Temperature of the earth—Temperature of the Air—Radiation—Foci of
Maximum Cold—Thermal Equator—Its Temperature, mean and absolute—
Isothermal Lines—Continental and Insular Climates—Extreme Climates—
Stability of Climate—Decrease of Heat in Altitude—Line of Perpetual
Snow—Density of the Atmosphere—The Barometer—Measurement of Heights—
Variations in Density and their Causes—Horary Variations—Independent
Effect of the dry and aqueous Atmospheres—Mean height of Barometer in
different Latitudes—Depression in the Antarctic Ocean and in Eastern
Siberia—Barometric Storms—Polar and Equatorial Currents of Air—
Trade-Winds—Monsoons—Land and Sea Breezes—Gyration of the Winds in the
Extra-Tropical Zones—Winds in Middle European Latitudes—Hurricanes—The
Laws of their Motion—Their Effect on the Barometer—How to steer clear
of them—The Storm-Wave—Storm-Currents—Arched Squalls—Tornadoes—
Whirlwinds—Water Spouts.
THE atmosphere completely envelops the earth to the height of about 20
miles; it bulges at the equator, and is flattened at the poles, in
consequence of the diurnal rotation. It is a mixture of water in an
invisible state and of air; but the air is not homogeneous; 100 parts of
it consist of 79 parts of hydrogen or azotic gas, and 21 of oxygen, the
source of combustion and animal heat. Besides these, there is a little
ammoniacal vapour, and a small quantity of carbonic acid gas, which is
sufficient to supply all the vegetation on the earth with wood and
leaves. No doubt exhalations of various kinds ascend into the air, such
as those which produce miasmata, but they are in quantities too minute
to be detected by chemical analysis, so that the atmosphere is found to
be of the same composition at all heights above the sea hitherto
attained.[126]
The temperature of the earth’s surface, and the phenomena of the
atmosphere, depend upon the revolution and rotation of the earth, which
successively expose all the parts of the earth, and the air which
surrounds it, to a perpetual variation of the gravitating forces of the
two great luminaries, and to annual and diurnal vicissitudes of solar
heat. Atmospheric phenomena are consequently periodical and connected
with one another, and their harmony, and the regularity of the laws
which govern them, become the more evident in proportion as the mean
values of their vicissitudes are determined from simultaneous
observations made over widely-extended tracts of the globe. The
fickleness of the wind and weather is proverbial, but, as the same
quantity of heat is annually received from the sun, and annually
radiated into space, it follows that all climates on the earth are
stable, and that their changes, like the perturbations of the planets,
are limited, and accomplished in fixed cycles, whose periods are still
in many instances unknown. It is possible, however, that the earth and
air may be affected by secular variations of temperature during the
progress of the solar system through space, or from periodical changes
in the sun’s light and heat, similar to those which take place in many
of the fixed stars. The secular variation in the moon’s mean distance
will no doubt alter the amount of her attractive force, though probably
by a quantity inappreciable in the aërial tides; at all events,
variations arising from such circumstances could only become perceptible
after many ages.
From experiments made by M. Peltier it appears that, if the absolute
quantity of heat annually received by the earth were equally dispersed
over its surface, it would, in the course of a year, melt a stratum of
ice 46 feet deep covering the whole globe. It is evident that, if so
great a quantity of heat had been continually accumulated in the earth,
instead of being radiated into space, it would have been transmitted
through the surface to the poles, where it would have melted the ice,
and the torrid zone, if not the whole globe, would by this time have
been uninhabitable. In fact, every surface absorbs and radiates heat at
the same time, and the power of radiation is always equal to the power
of absorption, for, under the same circumstances, bodies which become
soon warm also cool rapidly, and the earth, as a whole, is under the
same law as the bodies at its surface.
Although part of the heat received from the sun in summer is radiated
back again, by far the greater part sinks into the earth’s surface, and
tempers the severity of the winter’s cold while passing through the
atmosphere into the etherial regions.
The power of the solar rays depends on the manner in which they fall, as
may be seen from the difference of climates. The earth is about
3,000,000 of miles nearer to the sun in winter than in summer, but the
rays strike the northern hemisphere more obliquely in winter than in the
other half of the year.
