Analysis of the Atmosphere—Its Pressure—Law of Decrease in
Density—Law of Decrease in Temperature—Measurement of Heights
by the Barometer—Extent of the Atmosphere—Barometrical
Variations—Oscillations—Trade-Winds—Cloud-Ring—Monsoons—Rotation of
Winds—Laws of Hurricanes.
THE atmosphere is not homogeneous. It appears from analysis that, of 100
parts, 99·5 consist of nitrogen and oxygen gases mixed in the
proportions of 79 to 21 of volume, the remainder consists of 0·05 parts
of carbonic acid and on an average 0·45 of aqueous vapour. These
proportions are found to be the same at all heights hitherto attained by
man. The air is an elastic fluid, resisting pressure in every direction,
and is subject to the law of gravitation. As the space in the top of the
tube of a barometer is a vacuum, the column of mercury suspended by the
pressure of the atmosphere on the surface of that in the cistern is a
measure of its weight. Consequently every variation in the density
occasions a corresponding rise or fall in the barometrical column. At
the level of the sea in latitude 42°, and at the temperature of melting
ice, the mean height of the barometer is 29·922 or 30 inches nearly. The
pressure of the atmosphere is about fifteen pounds on every square inch;
so that the surface of the whole globe sustains a weight of
11,671,000,000 hundreds of millions of pounds. Shell-fish, which have
the power of producing a vacuum, adhere to the rocks by a pressure of
fifteen pounds upon every square inch of contact.
The atmosphere when in equilibrio is an ellipsoid flattened at the poles
from its rotation with the earth. In that state its strata are of
uniform density at equal heights above the level of the sea; but since
the air is both heavy and elastic, its density necessarily diminishes in
ascending above the surface of the earth; for each stratum of air is
compressed only by the weight above it. Therefore the upper strata are
less dense because they are less compressed than those below them.
Whence it is easy to show, supposing the temperature to be constant,
that if the heights above the earth be taken in increasing arithmetical
progression, that is, if they increase by equal quantities, as by a foot
or a mile, the densities of the strata of air, or the heights of the
barometer which are proportionate to them, will decrease in geometrical
progression. For example, at the level of the sea if the mean height of
the barometer be 29·922 inches, at the height of 18,000 feet it will be
14·961 inches, or one half as great; at the height of 36,000 feet it
will be one-fourth as great; at 54,000 feet it will be one-eighth, and
so on. Sir John Herschel has shown that the actual decrease is much more
rapid, and that, in any hypothesis that has been formed with regard to
the divisibility of the aërial atoms, a vacuum exists at the height of
80 or 90 miles above the earth’s surface, inconceivably more perfect
than any that can be produced in the best air-pumps. Indeed the decrease
in density is so rapid that three-fourths of all the air contained in
the atmosphere is within four miles of the earth; and, as its
superficial extent is 200 millions of square miles, its relative
thickness is less than that of a sheet of paper when compared with its
breadth. The air even on mountain tops is sufficiently rare to diminish
the intensity of sound, to affect respiration, and to occasion a loss of
muscular strength. The blood burst from the lips and ears of M. de
Humboldt as he ascended the Andes; and he experienced the same
difficulty in kindling and maintaining a fire at great heights which
Marco Polo, the Venetian, felt on the mountains of Central Asia. M.
Gay-Lussac ascended in a balloon to the height of 4·36 miles, and he
suffered greatly from the rarity of the air. It is true that at the
height of thirty-seven miles the atmosphere is still dense enough to
reflect the rays of the sun when 18° below the horizon; but the tails of
comets show that extremely attenuated matter is capable of reflecting
light. And although, at the height of fifty miles, the bursting of the
meteor of 1783 was heard on earth like the report of a cannon, it only
proves the immensity of the explosion of a mass half a mile in diameter,
which could produce a sound capable of penetrating air three thousand
times more rare than that we breathe. But even these heights are
extremely small when compared with the radius of the earth.
