WIND AND PRESSURE OF THE GLOBE
CAUSE OF LOCAL WINDS AND OF GENERAL CIRCULATION
=General Circulation.= Differences in temperature, changing the
specific gravity of the air, are the cause of the general circulation
of the atmosphere about the earth, modified by the rotation of the
earth; likewise the local circulation between land and water is
caused by the different quantities of heat radiated by the two widely
differing forms of matter, each attaining to a different temperature
under the influence of the same solar radiation; and the inflow of
winds to the cyclone and the outflow from the anti-cyclone are due to
the same forces that cause the general and the local circulations.
If there were no difference in temperature between the equator and
the poles the atmosphere would soon adjust itself in accordance with
the laws of gravity, modified by the centrifugal force developed from
the rotation of the earth, and the atmosphere forever would be at
rest relative to the earth, moving with it as if it were a part of
the solid sphere throughout its diurnal rotation on its axis and its
annual movement about the sun. But there is a decided difference in
temperature between the equator and the poles and between land and
water surfaces; hence a general circulation, modified and distorted
by numerous local movements, which, in turn, may be modified by the
height of hills and mountains and the direction of their trend.
[Illustration: FIG. 12.—Trade wind circulation and reason for belts
of high pressure at latitudes 30° N. and S. that extend around globe
as shown by Figure 13.]
Let us trace a current of air through its course as shown in
Figure 12 and the reason for the blowing of the trade winds will
be apparent, as will the reason for the location of a belt of high
pressure at latitudes 30° north and south encircling the globe. At
the equator there is a belt of calms. Here the air gently ascends
under the intense heat of vertical sunshine. It is humid, for
there is much water surface in the region of the equator, and the
air carries vast quantities of water vapor aloft, later to be
precipitated as torrential rains in the Tropical Zone, as the air
cools by expansion in its ascent. This air expands or bulges upward
and overflows aloft northward and southward, causing low air pressure
at the equator, because of the quantity of air moved to other
latitudes, which more than compensates for the amount banked up over
the equator by the centrifugal force of the earth’s rotation.
[Illustration: CHART 1.—HIGH AND LOW CENTERS OF ACTION AND PREVAILING
WINDS OF THE GLOBE FOR JULY (Buchan).]
Since air, passing away from the equator, must pass successively
over parallels of latitude having less easterly velocity than that
with which it started its journey, it runs ahead of the earth, and,
relative to the surface of the earth, has a direction from the
southwest north of the equator, and from the northwest south of the
equator. Our current was divided at an altitude probably of six miles
above the equator, one half following the northern and the other half
the southern circuit. It was cooled by elevation and by radiation
outward to space and as a result gained in weight and gradually
descended, reaching the earth at about latitudes 30° north and south,
and causing an accumulation of air at those latitudes and the belt
of high pressure that irregularly surrounds the earth. In descending
in the belt the air breaks up into a number of anti-cyclonic systems,
sub-permanent highs or Centers of Action, which have so much to do
with initiating the migratory Highs and Lows that create the weather
of the earth, as will be fully explained in the Chapter on Weather
Forecasting. The intensity of these centers of action is modified and
their geographic positions shifted with change of season. (See Charts
1 and 2.)
[Illustration: CHART 2.—HIGH AND LOW CENTERS OF ACTION AND PREVAILING
WINDS OF THE GLOBE FOR JANUARY (Buchan).]
=Trade Winds.= But to return to the current that we left as it
divided above the equator (Figure 12) and descended on an inclined
plane to latitudes 30° north and south. It is cooler and dryer and
heavier than when it started to ascend and it has lost the thousand
miles per hour and more easterly velocity that it had at the equator
and now only has the velocity that belongs to latitude 30°; therefore
as it moves toward the equator from either side it lags behind
latitudes whose easterly velocity is greater, and it takes up a
direction partly toward the west, which, relative to the earth, makes
it a northeast wind in the Northern Hemisphere and a southeast wind
in the Southern Hemisphere. And thus is established a circulation the
lower part of which is known as the “trade winds.” (Figure 13.)