Diurnal variations of heat are perceptible only to a small distance
below the surface of the ground, because the earth is a bad conductor:
the annual influence of the sun penetrates much farther. At the equator,
where the heat is greatest, it descends deeper than elsewhere with a
diminishing intensity, but there, and everywhere throughout the globe,
there is a stratum, at a depth varying from 40 to 100 feet below the
surface of the ground, where the temperature never varies, and is nearly
the same with the mean temperature of the country over it. This zone,
unaffected by the sun’s heat from above, or by the internal heat from
below, serves as an origin whence the effects of solar heat are
estimated on one hand, and the internal temperature of the globe on the
other. Below it the heat of the earth increases, as already mentioned,
at the rate of one degree of Fahrenheit’s thermometer for every 50 or 60
feet of perpendicular depth; were it to continue increasing at that
rate, every substance would be in a state of fusion at the depth of 21
miles; hitherto, however, the experiments in mines and Artesian wells,
whence the earth’s temperature below the constant stratum is
ascertained, have not been extended below 1700 feet.
M. de Beaumont has estimated by the theory of Fourier, from the
observations of M. Arago, that the quantity of central heat which
reaches the surface of the earth is capable, in the course of a year, of
melting a shell of ice covering the globe a quarter of an inch
thick.[127]
The superficial temperature of the earth is great at the equator, it
decreases gradually towards the poles, and is an exact mean between the
two at the 45th parallel of latitude; but a multitude of causes disturb
this law even between the tropics. It is affected chiefly by the unequal
distribution of land and water, by the height above the sea, by the
nature of the soil, and by vegetation, so that a line drawn on a map
through all the places where the mean temperature of the earth is the
same would be very far from coinciding with the parallels of latitude,
but would approximate more to them near the equator. Between the tropics
the temperature of the earth’s surface is greater in the interior of
continents than on the sea-coasts and islands, and in the interior of
Africa it is greater than in any other part of the globe.
Temperature depends upon the property all bodies possess, more or less,
of perpetually absorbing and emitting or radiating heat. When the
interchange is equal, the temperature of a substance remains the same;
but when the radiation exceeds the absorption, it becomes colder, and
_vice versâ_. The temperature of the air is certainly raised by the
passage of the solar heat through it, because it absorbs one-third of it
before reaching the earth, but it is chiefly warmed by heat transmitted
and radiated from the earth. The radiation is abundant when the sky is
clear and blue, but clouds intercept it; so that a thermometer rises in
cloudy weather, and sinks when the air becomes clear and calm; even a
slight mist diminishes radiation from the earth, because it returns as
much heat as it receives. The temperature of the air is subject to such
irregularities from these circumstances, and from the difference in the
radiating powers of the bodies at the surface of the globe, that it is
necessary to find, by experiment, the mean or average warmth of the day,
month, and year, at a great variety of places, in order to have a
standard by which the temperature in different parallels of latitude may
be compared.
The mean diurnal temperature of the air, at any place, is equal to half
the sum of the greatest and least heights of the thermometer during 24
hours, and, as the height of the thermometer is twice in the course of
that time equal to the mean temperature of the place of observation, it
might seem easy to obtain its value; yet that is not the case, for a
small error in observation produces a very great error in such minute
quantities, so that accuracy can only be attained from the average of a
great number of observations, by which the errors, sometimes in excess
and sometimes in defect, neutralize or balance each other. The mean
value of quantities is a powerful aid to the imperfections of our nature
in arriving at truth in physical inquiries, and in none more than in
atmospheric phenomena: almost all the certain knowledge man has acquired
with regard to the density and temperature of the air, winds, rain, &c.,
has been acquired by that method.