The density of the air is modified by various circumstances, chiefly by
changes of temperature, because heat dilates the air and cold contracts
it, varying 1/480 of the whole bulk when at 32° for every degree of
Fahrenheit’s thermometer. Experience shows that the heat of the air
decreases as the height above the surface of the earth increases. It
appears that the mean temperature of space is 226° below the zero point
of Fahrenheit by the theories of Fourier and Pouillet, but Sir John
Herschel has computed it to be -239° Fahr. from observations made during
the ascent in balloons. Such would probably be the temperature of the
surface of the earth also, were it not for the non-conducting power of
the air, whence it is enabled to retain the heat of the sun’s rays,
which the earth imbibes and radiates in all directions. The decrease in
heat is very irregular; each authority gives a different estimate,
because it varies with latitude and local circumstances, but from the
mean of five different statements it seems to be about one degree for
every 334 feet; the mean of observations made in balloons is 400 feet,
which is probably nearer the truth. This is the cause of the severe cold
and perpetual snow on the summits of the alpine chains. In the year 1852
four ascents in a balloon took place from the meteorological observatory
at Kew, in which the greatest height attained was 22,370 feet. The
observations then made by Mr. Welsh furnished Sir John Herschel with
data for computing that the temperature of space is minus 239°, that is
239° below the zero point of Fahrenheit, that the limiting temperature
of the atmosphere is probably 77-1/2 degrees below that point at the
equator, and 119-1/2 below it at the poles, with a range of temperature
from the surface of 161-1/2° in the former case, and 119-1/2° in the
latter. During these ascents it was found that the temperature of the
air decreases uniformly up to a certain point, where it is arrested and
remains constant, or increases through a depth of 2000 or 3000 feet,
after which it decreases again according to the same law as before.
Throughout this zone of constant temperature it either rains, or there
is a great fall in the dew point; in short, it is the region of clouds,
and the increase of temperature is owing to the latent or absorbed heat
set free by the condensation of the aqueous vapour. In the latitude of
Kew the cloud region begins at altitudes varying between 2000 and 6500
feet, according to the state of the weather.
Were it not for the effects of temperature on the density of the air,
the heights of mountains might be determined by the barometer alone; but
as the thermometer must also be consulted, the determination becomes
more complicated. Mr. Ivory’s method of computing heights from
barometrical measurements has the advantage of combining accuracy with
the greatest simplicity. Indeed the accuracy with which the heights of
mountains can be obtained by this method is very remarkable. Admiral
Smyth, R.N., and Sir John Herschel measured the height of Etna by the
barometer, without any communication and in different years; Admiral
Smyth made it 10,874 feet, and Sir John Herschel 10,873, the difference
being only one foot. In consequence of the diminished pressure of the
atmosphere water boils at a lower temperature on mountain tops than in
the valleys, which induced Fahrenheit to propose this mode of
observation as a method of ascertaining their heights. It is very
simple, as Professor Forbes ascertained that the temperature of the
boiling point varies in arithmetical proportion with the height, or 5495
feet for every degree of Fahrenheit, so that the calculation of height
becomes one of arithmetic only, without the use of any table.
The mean pressure of the atmosphere is not the same all over the globe.
It is less by 0·24 of an inch at the equator than at the tropics or in
the higher latitudes, in consequence of the ascent of heated air and
vapour from the surface of the ocean. It is less also on the shores of
the Baltic Sea than it is in France, and it was observed by Sir James C.
Ross that throughout the whole of the Antarctic Ocean, from 68° to 74°
S. latitude, and from 8° to 7° W. longitude, there is a depression of
the barometer amounting to an inch and upwards, which is equivalent to
an elevation above the sea level of 800 feet. A similar depression was
observed by M. Erman in the sea of Ochotzk, and in the adjacent
continent of eastern Siberia. Sir John Herschel assigns as the cause of
these singular anomalies the great system of circulation of the trade
and antetrade winds, in both hemispheres, reacting upon the general mass
of the continents as obstacles in their path, which is modified by the
configuration of the land.
