pp. 36, 207, 303.) These fogs have been so dense there that every
kind of locomotion was dangerous, even _with lanterns, at mid-day_.
The next in order, ascending, are the storm scud, which float in the
north-east or easterly, south-east or southerly wind, before and during
storms.
These, as the reader will hereafter see, are, _practically_, very
important forms of cloud condensation--although they have found no place
in any practical or scientific description given of the clouds, and are
not upon the cuts. They are patches of foggy seeming clouds of all sizes,
more or less connected together by thin portions of similar condensation,
often passing to the westward, south-westward, north-westward, or
northward with great rapidity. Their average height is about half a mile,
but they often run much lower. They are usually of an "ashy gray" color.
The annexed cut shows one phase of them, from among many taken by
daguerreotype. The arrows pointing to the west show the scud distinguished
from the smooth partially formed stratus above. This view was taken a few
hours prior to the setting in of a heavy S. E. rain storm. It is a
northerly view.
[Illustration: Fig. 7.]
At about the same height, but in a _different state of the atmosphere_,
float the peculiar fair-weather clouds of the N. W. wind. They usually
form in a clear sky, and pass with considerable rapidity to the S. E.
Sometimes they are quite large, approaching the cumulus in form, and
white, with dark under surfaces, and at others, in the month of November
particularly, are entirely dark, and assume the character of squalls and
drop flurries of snow; and then resemble the nimbus of Howard. They assume
at different times and in different seasons, different shapes like those
of the scud, the cumulus, or the stratus.
[Illustration: Fig. 8.]
They form and float in the peculiar N. W. current which is usually a
fair-weather wind, and are never connected with storms. In mild weather
they are usually white, and in cold weather sometimes very black, and at
all times differ _in color_ from the ashy gray scud of the storm. This
variety is not represented upon the general cuts. The annexed diagram
shows one phase of them, but they are readily observable at all seasons of
the year, when the N. W. wind is prevailing; differing in appearance
according to the season. Let these, as well as the storm scud, be
carefully observed and studied by the reader, and let no opportunity to
familiarize himself with their appearance be lost. A brief glance at
each recurrence of easterly or north-westerly wind will suffice.
[Illustration: SUMMER CUMULI.]
The _cumuli_ appear in isolated clouds of every size, or in vast clouds
composed of aggregated masses, as the peculiar cloud of the thunder
shower. They form as low down as the scud or fair-weather cloud of the N.
W. wind, which, for convenience, I will call N. W. _scud_; and often in
violent showers, and particularly in hail storms, extend up as far as the
density of the atmosphere will permit them to form. Professor Espy thinks
he has measured their tops at an altitude of ten miles. Others have
estimated their height, when most largely developed, at twelve miles; but
it is very doubtful whether the atmosphere can contain the moisture
necessary to form so dense a cloud at that elevation. It is their immense
height, however, whether it be six, or eight, or ten miles, together with
the sudden and violent electric action, condensing suddenly all the
moisture contained in the atmosphere within the space occupied by the
cloud, which produces such sudden and heavy falls of rain or hail. As the
rain drops or hail, when formed at such an elevation, in falling through
the partially condensed vapor of the cloud must necessarily enlarge by
accretion from the particles with which they come in contact, and probably
also by attraction, their size when they reach the earth, though
frequently very considerable, is not a matter of astonishment. The cumulus
is represented in the general plate with sufficient accuracy to show its
peculiar character.
In summer, when the air is calm, the weather warm, and no storm is
approaching, there is always, in the day time, a tendency to the formation
of cumuli. This tendency exhibits itself about ten o'clock in the
forenoon, and they gradually form and enlarge until about two in the
afternoon; and after that, if they do not continue to enlarge and form
showers, they melt away and disappear before nightfall. Sometimes in July
and August the atmosphere will be studded with them at mid-day, floating
about three-quarters of a mile from the earth (in a level country), gently
and slowly away to the eastward. At times it may seem as if they must
coalesce and form showers, yet they frequently do not, but gradually melt
away, as before stated.
The cumulus is the principal cloud of the tropics, and is not often seen
with us except in summer, or when our weather is tropical in character.
The engraving on the preceding page, shows a phase of these fair-weather
summer cumuli.
The last in order occupying (with their compounds) the higher portions of
the atmosphere, are the cirrus and stratus. The cirrus is often the
skeleton of the other, and precedes it in formation.
These are the proper clouds of the storm, in our sense of the term. While,
however, the cirrus remains a cirrus, it furnishes no rain. When it
extends and expands, and its threads widen and coalesce into cirro-stratus
and stratus, or it induces a layer of stratus below it, the rain forms.
The following is Dr. Howard's description of cirrus: "Parallel, flexuous
or diverging fibers, extensible by increase in any or in all directions.
Clouds in this modification appear to have the least density, the greatest
elevation, and the greatest variety of extent and direction. They are the
earliest appearance after serene weather. They are first indicated by a
few threads penciled, as it were, on the sky. These increase in length,
and new ones are in the mean time added to them. Often the first-formed
threads serve as stems to support numerous branches, which in their turn,
give rise to others."
The illustrations in the general cut are imperfect, and do not represent
the delicate fibers of the cloud, for it is a difficult cloud to
daguerreotype or engrave, but the representation is sufficiently accurate
to give the reader a general idea of the different varieties, and enable
him to discover them readily by observation. They are the most elevated
forms, always of a light color, and often illuminated about sunset by the
rays of the sun shining upon their inferior surface; the sun, however,
often illuminates, in like manner, the dense forms of cirro-stratus, and
the latter, from their greater density, are susceptible of a brighter and
more vivid illumination.
The stratus is a smooth, uniform cloud--the true rain cloud of the storm;
often forming without much cirrus above, or connected with it. It may be
seen in its partially formed state in the bank in the west, at nightfall,
or in the circle around the moon in the night. When it becomes
sufficiently condensed, rain always falls from it, but in moderation. If
there be large masses of scud running beneath it for its drops to fall
through (especially as is sometimes the case, in two or more currents),
the rain may be very heavy. But more of this hereafter.
[Illustration: Fig. 10.]
The annexed cut shows the forming stratus, light and thin, passing to the
east, as indicated by the short arrows just before a storm, while the scud
beneath is running to the west.
It was copied from a daguerreotype view, facing northwardly.
Intermediate between the fibrous, tufted, cirrus, and the smooth uniform
stratus, there is a variety of forms partaking more or less of the
character of one or the other, and termed _cirro-stratus_. No single
correct representation of cirro-stratus as a distinct cloud, can be
given--but several varieties will be hereafter alluded to, under the head
of prognostics. Several modifications are represented with tolerable
accuracy upon the cuts.
The cirro-cumulus is a collection in patches of very small distinct heaps
of white clouds; they are called fleecy clouds, from their resemblance to
a collection of fleeces of wool, and are imperfectly represented on the
general cut. They do not appear often, and are usually _fair-weather
clouds_.
This form has none of the characteristics of the cumulus, and does not
appear in the same stratum. It was probably called cumulus because its
small masses are distinct, as are those of the ordinary cumulus. It occurs
in the same stratum as cirro-stratus, and properly belongs to that
modification. I retain the name inasmuch as the cloud is of some practical
importance.
The cumulo-stratus is seldom seen in our climate, as it is represented in
the cut. Stratus condensation _above_, and in connection with cumulus
condensation, is not uncommon, but that precise form is rare.
This, too, is practically of no consequence, and I shall take no further
notice of it.
Recapitulating, I give (in a tabular form) the three principal strata and
their modifications, located with sufficient accuracy for illustration.
The clouds which are found in an upper or lower portion of a stratum are
so represented by the location of their names; those which appear at all
heights in the stratum, with the names across. The elevation is the
average one--although there is no limit to the cirrus above, except the
absence of sufficient moisture. It was seen by Guy Lussac, and has been by
other aeronauts, at an elevation of five miles, or more, when too delicate
to be visible below.
+------------------------------------------------+
| | 3 miles.
|Cirrus. |
| Cirro-cumulus. |
| Cirro-stratus. |
| |
Primary | { Cumulus extending up |
stratum. | { in violent showers. |
| |
| |
|Stratus. |
|------------------------------------------------|
| | 1-1/2 miles.
Scud & |N. W. scud. { Cumulus Storm scud. |
cumulus |Fair-weather { ordinarily and |
stratum. | { its base always. |
| |
|------------------------------------------------|
| | 1/2 mile.
Fog |High fog. Storm fog. |
stratum. | |
| Low fog at the surface of the earth. |
| |
+------------------------------------------------+
With the assistance of this table of elevations, and a careful
observation, the reader can soon become familiar with the forms of clouds
and their relative situations.
CHAPTER III.
Having thus taken a brief view of the different clouds, let us return to
the inquiry, from what ocean, and by what machinery, _our_ "rivers
return."
Not wholly or mainly from the North Atlantic, although it lies adjacent to
us, and they often _seem_ to do so; for, first, all storms, showers, and
clouds, which furnish, _independently_, any appreciable quantity of rain
to the United States, and even adjacent to the Atlantic, or indeed to the
Atlantic itself, come from a westerly point, and pass to the eastward.
_This is a general, uniform, and invariable law, although there is in
different places, and in the same place at different times, some variation
in their direction; ranging in storms from W. by S. to S. S. W., and in
showers between S. W. and N. W., to the opposite easterly points of the
compass; the most general direction, east of the Alleghanies, being from
W. S. W. to E. N. E._
But do we not see, you inquire--at least those of us who live east of the
Alleghanies--that when it rains, the wind is from the eastward; and that
the _clouds_ follow the wind from the east to the west? You do indeed,
generally, in all considerable storms, observe that the wind blows from
some easterly point, and that _seeming_ clouds are blown by it to the
westward; but what you see, and call clouds, are not the clouds which
furnish the rain. Far above the seeming clouds you notice, directly over
your head when it rains or snows, are the rain or snow clouds, dense and
dark, passing to the eastward, how strong soever the wind may blow from
the quarter to which they tend, or any other quarter, between you and
them. What you see below them are _scud_. So the sailors call them, and so
I have termed them. It is a "dictionary name," and a good one, expressive
of a distinction between them and _clouds_. They are thin, and the sun
shines through them, although with some difficulty, when the rain clouds
above are absent or broken. _This east wind and the scud are not the
storm, or essential parts of it._ Storms occasionally exist, particularly
in April, without either. They are but _incidents_, _useful_, but not
_necessary incidents_, as all surface winds are.
If you could see a section of the storm, you would see the rain cloud
above, moving to the east, and the scud beneath running to the west, as
indicated by the arrows in the cut on page 40. Opportunities frequently
occur when these appearances may be seen. Storms are sometimes very long,
a thousand miles, perhaps, from W. S. W. to E. N. E., and not more than
one to three hundred miles wide from S. E. to N. W., and their sides,
particularly the northern ones, regular, and without extensive partial
condensation. Then the storm cloud above, moving to the eastward, and the
scud running under to the westward, may be seen as in the cut.
So they may be seen before, at the commencement, and at the conclusion of
easterly storms, in a majority of cases, and the reader is desired to
notice them particularly as opportunities occur.
The term _running_, too, is a very expressive one, used by sailors as
applicable to _scud_. For while the forming or formed storm clouds may be
moving moderately along, at the rate of twelve to fifteen or twenty miles
an hour, from about W. S. W. to E. N. E., the scud may be running under
them in a different direction--opposite, or diagonal, or both--at the rate
of twenty, fifty, sixty, and, in hurricanes, even ninety miles an hour.
You have doubtless seen these scud running from N. E. to S. W., and
without dropping any moisture, a day or sometimes two days, before the
storm coming from the S. W. reached the place where you were; and then,
sometimes the storm cloud slipped by to the southward, and the expected
storm at that point proved "a dry northeaster." Sometimes the
condensation, although sufficiently dense to influence and attract the
surface atmosphere, and create an easterly wind and scud, does not become
sufficiently dense to drop rain, and then, too, we have a dry northeaster,
which may melt away or increase to a storm after it has passed over us. _I
have never seen, except, perhaps, in a single instance, one of these
masses of scud, however dense, which had not a rain (stratus) cloud above
it, drop moisture enough to make the eaves run._ So you see it may be
true, and if you will examine carefully, you may satisfy yourself that it
is true, that the storms all move from a westerly point to the eastward,
notwithstanding the wind under them is blowing, and the scud under them
are running to the westward.
There are many other methods by which the reader may determine this matter
himself. He may catch an opportunity for a view, when there is a break in
the stratus cloud above, and the sun or moon, no longer obscured by the
_storm cloud_, shines through the scud beneath. Then he may see they are
moving in different directions. _The upper cloud, if there be any of it
left, always to the eastward._
Again, we may see the storm approach from the westward, as it often does,
before the wind commences to blow, and the scud to run from the eastward;
particularly snow storms in winter, and the gentle showers and storms of
spring.