The mean temperature of any one month at the same place differs from one
year to another, but the mean temperature of the whole year remains
nearly the same, especially when the average of 10 or 15 years is taken;
for although the temperature in any one place may be subject to very
great variations, yet it never deviates more than a few degrees from its
mean state.[128]
The motion of the sun in the ecliptic occasions perpetual variations in
the length of the day, and in the direction of his rays with regard to
the earth; yet, as the cause is periodic, the mean annual temperature
from the sun’s motion alone must be constant in each parallel of
latitude. For it is evident that the accumulation of heat in the long
days in summer, which is but little diminished by radiation during the
short nights, is balanced by the small quantity of heat received during
the short days of winter and its radiation in the long frosty and clear
nights. Were the globe everywhere on a level with the surface of the
sea, and of uniform substance, so as to absorb and radiate heat equally,
the mean heat of the sun would be regularly distributed over its surface
in zones of equal annual temperature parallel to the equator, and would
decrease regularly to each pole. The distribution of heat, however, in
the same parallel is very irregular in all latitudes, except between the
tropics, from the inequalities in the level and nature of the surface of
the earth, so that lines drawn on a map through all places having the
same mean annual temperature are nearly parallel to the equator only
between the tropics; in all other latitudes they deviate greatly from
it, and from one another.[129] Radiation is the principal cause of
temperature; hence, the heat of the air is most powerfully modified by
the ocean, which occupies three times as much of the surface of the
globe as the land, and is more uniform in its surface, and also in its
radiating power. On the land the difference in the radiating force of
the mountains and table-lands from that of the plains—of deserts from
grounds covered with rich vegetation—of wet land from dry, are the most
general causes of variation; the local causes of irregularity are beyond
enumeration.
There are two points in the northern hemisphere, both in the 80th
parallel of latitude, where the cold is more intense than in any other
part of the globe with which we are acquainted. One north of Canada in
100° W. long. has a temperature of -3°·5 of Fahrenheit; while, at the
Siberian point, in 95° E. long., the temperature of the air is +1°;
consequently it is four and a half degrees warmer than that north of
Canada—a difference that has an influence even to the equator, where the
mean temperature of the air is different in different longitudes.
The line of the maximum temperature of the atmosphere, or the
atmospheric thermal equator, which cuts the terrestrial equator in the
meridians of Otaheite and Singapore, passes through the Pacific in its
southern course, and through the Atlantic in its northern, has a mean
temperature of 83°·84 of Fahrenheit. But by the comparison of many
observations the mean equatorial temperature of the air is 82°·94 in
Asia, 85°·10 in Africa, and 80°·96 in America: thus, it appears that
tropical Africa is the hottest region on earth. Moreover, the atmosphere
in the tropical zone of the Pacific, when free from currents, is two
degrees and a quarter warmer than the corresponding zone in the
Atlantic, which is 82°·40.
On account of the great extent of ocean, the isothermal lines in the
southern hemisphere coincide more nearly with the parallels of latitude
than in the northern. In the Antarctic Ocean the only flexure is
occasioned by the cold of the south polar current, which flows along the
western coast of the American continent. In the northern hemisphere the
predominance of land and its frequent alternations with water, the
prevalence of particular winds, irregularities of the surface, and the
difference in the temperature of the points of maximum cold, cause the
isothermal lines to deviate more from the parallels of latitude. They
make two deep bends northward, one in the Northern Atlantic and another
in the northeast of America, and at last they separate into two parts,
and encircle the points of maximum cold.
Professor Dove has discovered that, in consequence of the excess of land
in the northern hemisphere, and the difference in the effect produced by
the sun’s heat according as it falls on a solid or liquid surface, there
is an annual variation in the aggregate mean temperature at the surface
of the earth, whose maximum takes place during the sun’s northern
declination, and its minimum during its southern.[130]
Places having the same mean annual temperature, often differ materially
in climate: in some, the winters are mild and the summers cool, whereas
in others the extremes of heat and cold prevail: England is an example
of the first; Quebec, St. Petersburg, and the Arctic regions, are
instances of the second. The solar heat penetrates more abundantly and
deeper into the sea than into the land; in winter it preserves a
considerable portion of that which it receives in summer, and from its
saltness does not freeze so soon as fresh water; hence, the ocean is not
liable to the same changes of temperature as the land, and by imparting
its heat to the winds it diminishes the severity of the climate on the
coasts and in islands, which are never subject to such extremes of heat
and cold as are experienced in the interior of continents. The
difference between the influence of sea and land is strikingly
exemplified in the high latitudes of the two hemispheres. In consequence
of the unbounded extent of the ocean in the south, the air is so mild
and moist that a rich vegetation covers the ground, while in the
corresponding latitudes in the north the country is barren from the
excess of land towards the Polar Ocean, which renders the air dry and
cold. A superabundance of land in the equatorial regions, on the
contrary, raises the temperature, while the sea tempers it.