There are various periodic oscillations in the atmosphere, which, rising
and falling like waves in the sea, occasion corresponding changes in the
height of the barometer, but they differ as much from the trade-winds,
monsoons, and other currents, as the tides of the sea do from the
Gulf-stream and other oceanic rivers. The sun and moon disturb the
equilibrium of the atmosphere by their attraction, and produce annual
undulations which have their maximum altitudes at the equinoxes, and
their minima at the solstices. There are also lunar tides, which ebb and
flow twice in the course of a lunation. The diurnal tides, which
accomplish their rise and fall in six hours, are greatly modified by the
heat of the sun. Between the tropics the barometer attains its maximum
height about nine in the morning, then sinks till three or four in the
afternoon; it again rises and attains a second maximum about nine in the
evening, and then it begins to fall, and reaches a second minimum at
three in the morning, again to pursue the same course. According to M.
Bouvard, the amount of the oscillations at the equator is proportional
to the temperature, and in other parallels it varies as the temperature
and the square of the cosine of the latitude conjointly; consequently it
decreases from the equator to the poles, but it is somewhat greater in
the day than in the night.
Besides these small undulations, there are vast waves perpetually moving
over the continents and oceans in separate and independent systems,
being confined to local, yet very extensive districts, probably
occasioned by long-continued rains or dry weather over large tracts of
country. By numerous barometrical observations made simultaneously in
both hemispheres, the courses of several have been traced, some of which
occupy twenty-four, and others thirty-six, hours to accomplish their
rise and fall. One especially of these vast barometric waves, many
hundreds of miles in breadth, has been traced over the greater part of
Europe; and not its breadth only, but also the direction of its front
and its velocity, have been clearly ascertained. Although, like all
other waves, these are but moving forms, yet winds arise dependent on
them like tide streams in the ocean. Mr. Birt has determined the periods
of other waves of still greater extent and duration, two of which
required seventeen days to rise and fall; and another which takes
fourteen days to complete its undulation, called by Mr. Birt the
November wave, passes annually over the British Islands, probably over
the whole of Europe and the seas on its northern coasts. Its crest,
which appears to be 6000 miles in extent, moves from N.W. to S.E. at the
rate of about 19 miles an hour; while the extent of its barometrical
elevation from its trough to its crest is seldom less than an inch,
sometimes double that quantity. The great crest is preceded and followed
at about five days’ interval by two lower ones, and the beginning and
end are marked by deep depressions. The researches of M. Leverrier leave
no doubt that the great Crimean storm of the 14th November, 1854, was
part of this phenomenon,[8] for even a very small difference of
atmospheric pressure is sufficient to raise a considerable wind. Since
each oscillation has its perfect effect independently of the others,
each one is marked by a change in the barometer, and this is beautifully
illustrated by curves constructed from a series of observations. The
general form of the curve shows the course of the principal wave, while
small undulations in its outline mark the maxima and minima of the minor
oscillations.
The trade-winds, which are the principal currents in the atmosphere, are
only a particular case of those very general laws which regulate the
motion of the winds depending on the rarefaction of the air combined
with the rotation of the earth on its axis. They are permanent currents
of wind between the tropics, blowing to the N.E. on the N. side of the
equator, and to the S.E. on the S. side.