Again, thunder storms, we know, come from the westward, and apparently
against an east wind. It is sometimes said they approach from the east,
but it is a mistake. During thirty years attentive observation in
different localities, I have never seen an instance. They sometimes _form_
over us, or just east of us, or one may form at the east and another at
the west, and as they _spread out in forming_, one may seem to be coming
from the east, or there may be an easterly current, with dense flocculent
scud at the under surface of the shower cloud running westward, but they
finally pass off to the eastward, and never to the westward. It is
possible that a _patch of scud_ may become sufficiently _dense_ and
_electrified_ to make a _shower_, but I have never observed one. Such an
_apparent_ instance may be found recorded in "Sillman's Journal," vol.
xxxix. page 57. I have seen the scud assume a distinct cumulus form, but
never to become sufficiently dense to make a thunder shower.
Thunder and lightning sometimes attend portions of regular storms in
spring and autumn, but the thunder is always heard first in the west, and
last in the east.
Again, there are admitted facts with which you are conversant, which prove
this proposition. When it has been raining all day, and just at night the
storm has nearly all passed over to the eastward, and the sun shines under
the western edge of it, and "_sets clear_," as it is termed--you say that
"_it will be clear the next day_." Why? Because the storm will not pass to
the westward, covering the sun and continuing, how strong soever the wind
may be from the east; and because it is passing, and will continue to pass
off to the eastward, leaving the sky clear. _The easterly wind will stop
as soon as the storm clouds have passed, and it will fall calm, or the
wind will "come out" from the westward._
So, too, when the clouds are dark in the west in the morning, and the sun
rises clear, but "_goes into a cloud_," as it is expressed, you say that
it will rain. And if the clouds are dense this generally proves true;
because there is a storm or shower approaching from the west, and passing
over to the east, the western edge of whose advance condensation has met
the sun in his coming, and obscured him from your vision.
When, too, it has been storming, and lights up in the N. W. you say it
will clear off; the N. W. wind will blow all the clouds away. It is,
indeed, generally true that when it so lights up it is about to clear off;
although it sometimes shuts down again, in consequence of the approach of
another storm from the westward, following closely behind the one which is
passing off. It is a great mistake, however, to suppose the N. W. wind
blows away the clouds. Watch the smooth stratus rain cloud at its lower
edge, where the clear sky is seen, and you will see that it is moving on
steadily to the N. E., in obedience to the laws of its current, and will
do so, even when its retreating edge has passed up to the zenith, and down
to the S. E.
The storm uncovers us from the N. W. by the contraction of its width, _or_
because it has a _southern lateral extension_ and _dissolution_, and not
by being blown away by the N. W. wind; although that wind, by its peculiar
fair-weather clouds, may be, perhaps, observed beneath, ready to follow
its retreating edge.
Again, when it has been clear all day, and the sun sets in a bank of
cloud, you say--"_it will rain to-morrow, the sun did not set clear_," and
unless that bank is a thunder cloud, merely, which will pass over or by
you, with or without rain, before morning, it is generally true that it
will. The bank will prove the eastern edge of an approaching storm.
From these generally admitted and understood facts, you may know that
storms pass from the west to the east.
This proposition is also proved by all the investigations of storms, which
have taken place since the settlement of this country. Storms of great
severity attract particular attention, and are said to "back up" against
the wind, because they are observed to commence storming first at the
westward, although the wind is from the eastward. Doubtless you recollect
many such instances recorded in the newspapers. No season occurs without
such notices.
Many storms have been investigated by Mr. Redfield, for the purpose of
sustaining his theory. Many others by Professor Espy, to sustain his. One
by Professor Loomis, with great research and ability--and some by others,
accounts of all which have been published; and every one yet investigated,
north of the parallel of 30 deg., has been shown to pass from a westerly to an
easterly point.
So, too, we may know it from analogy. The laws of nature are uniform.
There is a great end to be accomplished, _viz._: the distribution of forty
inches of water, at regular intervals, over a large extent of country. The
rivers are to return, and the clouds are to drop fatness, and seed time
and harvest are not to cease. It is to be done and is done, by means of
storms and showers, and pursuant to general laws, as immutable as the
result. Most of these storms and showers, it has been found, and may be
observed, move from the westward to the eastward. Then we may know, from
analogy, that they do so in obedience to a general, uniform law; and so I
might say with confidence, if our inquiry stopped here, it will ever be
found by those who may hereafter examine them.
But, 2d. There is a current in the atmosphere, all over the continent
north of the N. E. trades, but in great volume over the United States,
east of the meridian of 105 deg. W. from Greenwich--varying in different
seasons, and upon different parallels, and flowing near the earth, when no
surface wind interposes between them. In the vicinity of New York, the
usual course of this current is from about W. S. W. to E. N. E. In the
western and south-western portion of the United States, it is, doubtless,
more southerly--varying somewhat according to the season--and in other
sections varies in obedience to the general law of its origin, and
progress.
I have observed its course in many places, between the parallels of 38 deg.
and 44 deg. N. _This current comes from the South Atlantic Ocean._ It is our
portion of the aerial current, which flows every where from the tropics
toward the poles, to which I have already alluded in connection with the
distribution of heat. _It brings to us the twenty inches of rain which we
lose by the rivers, and by the westerly winds, which carry off a portion
of the local moisture of evaporation, and its action precipitates the
remaining portion of that moisture. It spreads out over the face of our
country, with considerable, but not entire uniformity. All our great
storms originate in it, and all our showers originate in or are induced
and controlled by it._
_From the varied action, inherent or induced, of this current, most of our
meteorological phenomena, whether of wet or dry, or cold or warm weather,
result_; and a thorough knowledge of its origin, cause, and the reciprocal
action between it and the earth, is essential to a knowledge of the
"_Philosophy of the Weather_."
Let us then go down to the "chambers of the south," to the inter-tropical
regions, of which we have said something in connection with a notice of
Southern Mexico, and see where, and how this great aerial current
originates.
CHAPTER IV.
Between the parallels of 35 deg. north latitude, and 35 deg. south
latitude--changing its location within this limit at different seasons of
the year--encircling the earth, and covering about one-half of its
area--we find the trade-wind region. In this region are the simple and
uniform arrangements, which extend every where, and produce all the
atmospheric phenomena. In the center of it we find that movable belt of
continual or daily rains, and comparative calms, particularly _near its
center_, about four hundred and fifty miles in width upon the Atlantic,
and over Africa, and the eastern portions of the Pacific, and something
more over South America and the West Indies, the western portion of the
Pacific and the Indian Ocean, to which we have already alluded. This belt
of rains and calms follows the trades and sun, in their transit north and
south, from one tropic to the other--its width and extension depending
upon the volume of trade-winds existing on the sides of it. Its southern
edge, when the sun is at the southern solstice, extends to 7 deg. south in the
Atlantic, to 10 deg. south in the Indian Ocean, and still further, probably,
over South America: on this point I do not pretend to be accurate, for
accuracy is not essential. When the sun is at the northern solstice the
southern edge is carried up as far as 12 deg. north, over the Atlantic, and
still further over the northern portions of South America, the West
Indies, and Mexico. It travels, therefore, from south to north, over from
twenty to forty degrees of latitude. The presence of this belt of rains
over any given portion of the inter-tropics, gives that portion its rainy
season, and its absence, as it moves to the north, or the south, gives the
portion from which it has moved, its dry season. It passes in its transit
twice each year over some portions of the country, Bogota, for instance,
and two corresponding rainy and dry seasons result. Its presence, and
character, and movements, are as fixed and regular, over from twenty-five
to forty degrees of the earth's surface, _and all around it_, as the
presence and movements of the sun over the same area.
At the northern edge of this movable belt of rain, and extending in some
places, particularly in the Pacific Ocean, north about 20 deg., or about one
thousand four hundred miles, and in other places a less distance, the N.
E. trade winds prevail, blowing toward and into it from N. N. E., N. E.,
and E. N. E., averaging about N. E. At the south line of this belt of
rains, extending south from twenty-five to thirty degrees, or from sixteen
hundred to two thousand miles, the S. E. trades blow toward and into it,
from the S. E., S. S. E., or E. S. E., averaging about S. E. Of course the
northern limit of the N. E. trades travels north and south with the belt
of rain, toward which it blows; and so the southern limit of the S. E.
trades travel in like manner with the rainy belt, or rather, to speak with
entire accuracy, the belt of rain moves with the trades, and the trades
follow the verticality of the sun. The following diagrams exhibit
approximately, and with sufficient accuracy for illustration, the
situations of the rainy belt and the trades, when at their northern and
southern limit, as well as the manner in which it must give certain
localities two rainy seasons each year, in its transit north and south.
At the northern and southern limits of the trade-winds, and extending from
them to the poles, are found the variable winds and irregular
extra-tropical rains, all over the earth, which are shown by the shading
on the maps. This line of extra-tropical rains descends to the south,
following the retreating trades as they descend in our winter, and recedes
north before the trades when they return in spring and summer, so that at
the outer limit of the trades respectively, toward the poles, the line of
extra-tropical rains will be found, receding or following that limit, as
the trades pass up and down with the sun. From the north pole to the
northern limit of the N. E. trade-winds, wherever found, whether at 38 deg.
north latitude, as in some places in summer when the sun is at the tropic
of Cancer; or whether at 20 deg. to 30 deg. north latitude, as in our winter, when
the sun is at the tropic of Capricorn; the extra-tropical rains prevail. A
state of things precisely similar exists between the south pole and the
southern limit of the S. E. trades. Between this northern limit of the
N. E. trades and the northern line of the inter-tropical belt of
rains, wherever situated (with two exceptions, to which we have alluded
and shall allude again), there is, for the time being, a dry season; and a
like dry season between the southern line of the belt of rains and the
southern limit of the S. E. trades. We have, therefore, extending around
the earth, a belt of daily tropical rains, near the center,--two belts of
drought which are mainly trade-wind surfaces, one on each side of the
central rainy belt,--extending to the outward limits of the trades and the
line of extra-tropical rains; and these rainy and dry belts, moving up and
down after the sun, a distance of from twenty to forty degrees of
latitude, each year.
[Illustration: Fig. 10. IN SUMMER.]
[Illustration: Fig. 11. IN WINTER.]
Such are the _main_ phenomena, _at the surface_, in the trade-wind region.
Ascending a step higher in the atmosphere, we find, above the
surface-trades, a counter-trade, running, not in the opposite direction,
but at right angles, or nearly so. The counter-trade which issues from the
northern side of the rainy belt, running to the N. W. or W. N. W., and the
counter trade which issues from the southern side, running to the S. W. or
W. S. W., varying, as the trades do in direction in different localities.
These counter-trades are continuations of the surface trades, which,
ascending in their course, have threaded their way through the opposite
trade in the rainy belt, and are continuing on at the same angle, and in
the same direction at which they blew upon the surface, and in obedience
to the same law. This is apparent from several considerations.
1st. They issue at the same angle, and over the top of the surface trades.
In the West Indies and elsewhere, this has been ascertained and proved by
the course of the storms, and the rotation of their surface winds, and
observation.
2d. We can not suppose the N. E. trade to be reflected, and turn back over
itself at a right angle. That would be impossible, even if there were a
wall of solid material there for it to blow against. Air is a peculiar
fluid, and it stratifies with astonishing ease. He who supposes that a
current of air put in motion can be turned aside by another current, or by
the atmosphere at rest, or can be made to mingle, is mistaken. It will
stratify, and force itself onward through the adjacent and opposing
atmosphere, and in a right line. I have observed some remarkable instances
of this character.
3d. The cause which operates to produce the surface trades, still operates
upon the current to carry it over into the other hemisphere; a
counter-trade, as we shall see. It is impossible, therefore, to believe
that the surface-trades as they arrive at the belt of rains and calms,
turn at a right angle, or at any angle, and return: and impossible to
doubt that they pass through each other in this belt, and out at the
opposite side, as upper currents, at the same angle at which they entered.
Of course the N. E. trade of the Atlantic becomes the N. E. counter-trade
of South America, carrying their storms in a S. W. direction, and the S.
E. trade of the Atlantic the S. E. counter-trade of the West Indies,
carrying all their storms in a N. W. direction; and what is true of them
is true of the trade winds _every where, all over the globe, over the land
and over the sea_.
Doubtless here some one will say, our upper current is a S. W. current.
True, the S. E. trade which enters the belt of rains, and issues out on
the north, a S. E. upper current or counter-trade, keeps that course until
it arrives at the northern limit of the surface trade, when, in _obedience
to another law_, which we shall notice, it gradually _decends near the
surface, curves to the eastward_, and becomes _the S. W. current which
passes over us_. And so we have the S. E. trade-wind of the South
Atlantic, with its moisture, warmth, electricity, and polarity, over, and
perhaps sometimes around us, dropping the electric rain which makes glad
our fields; giving us, when not prevented by other conditions, the balmy
air of spring, the Indian summer of autumn, and the mild mitigating
changes of winter; and thus, _our rivers, which run into the sea, return
to us again_.
But let us go back to the trade-wind region--the region of regularity and
uniformity--and examine somewhat more attentively its features, that we
may more fully understand the character of this counter-trade.
Here are 60 deg. at least of the 180 deg. of the earth's surface, and at its
largest diameter, covered in the course of the year, and of their travels,
by the trade-winds at the surface, the counter-trades above, and the belt
of rains and comparative calms, formed by the action of the opposite
trades, as they thread their way through each other, to assume the
relation of counter-trades. Truly the magnitude, simplicity, and
regularity of this machinery are most wonderful.