Professor Dove has shown, from a comparison of observations, that
northern and central Asia have what may be termed a true continental
climate, both in summer and in winter—that is to say, a hot summer and
cold winter; that Europe has a true insular or sea climate in both
seasons, the summers being cool and the winters mild; and that in North
America the climate is inclined to be continental in winter, and insular
in summer. The extremes of temperature in the year are greater in
central Asia than in North America, and greater in North America than in
Europe, and that difference increases everywhere with the latitude. In
Guiana, within the tropics, the difference between the hottest and
coldest months in the year is 2°·2 of Fahrenheit, in the temperate zone
it is about 60°, and at Yakutsk in Siberia 114°·4. Even in places which
have the same latitude as in northern Asia, compared with others in
Europe or North America, the diversity is very great. At Quebec the
summers are as warm as those in Paris, and grapes sometimes ripen in the
open air, yet the winters are as severe as those in St. Petersburg. In
short, lines drawn on a map through places having the same mean summer
or winter temperature are neither parallel to one another, to the
isothermal or geothermal lines, and they differ still more from the
parallels of latitude.[131]
Observations tend to prove that all the climates on the earth are
stable, and that their vicissitudes are only oscillations of greater or
less extent, which vanish in the mean annual temperature of a sufficient
number of years. There may be a succession of cold summers and mild
winters, but in some other country the contrary takes place; the
distribution of heat may vary from a variety of circumstances, but the
absolute quantity gained and lost by the whole earth in the course of a
year is invariably the same.
Since the air receives its warmth chiefly from the earth, its
temperature diminishes with the height so rapidly, that at a very small
elevation the cold becomes excessive, as the perpetual snow on the
mountain-tops clearly shows. The decrease of heat is at the rate of a
degree of Fahrenheit’s thermometer for every 334 feet.
The atmosphere, being a heavy and elastic fluid, decreases in density
upwards, according to a determinate law, so rapidly, that three-fourths
of the whole air it contains are within four miles of the earth, and all
the phenomena perceptible to us—as clouds, rain, snow, and thunder—occur
within that limit. The air even on the tops of mountains is so rare as
to diminish the intensity of sound, to affect respiration, and to
occasion a loss of muscular strength in man and animals.[132]
Since the space in the top of the tube of a barometer is a vacuum, the
column of mercury is suspended in the tube by the pressure of the
atmosphere on the surface of the mercury in the cistern: hence, every
variation in the density or height of the atmosphere occasions a
corresponding rise or fall in the barometric column. The actual mean
pressure of the atmosphere at the level of the sea is 15 pounds on the
square inch; hence, the pressure on the whole earth is enormous.
The decrease in the density of the air affords a very accurate method of
finding the height of mountains above the level of the sea, which would
be very simple, were it not for changes of temperature which alter the
density and interfere with the regularity of the law of its decrease.
But as the heat of the air diminishes with the height above the earth at
the rate of one degree of Fahrenheit’s thermometer for every 334 feet,
tables are constructed, by the aid of which heights may be determined
with great accuracy. In consequence of diminished pressure also, water
boils at a lower temperature on mountain-tops than at the level of the
sea, which affords another method of ascertaining heights.[133]
By the annual and diurnal revolutions of the earth, each column of air
is alternately exposed to the heat and cold of summer and winter, of day
and night, and also to variations in the attraction of the sun and moon,
which disturb its equilibrium, and produce tides similar to those in the
ocean. Those produced by the moon ebb and flow twice during a lunation,
and diurnal variations in the barometer, to a very small amount, are
also due to the moon’s attraction.[134] The annual undulations
occasioned by the sun have their greatest altitudes at the equinoxes,
and their least at the solstices, and the diurnal variations in the
height of the barometer, which accomplish their rise and fall twice in
24 hours, are chiefly due to the effects of temperature on the dry air
and moisture of the atmosphere, which, according to Mr. Dove’s
discoveries, produce independent pressures upon the mercurial column.