If currents of air come from the poles, it is clear that equilibrium
must be restored by counter-currents from the equator; moreover, winds
coming from the poles, where there is no rotation, to the equator, which
revolves from W. to E. at the rate of 1000 miles an hour, must of
necessity move in a direction resulting from their own progressive
motion and that of rotation; hence, in blowing towards the equator the
bias is to the E., and in blowing from it the bias is to the W. Thus as
N. and S. winds from the poles blow along the surface from the tropics
to the equator, in consequence of this composition of motions that from
the N. becomes the N.E. trade-wind, and that from the S. the S.E.
trade-wind. Now these winds being in contrary directions cross at the
equator, balance each other through about 6 degrees of latitude, and
produce a belt of calms of that breadth encircling the globe, known as
the calms of the equator, or the Variables of seamen. The heat of the
sun rarefies the air so much, that the trade-winds, after crossing at
the equator, ascend into the higher regions of the atmosphere, where
that from the N. goes to the tropic of Capricorn, and that from the S.
to the tropic of Cancer. But while travelling in these lofty regions
they become cold and heavy, and, sinking to the surface at the tropics,
each proceeds to the opposite pole from which it set out. Now, however,
they have a greater rotatory motion than the places they successively
arrive at, so the bias is to the W., and they become the N.W. and S.W.
extra-tropical winds.
If on arriving at the poles the air were to accumulate there, the
circulation of the winds would cease; but currents rise into the upper
regions, and flow back again to the tropics, where they sink down to
fill the vacuum caused by the great precipitation of vapour in these
regions, and then flow to the equator as trade-winds (N. 177). So the
currents of air cross again at the tropics and produce two belts of
calms which surround the globe, named by Lieutenant Maury the Calms of
Cancer and the Calms of Capricorn, but generally known to sailors as the
Doldrums. Thus the winds go from pole to pole and back again,
alternately as under and upper currents. In their circuits the winds
cross each other five times, producing regions of calms at the poles,
the tropics, and equator. The trade-winds generally extend for about 28°
on each side of the equator, but, on account of the greater quantity of
land in the northern hemisphere, the N.E. trade-wind is narrower than
the S.E.
The sun is perpetually raising enormous quantities of vapour from the
ocean which the trade-winds carry to the equator: it is condensed when
it rises with the air into the higher strata, and forms a ring of clouds
along the southern side of the belt of equatorial calms that surrounds
the earth, which, during the day, is perpetually pouring down torrents
of rain, while the sun continually beating upon its upper surface
dissolves the clouds into invisible vapour which is carried by the winds
and condensed into rain on the extra-tropical regions. The whole system
of trade-winds, equatorial and tropical calms, with the cloud ring,
follow the sun in declination; consequently in its journeys back and
forwards it annually travels over 1000 miles of latitude, and regulates
the dry and rainy season in the tropical parts of the earth.
The monsoons, which are periodic winds in the Indian Ocean, in part
depend upon this movement. For when the sun is in the northern
hemisphere the trade-winds come northward with him; and when his intense
heat expands the air over the Great Gobi and other arid Asiatic deserts,
it ascends; the N.E. trade-wind is drawn in to fill the vacuum and
ascends with it; then the S.E. trade-wind, being no longer met and
balanced by the N.E. trade, passes into the northern hemisphere, and as
it proceeds northward from the equator it is deflected to the west by
the rotation of the earth, combined with the indraught over the heated
deserts, and becomes the S.W. monsoon, which blows while the sun is
north of the equator, but as soon as he goes south, and no longer
rarefies the air over the Indian deserts, the S.E. trade-wind resumes
its usual course, and is then known as the S.E. monsoon. The influence
of the heated deserts is perceptible to the distance of 1000 miles from
the shore; the monsoons prevail with great steadiness over the Arabian
Gulf, the Indian Ocean, and part of the China Sea. At the change,
torrents of rain and violent thunderstorms accompany the conflict
between the contending winds.
The Sahara desert in North Africa, and those of Utah, Texas, and New
Mexico, occasion the monsoons which prevail in the North Atlantic and on
both sides of Central America, and the monsoons which blow to the north
of Australia show the sterility of the interior, even if other proofs
were wanting. From the powerful effect of the land in drawing off the
winds from their course, it may be seen why the N.E. trade-winds are
narrower than the S.E. trades.
In the extra-tropical winds in the North Atlantic, which blow from the
40th parallel to the pole, the north-westerly are to the easterly as 2
to 1: hence there would be an accumulation of air at the pole at the
expense of the equator, did not a current rise at the pole and return to
the equator through the high regions of the atmosphere, which confirms
the theory of the rotation of the wind.