There are, however, some _apparent_ anomalies which deserve attention.
Here are most distinctly marked the _rainy_ and _dry seasons_, existing
side by side. Here are the _rainless portions_ of the earth, already but
briefly alluded to; here the _monsoons_, and another peculiarity, _viz._:
the _gathering of the counter-trades_ upon the western sides of the two
great oceans, into two _aerial currents of greater volume_, _analogous_
somewhat to the two _gulf streams_ of those oceans. Let us examine these
anomalies.
The rainy and dry seasons depend, as we have seen, upon the transit north
and south of the rainy belt, or belt of comparative calms. Wherever this
belt may happen on any given day to be situated, each side of it the
trades prevail, it is dry, the earth is parched, and vegetation withers.
These changes are graphically described by Humboldt in his "Views of
Nature," as they occur on the northern portions of South America, as
follows: "When, beneath the vertical rays of the bright and cloudless sun
of the tropics, the parched sward crumbles into dust, then the indurated
soil cracks and bursts, as if rent asunder by some mighty earthquake. The
hot and dusty earth forms a cloudy vail, which shrouds the heavens from
view, and increases the stifling oppression of the atmosphere; while the
east wind (_i. e._ trade-wind), when it blows over the long heated soil,
instead of cooling, adds to the burning glow.
"Gradually, too, the pools of water, which had been protected from
evaporation by the now seared foliage of the fan-palm, disappear. As in
the icy north animals become torpid from cold, so here the crocodile and
the boa-constrictor lie wrapped in unbroken sleep, deeply buried in the
dried soil. Every where the drought announces death, yet every where the
thirsty wanderer is deluded by the phantom of a moving, undulating, watery
surface, created by the deceptive play of the reflected rays of light (the
mirage). A narrow stratum separates the ground from the distant
palm-trees, which seem to hover aloft, owing to the contact of currents of
air having different degrees of heat, and therefore of density. Shrouded
in dark clouds of dust, and tortured by hunger and burning thirst, oxen
and horses scour the plain, the one belowing dismally, the other with
outstretched necks snuffing the wind, in the endeavor to detect, by the
moisture in the air, the vicinity of some pool of water not yet wholly
evaporated.
"Even if the burning heat of day be succeeded by the cool freshness of the
night, here always of equal length, the wearied ox and horse enjoy no
repose. Huge bats now attack the animals during sleep, and vampyre-like
suck their blood; or, fastening on their backs, raise festering wounds, in
which mosquitos, hippobosces, and a host of other stinging insects, burrow
and nestle. Such is the miserable existence of these poor animals, when
the heat of the sun has absorbed the waters from the surface of the
earth.
"When, after a long drought, the genial season of rain arrives, the scene
suddenly changes. The deep azure of the hitherto cloudless sky assumes a
lighter hue. Scarcely can the dark space in the constellation of the
Southern Cross be distinguished at night. The mild phosphorescence of the
Magellanic clouds fades away. Even the vertical stars of the
constellations Aquila and Ophiuchus, shine with a flickering and less
planetary light. Like some distant mountain, a single cloud is seen rising
perpendicularly on the southern horizon. Misty vapors collect and
gradually overspread the heavens, while distant thunder proclaims the
approach of the vivifying rain. Scarcely is the surface of the earth
moistened, before the teeming steppe becomes covered with Killingiae, with
the many-panicled Paspalum, and a variety of grasses. Excited by the power
of light, the herbaceous Mimosa unfolds its dormant, drooping leaves,
hailing, as it were, the rising sun in chorus with the matin song of the
birds, and the opening flowers of aquatics. Horses and oxen, buoyant with
life and enjoyment, roam over and crop the plains. The luxuriant grass
hides the beautiful and spotted jaguar, who, lurking in safe concealment,
and carefully measuring the extent of the leap, darts, like the Asiatic
tiger, with a cat-like bound on his passing prey."
Such is Humboldt's description of the dry season on the Orinoco, and the
return of the belt of rains from the south.
Again, within this trade-wind region are the _rainless countries_. These
are portions of the earth which the equatorial rainy belt does not ascend
far enough north in summer to cover, nor does the southern edge of the
extra-tropical regular rains descend, in winter, far enough south to cover
them, and where, of course, rain seldom, if ever, falls. Such are the
central parts of the Desert of Sahara, Egypt, Arabia, portions of
Affghanistan, Beloochistan, and the western parts of Hindoostan, to the
north of the inter-tropical belt, and a similar state of things exists
south of the equator in parts of South America, Africa, and New Holland,
although upon a comparatively small surface.
Again, another anomaly is the gathering of the trade winds into greater
volumes, on the westerly side of the great oceans, and the consequent
carrying of the equatorial rainy belt up to the region of extra-tropical
rains, on the eastern side of the great continents of Asia and North
America, and the peculiar liability of these aerial gulfs to hurricanes
and typhoons. Such an aerial gulf gathers over the Caribbean Sea, and the
West Indies. Passing across the Gulf of Mexico, it enters over Texas, and
Louisiana, and the other southern states; its western edge passing north
in autumn and winter, on the eastern side of the highlands of Western
Texas, New Mexico, and the Great Desert; curving, as all counter-trades
do, to the eastward as soon as it passes the limit of the N. E. trades,
and spreading out over our favored country, leaving the evidence of its
pathway in the greater quantities of rain, which fall annually upon its
surface. This gathering deprives a portion of the Atlantic, north of the
tropics, of its share of the counter trade, and there, as every where,
where the volume of counter-trade is small, storms and gales are
infrequent, and of less force, and comparative calms prevail. That portion
of the Atlantic has long been known as "the horse latitudes," a name given
to it by our Yankee sailors, because, there, in former times, the
old-fashioned, low-decked, flat-bottomed, horse-carrying craft of New
England, bound for the West Indies, often floundered about in the calms
and baffling winds, until their animals perished for want of water, and
were thrown overboard. Lieutenant Maury, in his most praiseworthy and
exceedingly useful investigation of "The Winds and Currents of the Ocean,"
has defined the situation of these calms and baffling winds at different
seasons--for they move up and down, of course, with the motion of the
whole machinery--and enabled navigators to avoid them, by running _east_
before they attempt to make _southing_; and very materially shortened the
voyages to the equator.
A like gathering, in volume, of the S. E. trade, on the western side of
the Pacific, enters over Asia, and covers China and Malaysia, extending,
in its western course, nearly as far as the western edge of Hindoostan. In
this concentrated volume of counter-trade, and owing to its concentrated
action, form and float the typhoons of the China Sea, and of the Bay of
Bengal; and to this anomalous aerial gulf stream, the S. E. portions of
Asia, from the western desert of Hindoostan, to the eastern portion of
China, north of the rainy belt, owe their great supply of moisture and
fertility, and their peculiar climate. The western line of this volume of
counter-trade is marked by the eastern portion of the rainless region of
Beloochistan, and the north-western deserts of India, as the western edge
of our concentrated volume of counter-trade, is marked by the arid plains
of northern Mexico, western Texas, and New Mexico. On the south of the
equatorial rainy belt, there is no corresponding aerial gulf of equal
volume, as there is no corresponding gulf stream of equal magnitude. On
the western side of the Indian Ocean we find a gathering of the N. E.
trades from the Bay of Bengal and the Indian Ocean, in which form and
travel the hurricanes which prevail--traveling to the southward and
westward--about the Isle of France or Mauritius; and the lagullus oceanic
current, which runs down to the S. W. toward the Cape of Good Hope. But
the extension of South America to the eastward, under, or just south of
the N. E. trades, does not permit the formation of such a concentrated
volume on the western side of the Atlantic, nor is the strength or
regularity of the N. E. trades, on that ocean, equal to those of the S. E.
Nor is the magnetic intensity on the eastern and middle portions of the
Pacific, sufficient to produce such a concentration, in large volume,
there. The trades over that ocean, therefore, curve without concentration,
except a partial one, over the western groups of Polynesia, which the
Asiatic line of magnetic intensity approaches and where hurricanes are
sometimes found, until we arrive near the eastern line of magnetic
intensity, on the eastern side of Asia. We shall, hereafter, have occasion
to follow the anomalous concentrated volumes of the S. E. counter-trade,
of the northern tropic, on the western side of the great oceans, in
explanation of some of the phenomena which we find north of the trade-wind
region. Suffice it here to add, that if it were not for the concentration
of these counter-trades, on the western side of the great oceans, the
rainless region between the parallels of 20 deg. and 30 deg. would encircle the
earth; and China and the Eastern United States would have a distinctly
marked rainy and dry season, as have California, the Barbary States,
Syria, Persia, and other countries which lie north of the rainless region,
within the summer range of the N. E. trades, but also within the winter
descending range of the belt of extra-tropical rains.
Another anomaly which we find in the trade-wind region, is the monsoon.
There are several of them, but they are found, in the greatest strength
and regularity, in the Indian Ocean. Another, defined by the
investigations of Maury, is found on the west coast of Africa, extending
out over the Atlantic. Another prevails on the western coast of South and
Central America. The etesian winds of the Mediterranean are but the N. E.
trades, whose northern limit is carried up in summer, by the transit of
the connected machinery, to the north, over that sea. The N. E. and S. E.
monsoons, so called, of the Indian Ocean, are but the regular trades,
blowing when the belt of rains is absent, as they do all over the globe.
The N. W. monsoon, south of the equator, in the vicinity of New Holland;
the S. W. monsoon which blows from the Arabian Sea, in upon Hindoostan;
the S. W. monsoon of the Atlantic, south of the Cape De Verde Islands; and
the variable west monsoon winds of the west coast of Southern and Central
America, and Southern Mexico (known under several different names, but
chiefly by that of Tapayaguas), are all that deserve attention as such.
At first sight they appear to be anomalies, but the facts declare their
character with perfect certainty. First, they are not continuous, like the
trades, but _prevailing_ winds, and are _storm winds_; _they always blow
toward a region_, _or portion of the ocean_, _covered at the time by
clouds and falling weather_.
Second, they do not blow upon, or toward, heated surfaces of land or
water--_i. e._, toward the dry and parched surfaces, where the dry season
prevails, or from adjoining cold waters on to warm surfaces, but toward
the land or water _situated under the rainy belt_. They are therefore
incident storm winds, (as our easterly winds are incident storm winds) of
the rain clouds of the tropics. They blow in upon the land, under the belt
of rains, while that belt with its daily cloud, and inducing electric
action, is over it, and follow that belt in its transit north and south.
They blow from the warm south polar current of the Atlantic, which flows
N. W. from the coast of Africa, toward the inshore north polar current,
which is there flowing south, but under the belt of rains. In the Indian
Ocean they blow from the center of that ocean, and the Arabian Sea, toward
the belt which hangs over Hindoostan, from the S. W.; and when the rainy
belt travels south they still blow toward, and under it, from the Indian
Ocean, but of course from the N. W. The heated character of the waters of
the Indian Ocean and Arabian Sea, which receive no polar currents, but
heated waters from the Persian Gulf, and from rivers which flow into the
Bay of Bengal over the heated plains of a tropical country, explain this.
So, too, the monsoon of the Atlantic Ocean, does not blow north of the
Cape De Verde Islands,--where the heated surface of Sahara, burning with
the rays of a vertical sun, has a temperature sometimes ranging from one
hundred and forty to one hundred and sixty degrees--but remains under the
rainy belt, drawn from the heated waters which flow up from the South
Atlantic, and travels north as the rainy belt travels north in summer, and
south to the Gulf of Guinea, as that travels south in winter. The same is
true of the Pacific monsoon, the Tapayaguas, the least marked of all,
which blows in during the rainy season upon the west coast of Southern
Mexico, and of Southern and Central America. They are all incident rain or
storm winds, blowing in upon the land, or on to a colder surface of
different polarity, _during the rainy season_; and if it were possible to
catch one of our north-easters, in its passage over our country to the
eastward, and anchor it to the Alleghanies, "paying out" so to have it
reach in part over the Atlantic, and keep it there in operation six
months, we should have a continual easterly wind under it; a _monsoon_
more strongly marked than the monsoons of the Indian, or Atlantic Oceans.
_The received theory in relation to them is a fallacy._
Recapitulating, then, all the phenomena, we have,--_Surface-trades_,
blowing toward the center, passing through each other, and continuing on
as upper or counter-trades; a _belt of rains_, with calms near the center,
formed by the trades where they meet and pass through each other, which
travels with them north and south following the sun; _two belts of
drought_, following the belt of rains and the trades, and followed by the
_extra_-tropical line of rains, as it travels with the trades and the
rainy belt, leaving a part of the earth which the equatorial rainy belt
does not travel far enough north, nor the extra-tropical line of rains far
enough south to cover, and which is consequently a _rainless region_; _the
monsoons_, which are but incidents of the rainy belt, and the _gathered
volumes_ of counter-trade, on the west of the two great oceans, which
usurp the place of the N. E. trades, carrying the rainy belt up to the
region of extra-tropical rain, and preventing the rainless region from
encircling the earth.
Upon _what cause_ do these great central phenomena, so vast, so regular,
so wonderful, depend? What is the _motive power_ of this connected
atmospheric machinery, whose action and influence extend over the entire
globe?