A quantity of vapour is continually raised by the heat of the sun from
the surface of the globe, which mixes in an invisible state with the dry
air or gaseous part of the atmosphere. It is most abundant in the torrid
zone, and, like the heat on which it depends, varies with the latitude,
the season of the year, the time of the day, the elevation above the
sea, and also with the nature of the soil, the land, and the water.
There is no chemical combination between the aërial and aqueous
atmospheres, they are merely mixed; and the diurnal variations arise
from the superposition of two distinct diurnal oscillations, each going
through its complete period in 24 hours; one taking place in the aërial
atmosphere from the alternate heating and cooling of the air, which
produce a flux and reflux over the point of observation; the other
arising from the aqueous atmosphere, owing to the alternate production
and destruction of vapour by the heat of the day and the cold of the
night. The diurnal variations of the vapour have their maximum at or
near the hottest hour of the day, and their minimum at or near the
coldest, which is exactly the converse of the diurnal variations of the
dry air. On the whole, there are two maxima and two minima heights of
the barometer in the course of 24 hours from the combinations of these,
but in the interior of continents far from water, where the air is very
dry, there ought to be one maximum and one minimum during that period,
according to this theory.
Between the tropics the barometer attains its greatest height at nine or
half-past nine in the morning; it then sinks till four in the afternoon,
after which it again rises and attains a second maximum at ten or
half-past ten in the evening; it then begins to fall till it reaches a
second time its lowest point at four in the morning. The difference in
the height is 0·117 of an inch, which gradually decreases north and
south. Baron Humboldt mentions that the diurnal variations of the
barometric pressure are so regular between the tropics, that the hour of
the day may be inferred from the height of the mercury to within fifteen
or sixteen minutes, and that it is undisturbed by storm, tempest, rain,
or earthquake, both on the coasts and at altitudes 13,000 feet above
them. The mean height of the barometer between the tropics at the level
of the sea is 30 inches with very little fluctuation, but, owing to the
ascending currents of air from the heat of the earth, it is less under
the equator than in the temperate zones. It attains a maximum in western
Europe between the parallels of 40° and 45°; in the North Atlantic the
maximum is about the 30th parallel, and in the southern part of that
ocean it is near the tropic of Capricorn; the amplitude of the
oscillations decreases from the tropics to about the 70th parallel,
where the diurnal variations cease. They are affected by the seasons,
being greatest in summer and least in winter. It appears, also, that the
fluctuations are the reverse on mountain-tops from what they are on the
plains, and probably at a certain height they would cease
altogether.[135] It is a singular fact, discovered by our navigators,
that the mean height of the barometer is an inch lower throughout the
Antarctic Ocean and at Cape Horn than it is at the Cape of Good Hope or
Valparaiso: that difference in the pressure of the atmosphere is
probably connected with the perpetual gales off the extremity of South
America. M. Erman observed a similar depression near the Sea of Okhotsk
in eastern Siberia.
Besides the small horary undulations, there are vast waves moving over
the oceans and continents in separate and independent systems, being
confined to local yet very extensive districts, probably occasioned by
long-continued rains or dry weather over wide tracts of country. By
numerous barometrical observations made simultaneously in both
hemispheres, the courses of several have been traced, some of which take
24, others 36 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. The course of another wave has been made out from the Cape
of Good Hope, through many intermediate stations, to the observatory at
Toronto in Canada. Since every undulation has its perfect effect
independently of the others, each one is marked by a change in the
barometer, and this is beautifully illustrated by curved lines on paper,
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. Although,
like all other waves, these in the atmosphere are but waving forms, in
which there is no transfer of air, yet winds arise from them like
tide-streams in the ocean, and Sir John Herschel is of opinion that the
crossing of two of these vast aërial waves, coming in different
directions, may generate, at the point of intersection, those tremendous
revolving storms, or hurricanes, which spread desolation far and wide.