Navigators profit largely by availing themselves of the west winds
in the middle latitudes and of the east winds in the tropics. To
the daring and persistence of Columbus, and the force and constancy
of the trade winds which blew him westward, we owe the discovery of
America.
[Illustration: FIG. 13.—Average surface winds and pressure of the
globe.]
=Winds of Middle Latitudes.= Now study Figure 12 and associate the
information it conveys with that of Figure 13, and observe that from
the two belts of high pressure the air is pushed outward on both
sides. In each case it starts as a true north or south wind, but, due
to the rotation of the earth, is always and everywhere deflected to
the right in the Northern Hemisphere and to the left in the Southern
Hemisphere, and this deflection increases until what started as a
poleward wind in the middle latitudes soon becomes almost a due
west wind. In this region of west winds cyclonic storms are more
frequent than in any other part of the globe. Now get clear in the
mind the fact that no matter what may be the direction of the wind
inside a cyclonic or anti-cyclonic whirl (often one thousand miles in
diameter), the whirl is carried toward the east by the general drift
from the west of the winds between latitudes 30° and 60°, and toward
the west in the region of the trade winds.
=Low Barometer at the Poles.= Even though the air is contracted and
rendered denser by the great cold of the Arctic regions, the pressure
remains low because of the quantity of air driven equatorward by
the centrifugal force both of the earth and of the winds themselves
as they rim ahead of the earth and encircle the globe in the middle
latitudes.
=Data too Meager to Show Full Circulation Aloft of the Atmosphere
of the Globe.= Many charts have been published in the attempt to
show how the atmosphere circulates below and aloft through the whole
world. They only have speculative value, as our knowledge is too
limited to permit us to unravel the complexities of all the upper
movements.
=Rain Winds of the Tropics.= The trade winds, mostly moving over
water surfaces, are laden with moisture, but, gaining temperature as
they move towards the equator, their capacity to hold water vapor
steadily increases, and therefore they do not become rain winds
unless forced to ascend by the interposition of mountains, or until
cooled by ascension at the equator. In no part of the world does the
air rise so steadily and in such great volume as in the equatorial
belt of calms and low pressure. Hence this is the region of greatest
rainfall. During the two rainy seasons, spring and fall, the day
opens clear; near midday the clouds gather and rain falls early in
the afternoon; after which it quickly clears. This is so regular
a program that one lays his plans accordingly. There is almost no
rain in December and January; this is because the belt of calms and
the inflowing trade winds move northward and southward with the
migrations of the sun, and in December and January, the sun being
far south, the northern trades, with their rainless winds, cover
the equator and the region formerly occupied by the belt of calms.
In midsummer the sun is far north and then the southern trades move
up and give dryness to the equator. In the northern trades, of the
moderate amount of rain that falls, the greater quantity falls in
summer; in the southern trades the order is reversed.
=Rain of the High-Pressure Belts and of the Regions of West Winds.=
In the high-pressure belts the air is settling down and gaining heat
by compression and there is not much horizontal movement. These are,
therefore, regions of but little rainfall, and all the great deserts
occur in or near them. The belts of west winds are the regions of
most frequent cyclonic activities. Here the rainfall is quite equally
distributed throughout the year and is the result of the mixing of
the air by storms and its cooling by expansion as it is carried
upward in the migrating whirl.
=Circulation between Continents and Oceans.= In Chapter X, under the
sub-caption “Influence of Continents and Oceans on Climate”, the
circulation between them is well explained. In general the movement
is from the continent to the oceans in winter, with the air flowing
inward aloft to settle down and take the place of that which passes
out to sea. In summer the directions are reversed.
=Daily Variation in Coastal Winds.= In summer, when there are no
forceful storm winds blowing steadily from one direction for several
hours at a time, there will daily spring up gentle to fresh winds
from the surface of oceans and large lakes to the land, because of
the influence of the sun’s rays in heating the land to a higher
temperature than it does the water. These winds will not appear on
cloudy days and they will extend inland but a few miles.