"_Heat, heat_," say the text books, the Professors, the votaries of
meteorology. "All these phenomena are owing to the heat of the sun. It
heats the ocean and the earth--the air is thereby heated and rises, the
cold air rushes in from below, then the ascended current rolls off each
way at the top toward the pole, acquiring a westerly motion from the
rotation of the earth, slipping away from under it, and a different,
_viz._: an easterly motion, after reaching the latitude of 30 deg., from the
_same rotation_; and all the winds and disturbances of the atmosphere are
produced in the same way. They are produced by the action of heated
surfaces upon the adjacent atmosphere."
This is the great theory of meteorologists, by which they attempt to
account for the various atmospherical disturbances, of both tropical and
extra-tropical regions.
The whole theory is a fallacy--it will not stand the test of a careful
examination. The bases of the theory, which are assumed to be facts, are
not so. The agent has not the power claimed for it. A heated surface,
alone, never caused any considerable ascending current, or if it did,
never produced a mile of wind. I repeat it, the theory and all incidental
ones--the thousand explanatory and modifying theories, and
hypotheses--_the whole system_--is without foundation in fact, and will
not bear a critical examination.
Let us see if this language is stronger than the facts will warrant.
The theory assumes that both the land and water, under this central belt,
where the air is supposed to be rising are _materially hotter_ than the
land and ocean are on _either side of it_. Now, how much hotter are the
air and the land under the belt of rains and calms, upon Hindoostan, or
Africa, or South America, where the former is supposed to be acquiring
heat and expansion so rapidly, and to be ascending, than under, and in the
dry belts on either side? None; it is cooler by the thermometer--_much
cooler_.
The central belt of rains in midsummer over Africa, extends up as far as
17 deg. north latitude, and perhaps further. North of this line over the whole
surface of the desert, the Barbary States, a part of the Mediterranean,
and some portion of Italy, the dry season extends, and from the entire
surface the N. E. trade blow into the central belt.[1] Over the desert
they all pass. Now this desert is a sea of sand, under a vertical sun,
intensely heated, blistering the skin with which it comes in contact, and
often acquiring a temperature of 150 deg. to 160 deg. of Fahrenheit. Under the
central belt of rains neither the earth nor air exceed the temperature of
84 deg.. And yet the hot air of the desert does not ascend, but blows into
this cooler central belt; and when it is felt as it blows off the western
coast by the mariner, or even in Guinea, when the belt of rains has gone
south in winter, as it often is as the _harmattan_, it is suffocating and
intolerable. There, then, not only is it untrue, that the land and the air
over it under the rainy belt are hotter, but it is true that intensely
heated air blows horizontally from the Desert of Sahara. Nay, as it will
appear in the sequel, this hottest of all surfaces not only can not have a
vortex, but it can not induce a monsoon, and scarcely a sea breeze. The
same is true in a great degree of the surface, and the air over it, on
either side of the supposed vortex of the rainy belt upon South America.
See the description of Humboldt, already given, where the thermometer
stood as high as 115 deg. of Fahrenheit in the shade, while the N. E. winds,
the regular trades, were blowing over the land. And it is equally true of
Arabia, and indeed of every portion of the earth. There is not a spot upon
the globe where the land and the air are cooler _by the side_ of the
central belt of rains, than _under it_. _And the opposite is true every
where upon the land._
How much hotter is the ocean and air under this supposed vortex? But
little hotter than they are on the side where the sun is not vertical,
_and none on the other_. Let us be a little more particular. The
temperature of the Atlantic under the belt of rains in our winter, and on
the south of the belt at the latitude of 3 deg. south, and down to 9 deg. or more
south, is 82 deg.. The air may range a degree, or possibly two, higher than
the water at either point. On the north this difference is from nothing at
the meeting of the trades and belt of rains, to about 4 deg. at their northern
limit. This is too _trifling_ to be worth one moment's consideration. It
is less, far less than the difference between the water and air of the
Gulf Stream which runs along our coast, and the adjoining waters and air
over them. While on the south side of the belt of rains the _difference is
actually against the theory_--and the same state of things is reversed in
summer, when the sun is vertical at the north.
From the log of an intelligent shipmaster, found in the wind and current
charts of Lieutenant Maury, I abridge the following, which will illustrate
this. Captain Young in February, found the N. E. trades at about 17 deg. north
latitude, with the water at 75 deg. and air at 76 deg., trade-wind N. E.
At 12 deg. 16' the water was 75 deg. the air 76 deg. wind N. E.
Feb. 22d. 9 deg. 49' " 76-1/2 deg. " 77 deg. " N. E.
" 23d. 7 deg. 13' " 78 deg. " 78 deg. " N. E.
" 24th. no obs. " 79-1/2 deg. " 79 deg. " N. E.,
E. S. E. rain.
" 25th. 3 deg. 10' " 81 deg. " 83 deg. " E. S. E. rain.
" 26th. no obs. " 82 deg. " 82 deg. " S. E. to
E. S. E. hazy,
rain & sqs.
" 27th. 2 deg. 24' " 82 deg. " 82 deg. " calm,
with rain.
" 28th. no obs. " 82 deg. " 82 deg. " calm rain.
March 1st. 0 deg. 29' " 82 deg. " 82 deg. " E. S. E.
sqs. rain.
" 2d. 1 deg. 27' S. L. " 82 deg. " 82 deg. " S. E. sqs.
rain.
" 3d. 2 deg. 44' " 82 deg. " 83 deg. " S. E. &
S. S. E.
weather
settled.
" 4th. 4 deg. 17' " 82 deg. " 83 deg. " S. S. E. &
S. E. fair
weather.
" 5th. 6 deg. 08' " 82 deg. " 84 deg. " S. E. fair
wthr.
" 6th. 8 deg. 08' " 82 deg. " 84 deg. " S. E. &
E. S. E. fair
weather.
Here the air was seven degrees colder at the extreme limit of the N. E.
trades than in the _center_ of the belt of rains, as it is, usually, in
mid-winter, but not in summer. On the other hand, _after he left the
region of calms and rains_, where the water and air stood with almost
entire uniformity at 82 deg., on the 3d of March, and for three days
thereafter, during which he was in the S. E. trades with fair weather,
the water was the same as under the supposed vortex, _viz._, 82 deg., _and the
air rose to 83 deg. and 84 deg._! _This is demonstration._
I also take from a letter of Lieutenant Walsh to Lieutenant Maury,
relative to the cruise of the "Taney" the following, showing the warmth of
the Gulf Stream compared with the adjacent ocean.
"We first crossed the Gulf Stream on the 31st of October; we struck
it in latitude 37 deg. 22', longitude 71 deg. 26' as indicated by the
temperature of the water, which was as follows:
8 A.M. water at surface 66 deg.
9 " " " 73 deg.
10 " " " 76 deg.
11 " " " 77 deg.
77 deg. was the highest temperature found in crossing at this time.
Re-crossing it in May, in latitude 35 deg. 30', longitude 72 deg. 35', he
found the water as follows:
8 A.M. water at surface 71 deg. 8'
9 " " " 73 deg.
10 " " " 75 deg. 5'
11 " " " 78 deg. 5'
12 M. " " 78 deg. 5'
79 deg. being the highest temperature found."
The average difference between the temperature of the water of the Gulf
Stream and the adjoining ocean, at the line of division, is about ten
degrees, increasing to more than twenty on approaching the coast, and
within one hundred miles--a far greater difference than is ever found on
the winter side of the inter-tropical rainy belt.
It is not only not so, then, that the surface of the ocean is materially
warmer under the belt of rains than the adjoining surface under the
trades, especially on the summer side, but if it were so, the trades would
not be created thereby, any more than upon the Gulf Stream. And the
opposite is true of the land where the line of calms, and rains, and
drought meet, all around the globe. The fact assumed is therefore untrue.
The hottest surfaces, even at the rainless portion, where there is no
vortex, no storm, and no wind but the continual uniform N. E. horizontal
trade-wind, _never_ created, by reason of the heat alone, a mile of wind,
a storm or shower.
But, again, the belt of calms, where the air is supposed to rise and
create a suction which draws the trades on either side a distance of from
one thousand to two thousand miles, an average of three thousand miles in
all, at least, is not itself, on an average, over five hundred miles in
breadth from north to south. What a wonder of meteorology is here!
With a breadth of five hundred miles, the rising of the atmosphere is
supposed to be so rapid and of such immense volume that it draws the
surface atmosphere, one thousand to fifteen hundred miles on one side and
two thousand on the other, with a uniform steady velocity of twenty miles
per hour. Is this vast suction found by the unlucky mariner who may be
drawn within the vortex? _Not at all._ He finds no rapid suction there,
but _horizontal currents_, not steady, indeed, like the trades, and
sometimes calms _at the center_, but still the _currents are there_, and,
_except near the center, there as squalls, showers, and baffling winds
and as monsoons_.
Again, is there at the mouth of this vortex, or as you approach it, an
increased rapidity in the trade corresponding to the magnitude of its
influence? Does the trade become a hurricane as it approaches the spot
where it is to supply the place of that which has suddenly "expanded by
heat, and been forced to rise, boil over, and run off at the top in turn?"
Not at all. It blows gently, even up to the very line of the rainy belt,
and becomes squally and baffling, falls gradually calm near the center, or
changes to a monsoon.
But, again, the belt of rains is so far from being a belt of calms
strictly, that its monsoons in the Indian, Atlantic, and Pacific Oceans,
at times, extend hundreds of miles out over the ocean. That of the
Atlantic, triangular, with its base resting on Africa, according to
Lieutenant Maury, extends sometimes almost to the coast of South America,
a distance of one thousand miles, and thus under the supposed ascending
vortex. Where is the great uprising suction during the prevalence of this
extensive surface horizontal monsoon beneath it? Manifestly it does not
exist. Nay, that monsoon is blowing from the warm current which sets up
from the Cape of Good Hope toward the Caribbean Sea, and over the cold
north polar current, which runs down between the continent and the Cape de
Verdes. Equally untrue is the presumption that the air rises over heated
portions of the earth elsewhere, and by reason of such heating.
_Perpendicular currents of the atmosphere are rarely seen, never
extensive, or attaining any considerable altitude._ I have watched for
them thirty years. I have seen currents of air ascend, with their moisture
condensing as they ascended, and unite with the under surface of a highly
electrified cloud--the advance condensation of a thunder shower--but that
cloud was moving horizontally at a distance of from one to two thousand
feet above the surface of the earth, and did not rise. I have seen patches
of scud rising from the surface during the intervals of a showery and
highly electrified storm, toward, and uniting with, the clouds above, when
very low, as I have seen them approach and unite horizontally; and
doubtless there is a tendency upwards of the wind, created and attracted
by the summer shower, as may be seen in the ascending dust before the
rain, but I have never been able to detect an ascending current, except as
induced and attracted by a cloud above moving horizontally, in the hottest
day or dryest time. None of the clouds of our climate, even when the earth
is heated and parched by a two months' unbroken drought, can be detected
rising above the strata in which they form. I have watched the cumuli at
such periods when they filled the air, and can assert that they never
rise. The atmosphere moves, invariably, in horizontal strata, and the
whole theory of ascending currents is fallacious.
But let us look still further at the tropical currents. The true harmattan
of north-western Africa (for the term is sometimes misapplied), hot and
blistering, generated upon the sand of the desert--why does it blow from
Sahara horizontally, on or over cooler surfaces, following the belt of
rains as a N. E. trade? Why does it not ascend? The sirocco of north
Sahara, the kamsin or chamsin of eastern Sahara, and the simoon of Arabia,
which blow hot and suffocating from those deserts--why do they blow _from_
heated surfaces and _horizontally over_ cooler ones? Why do they not
ascend? Arabia is surrounded on three sides by seas and gulfs, from which
evaporation is rapid. Her interior deserts are extensive and intensely
hot--why are they rainless? Why do they not have a _vortex_, a _monsoon_,
or even a _shower_? Because there is no such law or action as this theory
supposes. Those winds blow horizontally in obedience to other laws, and
under the control of other and more powerful agents. But further still,
what heating and ascending process is it that makes the variable winds
north of the tropics? that brings in the warm air and fog of the Gulf
Stream upon our _snow-clad coast_, in mid-winter, to increase the January
thaw? Nay, what heating process is it that disturbs the calms of the polar
regions with fresh breezes and gales, sometimes of the force of 6, when
the _sun does not shine_, the thermometer is from 20 deg. to 40 deg. below zero,
the _earth and sea one frozen surface_, and the hardy explorer dressed in
furs, barely lives in his cabin covered by an embankment of snow, and
heated by a stove?
Gentlemen, meteorologists, it will not do. The theory is unsound; the
assumed facts do not exist. The whole universe has not an agent, organic
or inorganic, which can play such absurd and inconsistent pranks in the
face of its Creator, as your various and complicated theories assign to
caloric.
Away with the theory and all its incidental and complicated and mystified
hypotheses, they rest like a pall upon the science;--away with the whole
system, and let us seek some agent whose _power_ and _adaptation_
correspond with the _extent_, and _simplicity_, and _magnificence_ of the
phenomena, and, in some degree, with the _power_ and _wisdom of their
Author_.