The air expands and becomes lighter with heat, contracts and becomes
heavier with cold, and, as there are 82 degrees of difference between
the equatorial and polar temperature, the light warm air at the equator
is constantly ascending to the upper regions of the atmosphere, and
flowing north and south to the poles, from whence the cold heavy air
rushes along the surface of the earth to supply its place between the
tropics, for the same tendency to restore equilibrium exists in air as
in other fluids. These two superficial currents, which have no rotatory
motion when they leave the poles, are deflected from their meridional
paths by friction from the continually increasing velocity of the
earth’s rotation, as they come nearer and nearer to the tropics; and, as
they revolve slower than the corresponding parts of the earth at which
they arrive, the bodies on its surface strike against them with the
excess of their velocity, so that the wind appears, to a person who
thinks himself at rest, to blow in a direction contrary to that of the
earth’s rotation. For that reason the current from the north pole
becomes a north-east wind before arriving at the tropic of Cancer, and
that from the south pole becomes a south-east wind before it comes to
the tropic of Capricorn, their limit being the 28th parallel of latitude
on each side of the equator. In fact, the difference of temperature puts
the air in motion, and the direction of the resulting wind, at every
place, depends upon the difference between the rotatory motion of the
wind and the rotatory motion of the earth—the whole theory of the winds
depends upon these circumstances.
Near the equator the trade-winds, north and south of it, so completely
neutralize each other, that far at sea a candle burns without flickering
[_i. e._ when it is flat calm]. This zone of calms and light breezes,
known as the _Variables_, which has a breadth of about five degrees and
a half, is subject to heavy rains and violent thunder-storms. On account
of the arrangement of land and water, it does not coincide with the
equator, but its centre runs along the sixth parallel of north latitude;
however, it changes in position and extent with the declination of the
sun, but never crosses the line.
Though the trade-winds extend to the 28th degree on each side of the
equator, their limits vary considerably in different parts of the ocean,
moving two or three degrees to the north or south, according to the
position of the sun; and in the Atlantic the north-east trade-wind is
less steady than the south-east.[136] These perennial winds are known by
recent observations to be less uniform in the Pacific than in the
Atlantic; they only blow permanently over that portion between the
Galapagos Archipelago, off the coast of America, and the Marquesas. In
the Indian Ocean the south-east trade-wind blows from a few degrees east
of Madagascar to the coast of Australia, between 10° and 28° S. lat. The
trade-winds are only constant far from land, because continents and
islands intercept them, and change their course. On that account the
numerous groups of islands westward from the Marquesas change the
trade-winds into the periodical monsoons, which are steady currents of
air in the Arabian Gulf, the Indian Ocean, and China Sea, arising from
diminished atmospheric pressure at each tropic alternately, from the
heat of the sun, thereby producing a regular alternation of north and
south winds, which, combining with the rotation of the earth on its
axis, become a north-east wind in the northern hemisphere, and a
south-east in the southern. The former blows from April to October, the
latter from October to April; the change is accompanied by heavy rain
and violent storms of thunder and lightning. The ascent of the warm air
between the tropics occasions a depression of the barometer amounting to
the tenth of an inch, which is a measure of the force producing the
trade-winds. In both hemispheres there is a regular variation in the
mean height of the barometer within the zone in which these great aërial
currents flow; it is higher at their polar limits, and decreases with
extreme uniformity towards their equatorial boundaries, the difference
in both hemispheres being 0·25 of an inch.
The unequal temperature of the land and sea causes sea-breezes which
blow towards the land during the day, and land-breezes which blow
sea-ward in the night; they are not perceptible in the mornings and
evenings, because the temperature of the land and water is then nearly
the same.
The trade-winds and monsoons are permanent, depending on the apparent
motion of the sun; but it is evident from theory that there must be
partial winds in all parts of the earth, occasioned by the local
circumstances that affect the temperature of the air. Consequently, the
atmosphere is divided into districts, both over the sea and land, in
which the winds have nearly the same vicissitudes from year to year. The
regularity is greatest towards the tropics, where the causes of
disturbance are fewer. In the higher latitudes it is more difficult to
discover any regularity, on account of the greater proportion of land,
the difference in its radiating power, and the greater extremes of heat
and cold. But even there a degree of uniformity prevails in the
succession of the winds; for example, in all places where north and
south winds blow alternately, a vane veers through every point of the
compass in the transition, and in some places the wind makes several of
these gyrations in the course of the year.[137] The south-westerly
winds, so prevalent in the Atlantic Ocean between the 30th and 60th
degrees of north latitude, are produced by the upper current being drawn
down to supply the superficial current which goes towards the equator,
and, as it has a greater rotatory motion than the earth in these
latitudes, it produces a south-westerly wind. On this account the
average voyage from Liverpool to New York in a sailing vessel is 40
days, while it is only 23 days from New York to Liverpool. For the same
reason the average direction of the wind in England, France, Germany,
Denmark, Sweden, and North America, is some point between south and
west. North-westerly winds prevail in the corresponding latitudes of the
southern hemisphere from the same cause. In fact, whenever the air has a
greater velocity of rotation than the surface of the earth, a wind more
or less westerly is produced; and when it has less velocity of rotation
than the earth, a wind having an easterly tendency results. Thus, there
is a perpetual change between the different masses of the atmosphere,
the warm air tempering the cold of the higher latitudes, and the cold
air mitigating the heat of the lower; it will be shown afterwards that
the aërial currents are the bearers of principles on which the life of
the animal and vegetable world depends.