=Monsoon Winds.= During winter the vast continent of Eurasia (Europe
and Asia) cools to such an extremely low temperature as to develop a
High, or center of action, of great energy and extent, which drives
a steady dry monsoon into the Indian Ocean and China Sea. Unlike the
trade winds, these winds reverse their direction in the summer; then
the intense heat of the continent to the north develops an extensive
Low, which draws the ocean winds inland and extends its influence so
far south as to attract the southeast trade winds of the Southern
Hemisphere and, turning them so that they flow from the southwest,
continue them far into the interior of Asia. Since the summer monsoon
blows from a tropical sea it comes heavily laden with water vapor
and as it rises over the mountains of the great Himalayan system
copious rains are precipitated. In Australia, Africa, South America,
and some parts of the North American continent monsoon influence in
various degrees is felt, but in no place is the monsoon so important
as in the countries bordering the Indian Ocean. (Charts 15 and 16.)
=Föhn Winds.= This is a hot wind that sometimes blows down a mountain
side in the Alps. In the Rocky Mountains it is called the Chinook
Wind. It is caused by moisture-laden air being drawn over a high
mountain so quickly that the heat liberated in condensation does not
have time to escape by radiation. The air cools by expansion as it
ascends on the west side of the mountain, but it gains this all back
by compression as it descends, and it has added to its temperature
much of the heat of condensation. It is dry and greedily evaporates
snow from the ground in winter, clearing off a deep covering within a
few hours.
[Illustration: FIG. 14.—How winds would blow into a cyclone on a
non-rotating earth.]
=How Winds Are Deflected by Earth’s Rotation.= Every free-moving
thing, whether wind or projectile, is deflected to the right of
its initial direction by the rotation of the earth in the Northern
Hemisphere and to the left in the Southern Hemisphere, unless the
object be moving exactly along the line of the equator. Winds moving
inward to a Low are therefore so deflected as to cause the cyclone
to gyrate in a direction contrary to the movements of the hands of
a watch. In an anti-cyclone the movement is with the watch. In the
Southern Hemisphere these wind directions are reversed.
Figure 14 gives an illustration of what would be the movement of
air inward to a cyclone on a non-rotating earth. The winds would
blow along radial lines for a time, but, converging together as they
began to ascend, they doubtless would soon set up a gyration about
the center. On a non-rotating earth this gyration would be clockwise
as often as it would be anti-clockwise, but on a rotating earth the
gyration can be in but one direction. (Figure 15.) Even tornadoes,
whose diameters of rotation are never but a few hundred feet, obey
this law. In little dust whirls, in which the movements of air may
be comprehended from the motion of the trash that is whirled about
and which are tornadoes in miniature, the direction of gyration may
be either way. They are too small for the deflecting force to be
appreciable, and it may be that the tornado is forced to take its
direction of gyration from the cyclone in whose southeast quarter it
has its origin.
[Illustration: FIG. 15.—Deflection of wind due to earth’s rotation.]
=How Wind Velocity Increases with Altitude.= Figure 16 shows how
the velocity of the wind increases with elevation in the free air
up to five thousand meters (about three miles). The average for the
year, for the summer and for the winter, is given. It increases most
rapidly up to six hundred meters in summer and up to eight hundred
meters in winter. From these two heights there is a steady and
pronounced slowing down of the wind up to one thousand meters; after
which it increases up to five thousand meters, and how far beyond we
know not. In winter there is a singular acceleration of velocity in
the stratum between two thousand and twenty-five hundred meters and
then no increase for the next five hundred meters; after which there
is a uniform and steady gain up to five thousand meters. Starting at
two hundred and seventy meters, the average velocity for the year is
3½ meters per second, or about 7¼ miles per hour. At five thousand
meters altitude the average for the year is 11¼ meters per second, or
about 27 miles per hour.
[Illustration: FIG. 16.—Annual, summer, and winter wind velocities,
with altitude. 1, 1850 feet; 2, 2467 feet; 3, 3083 feet; 4, 15,417
feet.]