CHAPTER V.
One, and the principal end attained by the power of the agent, is the
gathering of a volume of atmosphere from, or near, the _surface_ of the
land and sea, so as to ensure its possession of all the moisture of
evaporation which rises from the locality, and the highest degree of
temperature, and from a space ranging from one to two thousand miles in
width, in one hemisphere, and to carry it over into the other. Not over
the top, or upon the top, of the whole mass of atmosphere situated in the
opposite hemisphere--_out of reach of all influences from the earth_--but
through it, and curving gradually down near to, and within influential
distance of the surface of the earth, soon after it passes the outward
limit of its fellow trade; and to continue the current onward, leaving
portions of it and its heat and moisture on the way, but taking a
considerable volume up and around the magnetic poles--it being impossible
for the entire volume to be thus carried around the poles in consequence
of the diminished circumference of the earth. To this end it is obvious it
must possess _polarity_.
Another end to be attained is to combine the moisture of evaporation with
the air, so that the cold atmosphere through which, or the earth over
which it passes, may not be _continually condensing its moisture_, and
thereby _enveloping the earth in a perpetual mist_; but so that it may
part with it at _intervals_, making _cloudy_ and _clear days_; and part
with it in _portions_, so that a _regular_ and _necessary supply_ may be
furnished to the _entire hemisphere_, even up to the geographical poles.
Is there such an agent? There is, precisely and perfectly adapted to the
ends to be attained, ever there and ever active, and that agent is
_magnetism_.
[Illustration: Fig. 12.]
The earth is a magnet. It has its magnetic poles, and they are distinct
from its geographical ones; and there are two in each hemisphere. They are
situated from 17 deg. to 19 deg. distant from the geographical poles; and ours is
not far from longitude 97 deg. W. from Greenwich, and 71 deg. north latitude.
Navigators have gone north and north-west of it, and found its situation
by the declination of the needle. From these poles, lines of magnetic
intensity extend to the opposite and corresponding pole of the other
hemisphere, and upon or near those lines the needle points north without
variation; and toward these lines of no variation the needle every where,
on either side declines. The foregoing diagram shows the situation of our
magnetic pole and line of no variation, the dip of the needle by the
arrows, and the magnetic equator.
Recent discoveries have shown that the magnetic force is exerted in lines
and currents; that such currents, as physical lines of force, surround
magnets, and currents of electricity. Doubtless such lines of force exist
around the earth and the magnetic poles. There are also _longitudinal_
lines of force existing and active, between the poles, and extending from
one side of the center to the other, occupying nearly one third of the
magnet. If you take a large needle thoroughly magnetized, place it upon
paper and drop filings of iron upon it, they will become arranged about it
in circular and perpendicular, and also in _longitudinal lines_,
conforming to the currents.
[Illustration: Fig. 13.]
This experiment is illustrated in all our books on natural philosophy.
The foregoing diagram, copied from Olmstead's Philosophy, does not show as
accurately as Faraday's projection of the lines upon a globe-magnet the
comparative distance from the poles of the needle, at which the
longitudinal currents commence and terminate, and _where the filings will
not adhere_ to any considerable extent. The lines shown upon the needle
should bear the same proportion to its length as the trade-winds bear to
that of the earth, measured from pole to pole, and if the needle had a
globular form they would so appear.
These lines are made by currents arising from one side of the magnetic
equator, and passing over to the other. Doubtless, just such currents
rise, and pass over upon the earth.
Magnetic and electric currents carry the air with them. This is well
settled by experiment. _Oxygen_, too, is _magnetic_, and capable both of
receiving and retaining polarity and of combining with, or attracting and
retaining vapor, and of course the moisture of evaporation. Here then we
have a power existing, capable of producing the result--precisely, and
with evident wisdom adapted to its production--ever present and active;
and no other known agent can.
Is it not then the agent?
Let us look a little further. This result is affected by the action of the
sun: the trades with the central belts of rains travel north and south
after it; so does the sun affect the magnetic currents every where, even
the magnetic needle is daily affected by its action, as it increases the
intensity of the terrestrial magnetic currents, and hence its well
established diurnal oscillations.
Again, along the eastern lines of the continents which skirt the great
oceans on the west, run the northerly and southerly lines of no variation,
and of greatest magnetic intensity. Here are the trade currents gathered
into a volume, which curve and carry unusual fertility to South-eastern
Asia, and North America, and in those great aerial gulf streams we find
the _intense_ electric action which produces the typhoons of the former,
and the hurricanes of the latter. It may still be said that these
conditions and phenomena of the trade-wind region, are not produced by
magnetism or magneto-electricity, _but the objector can point to no other
adequate power_. That it must be heat, electricity, or magnetism, must be
admitted. There is no other power known. Heat demonstrably can not produce
them. Magnetism or electricity therefore must, and they are doubtless
states or phases of the same power, producing in their different states or
phases the different results. And even heat--atmospheric temperature, is
often, if not always the result of their action. In the present state of
science, it is enough for me that the _magnetic longitudinal currents are
there_; that they are _lines of force_ and _adequate_; that _oxygen is
magnetic_, and therefore the atmosphere must be affected by them--that so
far as we can reason from analogy, they ought to produce the effect upon
the atmosphere which we find produced, and until further light is thrown
upon the subject I shall presume that they do. Every step we take
hereafter in this investigation will confirm the presumption.
There is one peculiarity to be more particularly noticed before we leave
the trade-wind region, and we are now prepared to notice it.
The belt of rains, formed by the currents of the two trades, threading
their way through each other--how are they produced? Why should the place
where the currents thus pass through each other be a place of almost daily
precipitation? There is, in fact, no ascension, except that which the
currents have in their line of ascent to attain the elevation which the
magnetic law of the current requires.
The trades have passed over an evaporating surface and are charged with
moisture. This moisture they hold in magneto-electric combination.
_Evaporation_ does not depend upon _temperature_. Ice and snow evaporate
at all temperatures (Howard, vol. 1, p. 86). So the cold N. W. wind, full
of positive electricity, will lap up, as it were, the pools from the
earth, with astonishing quickness; and when this electricity is deranging
the action of the machinery and material of the manufacturer, he allays it
by a supply of moisture, with which the electricity can combine. Nor does
the air lose its moisture when below the freezing point. In all parts of
the atmosphere, as at the surface of the earth in winter, moisture is held
in large quantities in the coldest and severest weather; and it is not
till it moderates, and a perceptible _electric_ change takes place, that
it is precipitated as rain or snow. Doubtless there is an exposure of
considerable surfaces, of opposite currents, charged with opposite
polarity, and a constant depolarization where their surfaces meet. May
there not be a consequent dissolution of the electro-magnetic combination
between the air and moisture, or the excitation of that electric action
which attends or produces like rains every where? and hence the constant
precipitation. This is rendered probable, by the fact that precipitation,
at the meeting of the trades, takes place in level countries in the
day-time, between 10 A. M. and sunset, in showers, with thunder and
lightning, as with us in summer, although among the mountains the rain
sometimes falls in the night also. The precipitation in the heat of the
day is obviously induced by the action of the sun, although it is by no
means certain that the friction of the opposing surfaces does not assist
in the operation.
I am well aware that the lines of magnetic force curve upward and carry
the trades with them, and that, therefore, precipitation by condensation
from the mere cold of the upper stratum of the atmosphere is possible.
But, there are three reasons why I do not believe such to be the fact.
1st. Precipitation takes place in the day time mainly, and in sudden,
isolated, heavy showers and not in steady continuous rain. Nor is there
condensation or continual mist at other hours of the day.
2d. They occur at a time of day when the sun is affecting the magnetic
currents most powerfully, _viz._, between ten o'clock A. M. and sunset,
and mainly at the time of greatest heat.
3d. The counter-trades _do not precipitate_ after they leave the rainy
belt, although at a great elevation, until they reach the outward limits
of the trades; and they _do precipitate again_, although they gradually
descend _nearer the earth_, as soon as they become subject to the action
of the currents of an opposite magnetism. Their precipitation is partial
too, even then, and they carry a portion of their moisture through an
atmosphere of the coldest temperature up to the geographical poles.
A similar result attends the action of the sun in the extra-tropical
regions. Cumuli commence forming in the counter-trade, or at the line
between that and the surface current, at the same time of day that the
diurnal motion of the magnetic needle commences, or the rain clouds form
in the tropics; they continue to enlarge here as there, till about the
same hour of the day that the _needle_ obtains its maximum diurnal
variations; and when the influence of the sun upon the needle ceases, and
it returns to its original status, the cumuli disappear. Hail storms too,
it is said, always, or generally occur in the day time.
In like manner the sea-breezes and other fair-weather surface winds, rise
in the forenoon with the influence of the sun upon the magnetic currents
and the needle, and die away at nightfall when the influence ceases.
There are other electro-magnetic, or to speak more correctly,
magneto-electric, effects of the sun's action equally illustrative, which
tend to show that the precipitation at the passing of the trades, is the
result of their action upon each other, aided by the sun, to which we
shall allude when we come to speak of the causes and character of the
surface winds of the extra-tropical regions.
As, however, this takes place only, or mainly, where the threading
surfaces meet, it is but partial, and the body of the respective polarized
currents pursue their way unaffected, toward the opposite magnetic
pole--and there for the present we leave them.
Storms sometimes originate in these currents, when concentrated, as in the
West Indies, the China Sea, the Bay of Bengal, and Indian Ocean, while
passing through the rainy belt, and move with the current to the
north-west if issuing on the north side of it, and to the south-west if
issuing on the south side of it, until they respectively get beyond the
extreme limits of the trades, and then they curve to the eastward,
imbedded in and following their current. The peculiar extension of the
land to the east on the northern portions of South America, prevents the
gathering of an aerial gulf similar to the one which we have described to
the north-west, entering upon our division of the continent over the Gulf
of Mexico. It is otherwise in the Indian Ocean, and there the storms are
found issuing from the rainy belt on the southern side, sweeping over the
Mauritius and other islands of that ocean, and _often simultaneously_ with
storms issuing on the north over the Bay of Bengal. Colonel Reid mentions
instances and gives a diagram.[2]
These storms in milder forms issue from the rain belt at other points, and
may issue any where, but will always be found most extensive and most
violent, that is to say, as hurricanes and typhoons, in the concentrated
volumes of counter-trade on the western side of the great oceans, within a
few hundred miles of the lines of magnetic intensity and no variation, and
when they form in the rainy belt they are highly electric. Most
frequently, however, as we shall see, they form in these currents after
they have issued from the rainy belt, and after they have passed the
extreme limits of the trades and become subject to the circular and
perpendicular magnetic currents which exist north and south of the
longitudinal ones, and which when seen upon the magnetic needle, attract
the filings and cause them to adhere--although but slight attraction or
adhesion takes place where the longitudinal currents exist.
Such, then, are the atmospheric arrangements and phenomena of the
trade-wind region, and the cause that produces them; such is the character
and cause of the enlarged volume of counter-trade, which spreads out and
blows over our country as permanently as the S. E. trades blow on the
South Atlantic and South America, returning to us the rivers which had run
from us to the sea.
CHAPTER VI.
Coming back now, to a consideration of the course and functions of the
counter-trade after it leaves the northern limit of the surface-trades, we
find it curves to the eastward and gradually assumes about an E. N. E.
course, and becomes a W. S. W. current where it crosses the line of no
variation, and continues on until it passes off over the Atlantic; and
this course and curve is analogous to what may be found true of the
counter-trades every where. It is best illustrated by the course of all
the storms (in the American sense of the word, as distinguished from
thunder showers and other brief rains), which have been traced north or
south of the limits of the trades. It was found by Mr. Redfield in most of
the storms investigated by him, which originated within, or north of the
tropics.
Doubtless it was the actual course of the others, and that the
investigation was imperfect. All the great autumnal, winter, or spring
storms which have traversed the whole or any considerable portion of the
territory of the United States, east of New Mexico, which have been
investigated by Professors Espy, Loomis, Redfield, or others, have been
found to follow this course. A storm which passed over Madeira, appears
from the investigations of Colonel Reid to have followed the same law of
curvature.
And so, doubtless, did another which he has described as passing over the
Levant. The storms which supply the winter rains of California and Utah,
reach them by this law of curvature and progress, after the northern
limits of the trades have descended to the south with the sun, so that the
counter-trades of the Pacific may descend to the surface and curve in upon
them. But the absence of a concentration of the counter-trade, and its
deficient action because of its passage over mountain ranges, and their
location so near the northern limit of the trades that their storms can
not expand and become extensive, as well as their weaker magnetic
intensity, prevent their storms from becoming violent, and their supply of
rain is not large and much of it falls in showers. The same is true of the
Barbary States, of Syria, and Persia, and of Southern Europe; and indeed
of all the countries of the globe which lie between the winter and summer
extreme limits of the surface-trades, and without the limits of the two
concentrated counter-trades. Enough appears in the writings of the
meteorologists of Europe to show, that their long continued rains, which
are analogous to our storms and are _preceded by the formation of the true
cirrus of the counter-trade_, follow the same great law of curvature and
progress; although the presence of the Gulf Stream with its mass of south
polar waters on the western side of the British Islands, Denmark, and
Norway supplies them with showers, and fogs, and cumuli from the west and
north-west, and makes the mean of the surface winds of their storms
somewhat variant from ours. A like law reversed prevails in the southern
hemisphere. The storms of New Holland and the Indian Ocean, south of the
limits of the trade, curve to the eastward and travel about south-east,
their _south-west_ being a _clearing off wind_ as our _north-west_ is, and
_precisely similar in all its other characteristics_, where the relation
of magnetic intensity is the same.