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 forwards 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; the progressive motion may continue for days,
while the wind accomplishes many gyrations through all the points of the
compass in the same time. In the Atlantic the principal region of
hurricanes is to the east of the West Indian islands, and in the Pacific
it lies east of the island of Madagascar; consequently the former is in
the northern hemisphere, the latter in the southern; but in every case
the storm moves in an elliptical or parabolic curve. The West Indian
hurricanes generally have their origin eastward of the Lesser Antillas
or Caribbean islands, and the vertex of their path near the tropic of
Cancer, or about the exterior limit of the north-east trade-wind. As the
motion of the storm before it reaches the tropic is in a straight line
from S.E. to N.W., and after it has passed the tropic from S.W., to
N.E., the bend of the curve is turned towards Florida and the Carolinas.
In the South Pacific Ocean the body of the storms moves in an exactly
opposite direction. The hurricanes which originate south of the equator,
and whose initial path is from N.E. to S.W., turn at the tropic of
Capricorn, and then tend from N.W. to S.E., so that the bend of the
curve is turned towards Madagascar.
The extent and velocity of the Atlantic hurricanes are great; the most
rapid move at the rate of 43 miles an hour, the slowest 16. The
hurricane which took place on the 12th of August, 1830, was traced from
the eastward of the Caribbean islands to the banks of Newfoundland, a
distance of more than 3000 miles, which it passed over in six days.
Although that 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 great magnitude. Sometimes
they appear to be stationary, sometimes they stop and again proceed on
their course, like water-spouts. Hurricanes are occasionally
contemporaneous, and so near to one another as to travel in almost
parallel tracks. This happened in the China seas in October, 1840, when
the two storms met at an angle of 47°, and it was supposed that the ship
Golconda foundered in that spot with 300 people on board. A hurricane
has been split or divided by a mountain into two separate storms, each
of which continued its new course, and the gyrations were made 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, and the whirling motion begins in the higher regions of the
atmosphere before it is felt on the earth: 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 sometimes violent
electric explosions.
The rotation as well as the course of the storm is in a different
direction in the two hemispheres, though always alike in the same. In
the northern hemisphere the gyration is contrary to the movement of the
hands of a watch, that is to say, the wind revolves from east, through
the north, to west, south, and east again; while in the southern
hemisphere the rotation about the axis of the storm is in the contrary
direction. Hurricanes happen south of the equator between December and
April; in the West Indies between June and October. Rotatory storms
frequently occur in the Indian Ocean, and the typhoons of the China seas
are real hurricanes of great violence. Both conform to the laws of such
winds in the northern hemisphere. The Atlantic storms probably reach
Spain, Portugal, and the coast of Ireland. Two circular storms have
passed over Great Britain, and small ones often occur between the Chops
of the Channel and Madeira.
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 what
it had before: this happened in the island of St. Thomas on the 2d of
August, 1837, where the hurricane increased in violence till half-past
seven in the morning, when perfect stillness took place for 40 minutes,
after which the storm recommenced in a contrary direction. The breadth
of a hurricane is greatly augmented when its path changes its direction
in crossing the tropic. In the Atlantic, the vortex of one of these
tempests has covered an area from 600 to 1000 miles in diameter. The
breadth of the lull in the centre varies from 5 to 30 miles: the height
is from 1 to 5 miles at most; so that a person might see the strife of
the elements from the top of a mountain, such as Teneriffe or Mowna Roa,
in a perfect calm, for the upper clouds are frequently seen to be at
rest during the hideous turmoil in the lower regions.
