THE WINDS. 91

ences of density. caused by differences of tem-
perature.
d

eee

v

— Md
HEATED AREAZ
yy LY jy

Fig, 81. Origin of Winds,

The equilibrium of the atmosphere is disturbed
by differences of temperature as follows: When
any area becomes heated, as at a a, Fig. 81, the
air over it, expanding and becoming lighter, is
pressed upward by the colder air which rushes
in from all sides. Thus result the following
currents: ascending currents, b 6, over the heated
area ; lateral, surface currents, ¢ c, from all sides
toward the heated area; upper currents, d d, from
the heated area; and descending currents, ¢ e.

It is the lateral currents which flow toward or
from the heated area that are felt mainly as

winds. The ascending currents rise until they
reach a stratum of air of nearly the same den-
sity as their own, and then spread laterally in
all directions toward the areas where the air
has been rarefied by the movements of the lat-
eral surface currents, until they finally descend,
and recommence their motion toward the heated
area. These circulatory motions continue as long
as the heated area remains warmer than surround-
ing regions.

In speaking of winds, reference is always made to the
surface currents, unless otherwise stated.

242. Origin of the Atmospheric Circulation. —
The hottest portions of the earth are, in general,
within the tropics; hence in the equatorial regions
ascending currents continually prevail. To sup-
ply the partial vacuum so created, lateral sur-
face currents blow in toward the equator from
the poles, while the ascending currents, after
reaching a certain elevation, blow as upper. cur-
rents toward the poles. Thus result currents by
which the entire mass of the atmosphere is kept in
constant circulation, and an interchange effected
between the air of the equator and the poles.

The most important of these currents are the
following:

(1.) Polar currents, or the lateral surface cur-

 

rents, which flow from the poles to the equator;
and
(2.) Equatorial currents, or the upper currents,

‘which flow from the equator toward the poles.

It will be noticed that wherever the surface
wind blows in any given direction, the upper
wind blows in the opposite direction.

In several instances the ashes of volcanoes have been
carried great distances in directions opposite to that in which
the surface wind was blowing. The smoke from tall chim-
neys at first takes the direction of the surface wind, but
rising, is soon carried in the opposite direction by the
upper currents. The clouds are often seen moving in a
direction opposite to that indicated by vanes placed on
the tops of the houses.

A current of air is named according to the di-
rection from which tt comes; a current of water,
according to the direction in which it is going.
Thus, a north-east wind comes from the north-
east; a north-east current of water goes toward
the north-east.

243, Effect of the Earth’s Rotation on the
Direction of the Wind.—Were the earth at rest,
the equatorial and polar currents would blow due
north and south in each hemisphere; but by the
rotation of the earth they are turned out of their
course in a manner similar to the oceanic currents
already studied.

The polar currents, as they approach the equa-
tor, where the. axial velocity toward the east is
greater, are left behind by the more rapidly moy-
ing earth, and thus come, as shown in Fig. 83,
from the north-east in the northern hemisphere,
and from the south-east in the southern.

The equatorial currents, under the influence of
the earth’s eastward motion, are carried toward
the east as they approach the poles, and thus
come, as shown in Fig. 83, from the south-west in
the northern hemisphere, and from the north-west
in the southern.

Wherever the polar winds prevail, their direc-
tion, when unaffected by local disturbances, will
be north-east in the northern hemisphere, and south-
east in the southern. Near the equator their di-
rection is nearly due east.

Wherever the equatorial currents prevail, their
direction will be south-west in the northern hemi-
sphere, and north-west in the southern.

In Fig. 82, the equatorial currents are repre-
sented as continuing to either pole as upper cur-
rents, and the polar winds as surface currents to
the equator. If this were so, constant north-east-
erly winds would prevail in the northern hemi-