The storms of the Pacific on the S. W. coast of South America, in like
manner travel to the S. E., flooding the western slopes of the mountain
ranges with rain, and aggravated by the intensity of the magnetic currents
at the extremity of the continent in a high latitude, meet the mariner in
the face as he emerges from under the lee of the land and attempts to pass
the Horn. It will ultimately be shown that the precipitation which takes
place, as the storms and counter-trades pass north and east in the
northern hemisphere and south and east in the southern hemisphere, is
owing less to cold than increased magnetic intensity. And all this is the
result of one great uniform law, existing every where, varying in its
phenomena only in consequence of the difference in volume, and
magneto-electric intensity of the portions of the counter-trade, as of the
surface-trade at different places, and the different magnetic intensity of
the local perpendicular and circular currents of the earth over which they
pass, at different periods and at different points.
Mr. Redfield and Lieutenant Maury have assumed that our S. W. current
comes from the Pacific Ocean. Aside from the adverse evidence which the
investigations of the former in relation to the course of the West Indian
storms, and their curving over the continent, furnish to the contrary, and
that which has herein before been stated in relation to the law of
curvature, it is obvious they are mistaken, for another and conclusive
reason.
In order to reach us from the Pacific in a direction from S. W. to N. E.,
it must pass the table lands and mountain ranges of Mexico and New Mexico,
and it would supply them bountifully, even if it did not thereby leave us
comparatively rainless and sterile. Every where currents passing from the
ocean _over mountain ranges_ part with a large share of their moisture.
Thus the counter-trade which curves over the Andes and over Peru, is
deprived of its moisture and leaves the western coast rainless. So in
degree of the counter-trade which curves over the Himalaya and Kuenlon
Mountains, and from there passes over the Desert of Cobi, to the north and
east--it is deprived by those elevated ranges of its moisture. So the
mountains on the south-western coast of South America are drenched with
rain, while Patagonia, which lies on the east of them is comparatively
dry. And so of every other country similarly situated.
Now the mountain ranges and table lands of Mexico are not thus supplied
with moisture. For the space of four months in Southern and less in
Northern Mexico, and in summer, and while the belt of the tropics is
extended up over them, they have rain and in daily showers which _travel
up from the south_, indicating the course of the counter-trade. (See
Bartlett's Personal Narrative, vol. ii. p. 286.) At other seasons, and
while we are bountifully supplied, they are dry. In short, there are no
two portions of the earth that differ more widely in regard to their
supply of moisture, and all their climatic characteristics and relations.
It is therefore, according to all analogy, impossible that our
counter-trade should come from the South Pacific across the continent and
below 35 deg., and in this also those gentlemen are mistaken.
Messrs. Espy and Redfield recognizing the existence of "a prevailing" S.
W. current, but considering the surface-winds beneath it as the principal
actors in producing the atmospherical conditions and changes, have
attributed no office to that current, except that of giving direction and
progression to our storms. This is their great mistake. It plays no such
unimportant part in the philosophy of the weather, as we have already
incidentally seen, and will proceed still further to consider.
_All our storms originate in it._ This we may know from analogy.
_Where there is no counter-trade, outside of the equatorial belt of rains,
and within influential distance of the earth, there are neither storms nor
rain._ So, when, as we have seen, the concentration of the volume of
northern counter-trade in the West Indies, gathered by the hauling of the
S. E. trades more from the east, as they approach the central belt,
diminishing the volume of the counter-trade over the North Atlantic, the
calms and drought of the horse-latitudes are found. And when the
counter-trade is small in volume and weak in intensity, by reason of the
fact that the surface-trades from the opposite hemisphere which constitute
it, formed upon land where evaporation was small, as upon Southern Africa
and New Holland, or formed where the magnetic intensity was weak, or
passed over mountain ranges in their course, the annual supply of rain,
the ranges of the barometer, and the alternations of atmospheres
conditions are remarkably less.
We have already seen where the rainless portions of the earth are, and why
they are so; because those lying north of the northern limit of the
equatorial rainy belt were yet too far south to be covered by the line of
extra-tropical rains; or in other words, too far south to be uncovered by
the surface N. E. trades and the longitudinal magnetic currents, and to be
covered by the counter-trades in contact, or nearly so with the earth, and
influenced by the perpendicular north polar magnetic currents. Thus we
have seen that the rains of Southern Mexico were summer rains, due to the
northern extension of the equatorial rainy belt; those of California were
winter rains, due to the southern extension of the extra-tropical rains
following the N. E. surface trades. We have also briefly alluded to the
fact that either side of the equatorial rainy belt, evaporation is going
on for months under a vertical sun, without precipitation--unless it be
from an occasional brief storm of great intensity which originates in that
belt at the line of it, and passing on in the counter-trade, reverses, for
the time being, by its concentrated and powerful action, like a magnetic
body introduced into the field of another magnet, the surface-trades. Mere
evaporation then, does not produce the storm, or shower, or rain, where
most active in the dry torrid zone. It may be said that those dry portions
are, for the time being (as the rainless portions of the earth are
continually), within the operation of the surface-trades, and that
therefore the evaporated moisture is carried away by them toward the
equatorial rainy belt. Precisely so; but why carried away? Why should it
not condense, occasionally, at least, and drop the rain as it passes
along, if a great supply of moisture from excessive evaporation could
furnish rain. Perhaps it may still be said it is going from a cold to a
warm section. This is not true, as we have shown.
But, it may be said that the rainless regions at any rate receive no
moisture, and therefore can not supply any by evaporation. This would not
meet the case, as it would still be true that when the rainy belt has left
a given spot, the dry weather sets in with excessive evaporation, and the
north-east trades in summer, blowing from the countries lying north of the
rainless regions, and which have been supplied during the interval by the
extra-tropical rains, and are loaded with evaporation, are passing over
the rainless regions on their way to enter the central belt. So blow the
N. E. trades from the Mediterranean, and the Barbary States _over the
Desert of Sahara_ and into the rainy belt south of it; but drop no
moisture on their way, because exposed to no magnetic currents of an
opposite polarity.
But it is not true that all the rainless regions are without evaporation.
Egypt is an exception. The annual freshets of the Nile saturate its
central valley, and vast reservoirs of water are saved from it and let out
over its surface, and it all evaporates, but produces no rain. And so are
large quantities turned aside and scattered over the bottom lands of
Northern Mexico, and other countries, during the dry season, and their
evaporation furnishes no rain. Hygrometers and dew points are of no
consequence there--nor are they of any, on either side of the rainy belt,
where six perpendicular feet of moisture is evaporated in six months.
Again we have alluded to a strip of coast on the Pacific west of the
mountain ranges of South America, lying partly in Peru, partly in Bolivia,
and partly in Northern Chili, which, although long and narrow, washed by
the broad Pacific Ocean, is without rain. South America has no other
_wholly_ rainless region, so far as is known. A part of this region would
lie between the equatorial belt of rain, and the southern extra-tropical
one, and never be covered by either; but the volume of N. E. trades from
the Atlantic, although from the make of the land not concentrated to so
great an extent as the volume of S. E. trade on the north, and therefore
not so liable to hurricanes and other violent storms, is yet sufficiently
so to carry the southern line of the equinoctial rainy belt down in winter
to the summer line of extra-tropical rains, and give a supply of rain to
all the continent--leaving no strictly rainless region south of the
equatorial rainy belt and east of the Andes. Those mountains, however,
present a barrier to its south-western progress which it doubtless passes
to some extent, but deprived of its moisture, and unable to supply the
rainless coast region of Peru, Bolivia, and Northern Chili. There is,
therefore, a portion of this rainless line of coast which is within the
region of extra-tropical rains, over which a portion of the N. E. trades
of the Atlantic, as a counter-trade, should or do, curve, and where there
should therefore be extra-tropical rains. It is washed by the Pacific, an
evaporating surface, and westerly and south-west breezes are drawn in from
that ocean over it. Why then is it rainless? The only reason which can be
assigned why rain does not fall there is that the high mountain ranges of
the Andes intercept and perhaps in part divert the counter-trade, and
deprive that portion of it which passes them, of its moisture, by that
reciprocal action of opposite polarities which takes place whenever and
wherever the trade approaches so near the earth; and it curves over the
narrow line of coast with the feeble condensation, and imperfect forms,
and varied coloring which mark so peculiarly the rainless clouds of that
region. (See Stewart's Journal of a Voyage to the Sandwich Islands, page
72.)
Again, it is estimated, and on reliable data, that twelve perpendicular
feet of water are annually evaporated from the surface of the Red Sea,
between Nubia on one side, and Arabia on the other; yet they are both
rainless countries, except so far as the inter-tropical belt of rains
extends up on to a small portion of them. The moisture of evaporation,
floated up from a surface covered by the surface-trade is invariably so
combined as to remain uncondensed till it has passed south into the
equatorial rainy belt, and over to the opposite hemisphere, and been
exposed to the currents of an opposite magnetism.
Again, the N. E. trades extended up in summer over the Mediterranean Sea,
an evaporating surface, blow over the Barbary States in June and July, but
furnish no rain. And so of the S. E. or N. E. trades which blow over
Brazil and other countries in the absence north or south of the tropical
belt of rains.
It is obvious from these facts--and more like them might be cited--that
mere evaporation, however copious or long continued, does not make the
storm or shower in the locality where it takes place, and _without the
existence and influential agency_ of a counter-trade; and that _reciprocal
action_, whatever it may be, that takes place _between it and the earth_.
Again, our own experience is conclusive of this. We have no surface-trade
north of 30 deg., and yet a long drought and great evaporation may follow a
wet spring. Belts of droughts and frequent rains occur every year in
different portions of the country side by side, and _the dividing line
follows the course of the counter-trade_, and is sometimes distinctly
marked for weeks. When a change occurs in the counter-trade, whether from
causes existing there or the influence of terrestrial magnetism (in
relation to which we shall inquire hereafter), showers form or storms come
on: until it does they will not. Efforts at condensation will occasionally
appear, but they will be feeble and ineffectual, and occasion a repetition
of the axiom that "all signs fail in a drought." And we may know it from
direct observation.
The first indications of a storm, and of most if not all showers, are
observable in the counter-trade. These indications, so far as they are
visible, are of course to be looked for in the west; although the
direction and character of the surface-winds are often indicative of these
changes when not visible at the west as we shall see.
The indications are those of condensation, and vary very much in different
seasons of the year. It is not my purpose in this place to examine them
particularly. They will be alluded to hereafter under the head of
prognostics. Suffice it now to say, then, that whether it be the long
threads or lines of cirrus which occur in the trade in the winter after a
period of severe cold, following the interposition of a large volume of N.
W. cold air and the elevation of the counter-trade; or the forms of cirrus
which occur at other times and other seasons; or whether it be the
ordinary bank at night-fall, or the evening condensation which makes the
"circle" around the moon, or the morning cirro-stratus haze which
gradually thickens, passes over and obscures the sun, all which may be
followed by the easterly scud and winds: they are alike condensation in
the trade, the advance or forming condensation of a storm or showers.
The state of the weather, whether hot or cold, is extensively affected by
this trade current. As we have already suggested, the mere presence of the
sun in its summer solstice, or its absence in winter, is not an adequate
cause of all the sudden and various changes to which we are subject. The
state of the counter-trade, which is always over, or _within influential
distance of us_, and sometimes probably in contact with us--the nature of
the surface-winds which it is at any given time creating and attracting
around us, and the electric condition of the surface-atmosphere _induced_
by it, or by the immediate action of the earth's magnetism, produce those
sudden changes which mark our climate. When no intervening surface-winds
elevate it above us, and there is no storm or other condensation within
influential distance, it induces the gentle balmy S. W. wind of
spring--the cooling S. W. wind of summer--the peculiar Indian summer air
of autumn, or the comparatively moderate, although cold, open weather of
winter. If there be a partial tendency to condensation in it, the cumuli
form under the magnetic influence excited by the sunbeams from ten to
three o'clock in the day, and float gently away to the eastward,
disappearing before night-fall. If the disposition to condensation is
stronger, whether inherent or induced by an increased local activity of
terrestrial magnetism, these cumuli will increase toward night-fall, or
earlier, and terminate me showers; and if it is in a highly electrical
state, the still oppressive sultriness which precedes the tornado, and
that devastating scourge may appear. If this disposition to condensation
becomes extensive, cirri form and run into cirro-stratus, or they extend,
coalesce, and form stratus; the surface-wind will be attracted under them,
the thermometer fall in summer or rise in winter, and a storm begin.