The sudden fall of the mercury in the barometer in latitudes 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 rarified, and, as the atmosphere is disturbed to some distance
beyond the actual circle of gyration or the limits of the storm, the
barometer often sinks some hours before its arrival: it continues
sinking the first half of the hurricane, 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
where 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 to the seaman
knowledge to avoid 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 ought to 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 on 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.[138]
A heavy swell or storm-wave is peculiarly characteristic of these
tempests. In the centre of the hurricane the pressure of the atmosphere
is so much diminished by rotation, that the mercury in the barometer
falls from one to two, and even two and a-half inches. On that account,
the pressure of the ocean beyond the range of the wind raises the water
in the centre of the vortex about two feet above its usual level, and
proportionally to the degree of diminished pressure over the whole area
of the storm. This mass of water, or storm-wave, is driven bodily along
with, or before, the tempest, and rolls in upon the land like a huge
wall of water. It is similar to the earthquake wave, and is by no means
the heaping up of the water after a long gale. Ships have been swept by
it out of docks and rivers, and it has sometimes carried vessels over
reefs and banks so as to land them high and dry; this happened to two
ships on the coast of the Eastern Andaman islands, in 1844. Coringa, on
the Coromandel coast, is particularly subject to inundations from that
cause. In 1789, the town and 20,000 inhabitants were destroyed by a
succession of these great waves during a hurricane, and as many perished
there in 1839.
Besides storm-waves, storm-currents are raised, which revolve with the
rotation of the wind, and are of the greatest force near the centre of
the vortex.
The rise of the sea by the pressure of the surrounding ocean, and the
irresistible fury of the wind, makes a tremendous commotion in the
centre of the storm, where the sea rises, not in waves, but in pyramidal
masses: the noise during its passage resembles the deafening roar of the
most tremendous thunder; and in the typhoons in the China seas it is
like numberless voices raised to the utmost pitch of screaming. In
general, there is very little thunder and lightning; sometimes a vivid
flash occurs during the passage of the centre, or at the beginning of
the storm; yet in Barbadoes the whole atmosphere has been enveloped in
an electric cloud.
A thick lurid appearance, with dense masses of cloud in the horizon,
ominous and terrible, are the harbingers of the coming tempest. The sun
and clouds frequently assume a fiery redness, the whole sky takes a wild
and threatening aspect, and the wind rises and falls with a moaning
sound, like that heard in old houses on a winter’s night: it is akin to
the “calling of the sea,” a melancholy noise which, in a dead calm,
presages a storm on some parts of the English coast.
Those intensely violent gales, of short duration, called _arched
squalls_, because they rise from an arch of clouds on the horizon, are
not rotatory; they occur in the Straits of Malacca, attended by fierce
thunder and lightning and a lurid phosphorescent gleam. The
north-western gales in the Bay of Bengal, the tornadoes on the African
coast, and the pampéros of the Rio de la Plata, are of the same nature.
On an average, a strong gale moves at the rate of 40 miles an hour, a
storm at about 56, and hurricanes at 90.
Whirlwinds are frequent in tropical countries, especially in deserts;
sometimes several are seen at one time in the Arabian deserts, of all
sizes, from a few feet to some hundred yards in diameter. They occur in
all kinds of weather, by night as well as by day, and come without the
smallest notice, rooting up trees, overwhelming caravans, and throwing
down houses; and as they produce water-spouts when they reach the sea,
they dismantle and even sink ships. The water-spouts so frequently seen
on the ocean originate in adjacent strata of air of different
temperatures, running in opposite directions in the upper regions of the
atmosphere. They condense the vapour, and give it a whirling motion, so
that it descends tapering to the sea below, and causes the surface of
the water to ascend in a pointed spiral till it joins that from above,
and then it looks like two inverted cones, being thinner in the middle
than either above or below. When a water-spout has a progressive motion,
the upper and under part must move in the same direction, and with equal
velocity, otherwise it breaks, which frequently happens.