There are many proofs of the existence of the counter-currents above the
trade-winds. On the Peak of Teneriffe the prevailing winds are from the
west. Light clouds have frequently been seen moving rapidly from west to
east at a very great height above the trade-winds, which were sweeping
along the surface of the ocean in a contrary direction. Rains, clouds,
and nearly all the other atmospheric phenomena, occur below the height
of 18,000 feet, and generally much nearer to the surface of the earth.
They are owing to currents of air running upon each other in horizontal
strata, differing in their electric state, in temperature and moisture,
as well as in velocity and direction.
When north and south winds blow alternately, the wind at any place will
veer in one uniform direction through every point of the compass,
provided the one begins before the other has ceased. In the northern
hemisphere a north wind sets out with a smaller degree of rotatory
motion than the places have at which it successively arrives,
consequently it passes through all the points of the compass from N. to
N.E. and E. A current from the south, on the contrary, sets out with a
greater rotatory velocity than the places have at which it successively
arrives, so by the rotation of the earth it is deflected from S. to S.W.
and W. Now, if the vane at any place should have veered from the N.
through N.E. to E., and a south wind should spring up, it would combine
its motion with the former and cause the vane to turn successively from
the E. to S.E. and S. But by the earth’s rotation this south wind will
veer to the S.W. and W., and, if a north wind should now arise, it would
combine its motion with that of the west, and cause it to veer to the
N.W. and N. Thus two alternations of north and south wind will cause the
vane at any place to go completely round the compass, from N. to E., S.,
W., and N. again. At the Royal Observatory at Greenwich the wind
accomplishes five circuits in that direction in the course of a year.
When circumstances combine to produce alternate north and south winds in
the southern hemisphere, the gyration is in the contrary direction.
Although the general tendency of the wind may be rotatory, and is so in
many instances, at least for part of the year, yet it is so often
counteracted by local circumstances, that the winds are in general very
irregular, every disturbance in atmospheric equilibrium from heat or any
other cause producing a corresponding wind. The most prevalent winds in
Europe are the N.E. and S.W.; the former arises from the north polar
current, and the latter from causes already mentioned. The law of the
wind’s rotation was first described by Dr. Dalton, but has been
developed by Professor Dove, of Berlin.
Hurricanes are those storms of wind in which the portion of the
atmosphere that forms them revolves in a horizontal circuit round a
vertical or somewhat inclined axis of rotation, while the axis itself,
and consequently the whole storm, is carried forward along the surface
of the globe, so that the direction in which the storm is advancing is
quite different from the direction in which the rotatory current may be
blowing at any point. In the West Indies, where hurricanes are frequent
and destructive, they generally originate in the tropical regions near
the inner boundary of the trade-winds, and are caused by the vertical
ascent of a column of rarefied air, whose place is supplied by a rush of
wind from the surrounding regions, set into gyration by the rotation of
the earth. By far the greater number of Atlantic hurricanes have begun
eastward of the lesser Antilles or Caribbean Islands.
In every case the axis of the storm moves in an elliptical or parabolic
curve, having its vertex in or near the tropic of Cancer, which marks
the external limit of the trade-winds north of the equator. As the
motion before it reaches the tropic is in a straight line from S.E. to
N.W., and after it has passed it from S.W. to N.E., the bend of the
curve is turned towards Florida and the Carolinas. In the southern
hemisphere the body of the storm moves in exactly the opposite
direction. The hurricanes which originate south of the equator, and
whose initial path is from N.E. to S.W., bend round at the tropic of
Capricorn, and then move from N.W. to S.E.