Intense action and sudden cold may exist in and under this counter-trade
over the southern portion of the country, while all is calm, warm, and
balmy at the north. Heavy snow storms sometimes pass at the south when
there are none at the north, and a corresponding state of the weather
follows. If a large body of snow fall at the north, the winter is cold,
regular, and "old fashioned;" if little snow falls at the north and more
at the south, the winter at the north is open and broken. I have known the
ice make several inches thick at Baltimore and Washington, when none could
be obtained for the ice-houses on the Connecticut shore of Long Island
Sound. In short, although heat and cold are mainly dependent upon the
altitude of the sun, aided by the other arrangements we have alluded to,
yet the counter-trade, and the reciprocal action which takes place between
it and the earth, are most powerful agents, mitigating the rigors of
winter, bringing about the changes from cold to warm weather which the
sun is too far south to produce. And on the other hand, by this reciprocal
action, producing the electrical phenomena, the gusts, the tornadoes, the
hail storms, and the cool seasons of summer, and the period of intense
cold in winter.
_All our surface-winds, except the light, peculiar W. S. W. wind which is
felt where the counter-trade is in contact with the earth, and which is a
part of it, and perhaps the genuine N. W. wind which is very peculiar, are
incidents of the trade, and are due to its conditions and attractions._ We
have already said this was true of the easterly wind and scud of a
storm--it is alike true of all. The storm winds east of the Alleghanies
are usually, though not always, from the eastward. They are sometimes from
the southward, as they doubtless are still more frequently in the interior
of the continent.
There is occasionally a southerly afternoon wind, followed by short rains
in spring and fall, or a succession of showers in summer, which is rather
a precedent wind than a storm wind; blowing toward and under an advance
portion of the storm at the north, and hauling to the eastward when the
rain sets in, or to the westward when the showers reach us.
When there are no storms, or showers, or inducing electric action in the
counter-trade, within influential distance to disturb the surface
atmosphere, it is calm. If a storm approaches, or forms within inducing
distance, the surface atmosphere is _affected_ and _attracted toward the
storm_, from one or more points, and "blows," as we say, toward and under
it. It commences blowing first nearest the storm, and extends as the
storm travels, or becomes more intense and extends its inducing influence.
I have repeatedly noticed this in traveling on steamboats and railroads
running _toward_ or _from_, and in several instances _through_ a storm,
and telegraphic notices and other investigations prove it. The point from
which the surface atmosphere is attracted and blows, depends very much
upon the position of the storm in relation to bodies of water and the
point of observation, and its shape; and the force with which it may blow
will depend much upon its intensity.
Let us take an instance or two by way of illustration of all these points;
and as I have given instances of summer in the introduction, we will take
those of winter. It is January of an "old fashioned winter;" the snow is
about three feet deep in Canada, about one foot in Southern New York, and
a few inches in Philadelphia, and so extends west to the Alleghanies at
least. For several days the sky has been clear, the thermometer rising in
the day-time, in the vicinity of New York to about 25 deg. Fahrenheit, falling
at night to about 6 deg., with light airs from the N. W. during the middle and
latter part of the day; the counter-trade and the barometer both running
high; cold but pleasant, steady, winter weather. There is a warm
south-east rain and thaw coming, as one or more such almost invariably
occur in January. How coming? The sun is far south, and shines aslant, but
through a pure and windless atmosphere; he has tried for several days to
melt the snow from the roof; a few icicles are pendant from the eaves;
but the body of the snow is still there. How can a thaw come? not from the
sun, surely. No, indeed, not from the action of the sun directly, upon our
country, nor from the Atlantic or the Gulf Stream which is off our coast.
But a portion of the current of counter-trade is coming, heated by his
rays and the warm water in the South Atlantic, in an intense
magneto-electric state, capable of inducing an electro-thermal change in
the surface atmosphere which it approaches, and of being reciprocally
acted upon by the north polar terrestrial magnetism. It is now over
Northern Texas and Western Louisiana, it will be here day after tomorrow.
The day passes as the day previous had passed; the sleigh-bells jingle
merrily in the evening; the moon shines clear all night; the storm is
coming steadily on, but its influence has not reached us, and the morning
and midday are like those which preceded it. As nightfall approaches,
however, the thermometer does not fall as rapidly as on the day previous;
the sun shines dimly and through lines of whitish cirrus cloud extending
from the horizon at the west, appearing darker as the sun descends and
shines more _horizontally_ through them--perhaps mainly in the N. W.--and
which extend up and over toward the E. N. E. The air next the earth begins
to feel raw; it is changing, not from warm to cold, but _electrically_
from positive to negative; and dampening, from a tendency to condensation
by induction, as we shall see--the same condensation which in warm
weather may be seen on flagging stones, and walls, and vessels containing
cold water. The advance cirrus condensation of the storm is over us and
affecting us; the earth too is affecting the adjacent atmosphere by action
extended from beneath the storm. Still there is no wind, although sounds
seem to be heard a little more distinctly from the east, and so ends the
day. Evening comes, and the moon wades in a smooth bank of cirro-stratus
haze, with a very large circle around her; the cirrus bands of haze have
coalesced and formed a thin stratus. The storm is coming steadily on, its
condensation is seen to be thicker as it approaches, it is now raining
from one hundred to one hundred and fifty miles to the west, but we do not
know it.
That it is about to storm all believe, for all are conscious of a change.
The candle if extinguished will not relight as readily, if at all, on
being blown; there is a crackling almost too faint for snow in the fire;
the sun did not set clear; the old rheumatic joints complain, and the
venerable corns ache.
Morning comes, and the storm is on. The wind is blowing from the S. E.,
the scud are running rapidly from the same quarter to the N. W., the
thermometer continues rising, and it rains. The storm has reached us and
the thaw has commenced. Gradually, as the densest portion of the storm
cloud reaches us, it darkens; the scud are nearer the earth, and run with
more rapidity; the rain falls more heavily and continuously, and by the
middle of the day a thick fog has enveloped the earth; the wind is dying
away, and the trade itself, with its southern tendency to fog, has settled
near us; the barometer has fallen, the thermometer is up to fifty degrees,
the water is running down the hills, the snow is saturated with water and
is disappearing under the influence of the fog, the rain, and the warm
air. Evening comes; the south-east wind and the rain have ceased; the rain
clouds have passed off to the eastward; the fog has followed on and
disappeared; there is a light trade air from the S. W.; the moon shines
out, and a few patches of stratus, broken up into fragments and melting
away, are following on in the trade: the storm is past.
Hark! to the tones of Boreas as he bursts forth from the N. W., and
rushing, whistling, howling, dashes on between the trade and the earth,
following the storm. Now the barometer rises rapidly, the thermometer
falls, and in an incredibly short time all is congealed, and cold and
wintery as before. The cold N. W. wind has again interposed between the
trade and the earth; the trade is elevated a mile or more above it and is
entirely free from its influence and from condensation; the deep blue of a
sky "as pure as the spirit that made it" is over us, and steady winter
reigns again.
It is obvious that there was nothing in the action of the sun upon our
snow-clad country, to induce the thaw or the storm. It began, continued,
approached, and passed off to the N. E. in the counter-trade. The S. E.
wind which existed every where within its influence: in the interior
States, Missouri, Illinois, Indiana, Ohio, Michigan, and in Canada, as
well as upon the Atlantic coast, commencing in the former earlier than
upon the last, was the result of its induction and attraction. Of the N.
W. wind that followed we shall speak hereafter. If any one doubts whether
this be a true sketch let him examine the investigation of a storm
published by Professor Loomis, or observe for himself hereafter. If,
however, the storm of Professor Loomis is referred to, it should be
remembered that his notes show the occurrence of a slight distinct snow
storm at the N. W. stations one day in advance of the principal storm. The
latter appears first as rain at Fort Towson, on the nineteenth, moving
north and curving to the east--its center passing near St. Louis, and
south of Quebec, and the whole storm enlarging as it advanced.
Take another instance. Since the thaw it has not been quite as cold as
before; but the rain-soaked snow is hard and solid, the ground, where the
snow was blown or worn off, icy and slippery--the thermometer falls during
the night to about 12 deg., and rises to about 30 deg.; the sun makes no
impression upon the snow; the firmament is of the deepest blue, the
borealis at night vivid. "O, for a storm of some kind, to mitigate the
still severe cold;" for the thaw has made us more sensitive, and storm
winds do blow warm in their season. But patience, it will come. Another
day, or two, perhaps, pass: the sun rises as usual, the thermometer has
the same range still. "Long cold snap," we exclaim; "how long will it
last?"
A change is coming, but this time it will snow. About an hour or two after
sunrise the cirrus threads are discoverable again in the west, but now
they are most numerous in the S. W. As the day passes on they thicken and
advance toward the E. N. E., the sun begins to be obscured, the
thermometer rises, and it slowly "_moderates_." There is a snow storm
approaching from the S. W.
But the thermometer rises slowly; it must get up to 26 deg. or 28 deg. before it
can snow much. I have known in one instance, at Norwalk, a considerable
fall of snow, although much mingled with hail, when the thermometer stood
at 13 deg. above zero, and one, a moderate fall, some two inches, with it at
24 deg., but these were exceptions. The snow range of the thermometer on the
parallel of 41 deg. north latitude, and south of it, is from 26 deg. to 30 deg. above
0 deg.; when colder or warmer it may snow to whiten the ground, or perhaps
barely cover it, but usually rains or hails. We have seen that in the
polar regions, according to Dr. Kane, it is about zero, but the rise of
the thermometer there, previous to the snow, was about the same as here,
_i. e._, from 15 deg. to 25 deg.. This fact is instructive. Since the foregoing
was written, and on the 7th of February, 1855, a snow-storm of
considerable length set in, with the thermometer at 5 deg., and continued more
than twenty-four hours, the thermometer gradually rising. The snow was
very fine, like that described by Arctic voyagers as falling in extreme
cold weather.
As the dense and darker portions of the storm approach, and although the
sun is obscured, and the ground frozen, it continues to moderate, and at
evening, when the thermometer is up to 28 deg., and the dense portion of the
storm has reached us, gently and in calmness the snow begins to fall.
Perhaps a light air following the storm, or the presence of the trade near
the earth, at first inclines the snow-flakes to the eastward. This is
frequently so at the commencement of snow storms. Ere long, however, the
wind rises from the N. E., and the snow is driven against the windows,
rounded and hardened by the attrition of its flakes upon each other, in
their descent through the eddying and opposite currents. The next day we
rise to witness a heavy fall of snow, perhaps, and a continued driving N.
E. storm, in full blast; the snow whirling and settling in drifts under
the lee of every fence or building.
Can it be, you ask, that this driving wind is but an _incident_ of the
storm? the result of _attraction_, while the storm clouds are sailing
quietly and undisturbed on in the counter-trade above, directly over the
gale which is blowing below? It is even so. Nor has it "backed up," as it
is termed by those who have ascertained that it has commenced snowing
first, and cleared off first, at a point west of them. You saw, or might
have seen, the cirro-stratus cloud passing to the E. N. E. in the
afternoon, and until the snow-flakes filled the air, and the clouds became
invisible. You may still see that the wind will die away before the storm
breaks, and "come out" gently from the S. W., unless it should back into
the northward and westward, and in either event you may see the last of
the storm clouds, as you did see, or might have seen the first of them,
pass to the eastward. Toward night the wind dies away, and the storm
passes off abruptly, or the sky becomes clear in the N. W. Now you may see
the smooth stratus storm cloud, continuous, or breaking up into fragments
and passing off to the east, even at the edge which borders the clear sky
in the west or north-west, to be followed that evening or the next day, by
the north-west wind and its peculiar fair-weather scud.
I have given these as instances illustrating the manner in which rain and
snow storms originate the surface easterly winds in winter.
But it must not be supposed that they commence with precisely the same
appearances in every case in winter; much less in summer. There is very
great diversity in this respect, in different seasons, and in different
storms during the same season. A great many different and accurate
descriptions might be given, if time and space would permit, which all
would recognize as truthful. Very frequently in summer, and sometimes in
winter, the wind will set in from the eastward, and blow fresh toward a
storm, before the condensation in the trade, which forms the eastern and
approaching edge of the storm, has assumed the form of a distinct cloud.
Not unfrequently, when it is calm next the surface, a narrow stratum of
easterly wind, a half a mile or a mile above the earth, may be seen with a
continuous fog, condensing, but not in considerable patches like the
usual scud, running with great rapidity toward the storm. Such a stream of
fog blew with great rapidity for thirty-six hours toward the storm which
inundated Virginia and Pennsylvania, in 1852, and carried away the Potomac
bridge at Washington. Such a stream of fog was visible the evening before
the great flood of 1854, which inundated Connecticut, and curried away so
many railroad and other bridges. I have also seen such a stream of fog
running at about the same height, when it was calm at the surface, from
the S. W. toward a violent storm which formed over central New
England--and from the north toward a heavy storm passing south of us. Such
strata form, as far as I have been able to discover, the _middle current_
of storms which are accompanied with very heavy falls of rain. These
double currents are much more common than is supposed. East of the
Alleghanies, short and heavy rain storms, which commence north-east,
hauling to the south and lighting up about mid-day _after a very rainy
forenoon_, frequently have a S. E. or S. S. E. middle current of this
character, which involves the whole surface atmosphere when the storm has
nearly passed, and the N. E. wind dies away, and the wind seems to haul to
the S. S. E. and S.; so that it is rather the prevalence of a _different_
and _coexisting current_, than a hauling of the _same wind_, which marks
the period of lighting up in the south.