The extent and velocity of these storms are great; for instance, the
hurricane that took place on the 12th of August, 1830, was traced from
eastward of the Caribbee Islands, along the Gulf Stream, to the bank of
Newfoundland, a distance of more than 3000 miles, which it passed over
in six days. Although the hurricane of the 1st of September, 1821, was
not so extensive, its velocity was greater, as it moved at the rate of
30 miles an hour: small storms are generally more rapid than those of
greater dimensions.
The action of these storms seems to be at first confined to the stratum
of air nearest the earth, and then they seldom appear to be more than a
mile high, though sometimes they are raised higher; or even divided by a
mountain into two separate storms, each of which continues its new path
and gyrations with increased violence. This occurred in the gale of the
25th of December, 1821, in the Mediterranean, when the Spanish mountains
and the Maritime Alps became new centres of motion.
By the friction of the earth the axis of the storm bends a little
forward. This causes a continual intermixture of the lower and warmer
strata of air with those that are higher and colder, producing torrents
of rain and violent electric explosions.
The breadth of the whirlwind is greatly augmented when the path of the
storm changes on crossing the tropic. The vortex of a storm has covered
an extent of the surface of the globe 500 miles in diameter.
The revolving motion accounts for the sudden and violent changes
observed during hurricanes. In consequence of the rotation of the air,
the wind blows in opposite directions on each side of the axis of the
storm, and the violence of the blast increases from the circumference
towards the centre of gyration, but in the centre itself the air is in
repose: hence, when the body of the storm passes over a place, the wind
begins to blow moderately, and increases to a hurricane as the centre of
the whirlwind approaches; then, in a moment, a dead and awful calm
succeeds, suddenly followed by a renewal of the storm in all its
violence, but now blowing in a direction diametrically opposite to its
former course. This happened at the Island of St. Thomas on the 2nd of
August, 1837, where the hurricane increased in violence till half-past
seven in the morning, when perfect stillness took place for forty
minutes, after which the storm recommenced in a contrary direction.
The sudden fall of the mercury in the barometer in the regions
habitually visited by hurricanes is a certain indication of a coming
tempest. In consequence of the centrifugal force of these rotatory
storms the air becomes rarefied, and, as the atmosphere is disturbed to
some distance beyond the actual circle of gyration or limits of the
storm, the barometer often sinks some hours before its arrival, from the
original cause of the rotatory disturbance. It continues sinking under
the first half of the hurricane, is at a maximum sometimes of two inches
in the centre of gyration, and again rises during the passage of the
latter half, though it does not attain its greatest height till the
storm is over. The diminution of atmospheric pressure is greater and
extends over a wider area in the temperate zones than in the torrid, on
account of the sudden expansion of the circle of rotation when the gale
crosses the tropic.
As the fall of the barometer gives warning of the approach of a
hurricane, so the laws of the storm’s motion afford the seaman knowledge
to guide him in avoiding it. In the northern temperate zone, if the gale
begins from the S.E. and veers by S. to W., the ship should steer to the
S.E.; but, if the gale begins from the N.E., and changes through N. to
N.W., the vessel should go to the N.W. In the northern part of the
torrid zone, if the storm begin from the N.E., and veer through E. to
S.E., the ship should steer to the N.E.; but, if it begin from the N.W.,
and veer by W. to S.W., the ship should steer to the S.W., because she
is in the south-western side of the storm. Since the laws of storms are
reversed in the southern hemisphere, the rules for steering vessels are
necessarily reversed also. A heavy swell is peculiarly characteristic of
these storms. In the open sea the swell often extends many leagues
beyond the range of the gale which produced it.
Waterspouts are occasioned by small whirlwinds, which always have their
origin at a great distance from that part of the sea from which the
spout begins to rise, where it is generally calm. The whirl is produced
by two currents of air, which, running in opposite directions, compress
one another by their impetus, so that they rise in spiral eddies to the
clouds. They move slowly along the surface of the sea, sometimes in
vertical, and sometimes in twisted spirals, putting the sea into violent
agitation as they pass, and carrying the water aloft by the force of
gyration. Occasionally the eddies begin in the clouds and dip down to
the sea.