Sometimes the easterly wind will set in and blow a day or two before the
border of the storm reaches us. Sometimes the storm is passing, or will
pass, in its lateral southern extension, south of us, and the
condensation in the trade extends over us sufficiently dense to induce an
easterly current beneath it, but not dense enough to drop rain, and then
we have a dry north-easter. I can not, within the limits I have
prescribed, allude to all the peculiarities attending the induction and
attraction of an easterly wind, by the storm in the counter-trade. They
are readily noticeable by the attentive and discriminating observer, and
their existence and cause is all with which I have to do at present.
Winds from the north, or any point from N. N. E. to N. N. W., are
comparatively infrequent in the United States, east of the
Alleghanies--though it is otherwise in the vicinity of the great lakes.
Sometimes the wind "backs," as sailors term it, during a N. E. storm, from
the N. E. through the N. N. E., N., and N. N. W. to N. W. When this takes
place, it is toward the close of the storm. Occasionally, though very
rarely, it continues to storm after the wind has passed the point of N. N.
E., and until it gets N. W. I have known a few instances in the course of
thirty years, and but a few. They are exceptions--rare exceptions. When
the wind thus backs from the N. E. to the N. W. through the N., you may be
very certain that the body of the storm, or at least the point of greatest
intensity and greatest attraction, is at the time passing to the southward
of you. This is most commonly the course of the wind when the storm
extends far south and lasts several days, and does not extend north far,
or if so, with much intensity, beyond the point of observation. The
change of the wind is explained by the situation of the focus of intensity
and attraction, to the south of the observer, and its passage by on that
side.
Probably in locations further north and (as I think I have observed) south
of the lakes, it may be more frequent than upon the parallel of 44 deg. east
of the Alleghanies (which is as far north as I have observed), inasmuch as
the further north the locality, the more likely storms and other
disturbances in the counter-trade will be to pass to the southward of it.
Between the N. E. and S. E. the wind may blow from any point, before and
during storms, and in a clear day in the morning, as a light variable
breeze, or, after mid-day, toward approaching showers. I have known it
blow all day during a storm from due east; to change back and forth
between south-east and north-east, and to blow for hours from any
intermediate point--as different portions of the storm were of different
intensity, and exerted a more or less powerful inducing influence; and
doubtless this often takes place at sea. It depends upon the situation of
the focus of attraction of the storm, its shape relative to the particular
locality, and with reference to the atmosphere east of it, and peculiar
local magnetic action; or, as is sometimes the case in low latitudes, is
owing to the fact that the storm is made up of many imperfectly connected
showers, which have different force, and induce changeable and baffling
winds.
The inducing and attracting influence of the approaching storm is exerted
sooner, and with most force, upon the surface atmosphere, over bodies of
water like the ocean and the lakes. Thus, the wind will set from the
eastward toward an approaching storm out upon Long Island Sound, for hours
before it is felt upon either shore; and when all is calm in the evening
on land, and often before the moon forms a halo or circle in the milky
condensation of the approaching storm, or any sign of condensation is
visible, the breaking of the waves upon the shores may be heard. Doubtless
this may be observed on the shores of the Atlantic at other points.
This power of attracting the surface atmosphere from bodies of water like
the ocean and the great lakes, will account for two apparent anomalies,
mentioned by Mr. Blodget in a valuable and instructive article read to the
Scientific Convention, in 1853, regarding the annual fall of rain over the
United States.
First--the influence of mountains in extracting the water from the
atmospheric currents which pass over them, is well known and readily
explainable. Mr. Blodget, however, found that the source of our rains,
whatever it might be, when it reached the Alleghanies, was so far
exhausted of its moisture that those mountains extracted less from it than
fell to the westward, by some five to ten inches annually; and that the
fall of rain upon them was less than upon the Atlantic slope eastward of
them, to the ocean. This does not accord with observation elsewhere, but
is easily explained. As the storm approaches the ocean, it attracts in
under it the surface atmosphere of the ocean, loaded with vapor,
condensing in the form of fog and scud, as it becomes subject to the
increasing influence of the storm. Although the scud and fog would not of
itself make rain, it aids materially in increasing the quantity of that
which falls through it. The drops, by attraction and contact, enlarge
themselves as they pass through, in the same manner as a drop of water
will do in running down a pane of glass which is covered with moisture.
The small drop which starts from the upper portion of a fifteen-inch pane,
will sometimes more than double its size before it reaches the bottom. _It
is by this power of attracting the surface atmosphere, which contains the
moisture of evaporation, under it, and inducing condensation in it, that
the moisture of evaporation which rarely rises very far in the atmosphere
is made to fall again during storms and showers._ This attraction of a
moist atmosphere from the ocean accounts for the excess of rain on the
east of the Alleghanies, compared with its fall upon them. So the great
valley of the Mississippi is comparatively level, and less of its water
runs off than of that which falls upon the Alleghanies. There is,
therefore, more moisture of evaporation in the atmosphere of the former to
be thus precipitated and add to the annual supply of rain upon that
valley, and it exceeds that which falls upon the Alleghanies. Those
mountains, too, are elevated but about 1,500 feet above the table-lands at
their base, and exert little influence on the counter-trade. If they, were
6,000 or 8,000 feet high, a different state of things would exist.
Second--Mr. Blodget found the quantity of rain which fell in Iowa, and to
the south and west of the lake region, to be greater than fell over the
lake region itself. This is doubtless in part owing to the same cause. The
counter-trade, in a stormy state, attracts the surface atmosphere from the
lake region, with its evaporated moisture, before it arrives over it, and
therefore more rain falls S. W. of the lake region than upon it. This
power of attracting the surface wind of the ocean in under it, produces
the heavy gales which affect our coast, and which are rarely felt west of
the Alleghanies to any considerable degree; and a storm coming from the W.
S. W., extending a thousand miles or more from S. S. E. to N. N. W., may
have the wind set in violently at S. E. on the _southern coast first_, and
at later periods, successively, at points further north, and thus induce
the belief that the storm traveled from south to north.
Mr. Redfield finding that some of the gales which he investigated,
particularly that of September 3d, 1821, did not extend far inland, and
commenced at later periods regularly, at more northern points, concluded
that the gale traveled along the line of the coast to the northward. In
this, and in relation to the storm of 1821 (and perhaps some others), he
has been deceived. My recollections of that storm are accurate and
distinct. But I shall recur to this again when I come to speak of his
theory.
Toward storms, or belts of showers which would be storms if it were not
summer and the tropical tendency to showers active in the trade, which
pass mainly to the north of us, or commence north and pass over us,
condensing south while progressing east, the wind may commence blowing
before the body of the storm reaches us, from any point between south by
west and south east, particularly in the summer season and in the
afternoon. When the rain in a storm of this character sets in, in the
night, it will sometimes haul into the S. E., if the focus of attraction
be situated north of us, and so remain until just before the storm is to
break.
There are, however, a class of southerly summer winds which deserve more
particular notice. For two or three months in the year--say from the
middle of June to the 20th of August--storms on the eastern part of the
continent, except in wet seasons, are rare, and most of our rain is
derived from showers. During these periods belts of drought are frequent,
sometimes in one locality, and sometimes in another, extending with
considerable regularity from W. S. W. to E. N. E. in the course of the
counter-trade, while rain falls in frequent and almost daily showers to
the northward or southward of them. If the daily rains are at the north,
over the belt of drought, S. S. W. and S. W. by S. winds blow, sometimes
with cumuli or scud, during the middle of the day and afternoon, to
underlie the showery counter-trade on the north of the line of drought.
Thus, sometimes nearly every day for several days, the evaporated moisture
of the dry belt will be carried over to increase the store of those who
have a sufficient supply without. During the latter part of the afternoon
the clouds in the west may look very much like a gathering shower, but the
attractions of the counter-trade fifty or one or two hundred miles to the
north, will absorb them all, and at nightfall the wind will haul to the S.
W. on a line with the counter-trade, and die away.
If there be a drought on any given line of latitude, and frequent showers
or heavy rains at the south of it, although there may not be a like
surface-wind, with cumuli and fog, blowing from the north toward it, yet a
general, gentle set of the atmosphere, from the N. N. W., or N. W., or
other northerly point, toward the belt of rains, some distance above the
earth, will often be observable, with a barometer continually depressed,
and perhaps a cool atmosphere.
During set fair weather, when the attracting belt of rains is far north,
on the north shore of Long Island Sound, the wind, like a sea breeze, will
set in gently from about S. S. E. or S. by E. in the forenoon, blowing a
gentle breeze through the day, and hauling to W. S. W. on a line with the
trade at nightfall, and dying away. During a drought I have known this to
happen for seventeen successive days. It is obvious to an attentive
observer that this is the result of the influence of the sun in exciting
the magnetic influence of the earth, and producing a state of the trade
not unlike that which induces the formation of cumuli, and which attracts
the surface atmosphere from the Sound in over the land: for the _tendency
to cumulus condensation precedes the breeze_, and the breeze is often
wanting in the hottest days where no such tendency to the formation of
cumuli exists. The same is true of sea breezes elsewhere. They do not blow
in upon some of the hottest surfaces. Where they do exist, they do not
always blow, but are wanting during the hottest days; and careful
observers have identified their appearance with the formation of cumuli,
or other condensation, upon the hills inland. They are not, therefore, the
result of ascending currents of heated air.
The received theory regarding sea and land breezes is a mistaken one in
another respect. There is no such thing as a land wind corresponding in
force to, and the opposite of, the sea breeze--occasioned by the
comparative warmth of the ocean. These breezes blow mainly within the
trade-wind region. Of course they are either beneath the belt of rains or
the adjoining trades. They are said to be, and doubtless are, most active
and strongly marked on lines of coast, particularly the Malabar coast, and
where the trade-winds are drawing usually from them. In the day-time, when
the action of the sun increases the action of the magnetic currents upon
the land, or there are _elevations inland_ which approach the
counter-trade, and especially if it is elevated near the coast, as the
Malabar coast is by the Ghauts, the attraction of this atmosphere over it
_reverses the trade_, or inclines it in upon the land, and it blows in
obliquely or perpendicularly, according to the relative trending of the
coast and the direction of the surface-trade. Thus, where islands are
situated within the range of the trades, the latter will be _reversed_
during the day on the _leeward_ side, but continue to blow as land winds
during the night. So they are sometimes deflected in upon the land on the
sides, during the day, and in like manner return to their course in the
night. So, too, the north-east trades of Northern Africa, are occasionally
(though feebly where the coast is flat) deflected during the day-time, and
blow in as N. W. winds. Upon the southern coast of Africa the S. E. trade
is deflected, and blows in as a S. W. wind. Upon the south-western coast
of North America, the N. E. trades are deflected in like manner, and so
are the S. E. trades upon the western coast of South America. Where the
coast mountain ranges are very elevated, as upon the western coast of the
American continent, this attracting influence and consequent deflection
extends to a considerable distance seaward, and hence the westerly winds
of California, etc. It must be understood that we are now speaking of the
winds which blow within the range and during the existence of the
trade-winds or the presence of the dry belt--for the trades are not always
perceptible on the land. Captain Fitzroy thus describes the sea breezes of
the western coast of Peru, at 23 deg. south latitude. "The tops of the hills
on the coast of Peru are frequently covered with heavy clouds. The
prevailing winds are from S. S. E. to S. W., seldom stronger than a fresh
breeze, and often very slight. _Sometimes during the summer, for three or
four successive days, there is not a breath of wind, the sky is
beautifully clear, with a nearly vertical sun._ On the days that a sea
breeze sets in, it generally commences about ten in the morning, then
light and variable, but gradually increasing till one or two in the
afternoon. From that time a steady breeze prevails till near sunset, when
it begins to die away, and soon after the sun is down there is a calm.
About eight or nine in the evening _light winds_ come off the land, and
continue till sun-rise, when it again becomes calm until the sea breeze
sets in as before."
To illustrate this further, I take the following letter from Professor
Espy's Philosophy of Storms:
CLINTON HOTEL, N. Y., Dec. 20, 1839.
TO PROFESSOR ESPY,
DEAR SIR,--Understanding you are desirous of collecting curious
meteorological facts, I take the liberty of communicating to you what
I saw in the month of December, 1815, at the Island of Owhyhee. I lay
at that island in the Cavrico Bay,[3] in which Captain Cook was
killed, three weeks, and every day during that time, very soon after
the sea breeze set in, say about nine o'clock, a cloud began to form
round the lofty conical mountain in that island, in the form of a
ring, as the wooden horizon surrounds the terrestrial artificial
globe, and it soon began to rain in torrents, and continued through
the day. In the evening the sea breeze died away and the rain ceased,
and the cloud soon disappeared, and it remained entirely clear till
after the sea breeze set in next morning. The land breeze prevailed
during the night, and was so cool as to render fires pleasant to the
natives, which I observed they constantly kindled in the evening. I
was particularly struck with the phenomena of the cloud surrounding
the mountain, when none was ever seen in
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