THE WINDS. : 93

Zones of the Trades.—From the limits of the
zone of calms to about 30° on each side of the
equator the polar currents blow with great steadi-
ness throughout the year. The constancy in their
direction has caused these winds to be named
“trade winds,” from their great value to com-
merce. Their direction is north-east in the north-
ern hemisphere, and south-east in the southern.

Zones of the Calms of Cancer and Capricorn.
—Between the zones of the trades and the vari-
ables, where the interchange takes place between
the equatorial and polar currents, zones of calms
occur. Their boundaries are not well defined,
and. are dependent on the position of the sun.

Zones of the Variable Winds—Beyond the
limits of the preceding zones to near the latitude
of the polar circles, the equatorial and polar cur-
rents alternately prevail. Here the equatorial
and polar currents are continually striving for
the mastery, sometimes one and sometimes the
other becoming the: surface current. During
these conflicts the wind may blow from any
quarter; but when either current is once estab-
lished it often continues constant for some days.
This is especially the case over the ocean, where
the modifying influences are less marked.

Though the winds in these zones are variable,
still two directions predominate: south-west and
north-east in the northern hemisphere, and north-
west and south-east in the southern. Westerly
winds, however, occur the most frequently in
nearly all parts of these zones.

The equatorial currents are sometinfes called the Return
Trades, or the Anti-trades, because they blow in the oppo-
site direction to the trades.

Between about lat. 25° and 40°, N. and S., over parts of
the ocean, the winds are nearly periodical, blowing during
the hotter portions-of the year in each hemisphere from
the poles, and during the remainder of the year from the
equator. This zone is often called the Zone of the Sub-
tropical winds.

Polar Zones.—From the limits of the zones of
the variables to the poles, there are regions of pre-
vailing polar winds. These winds are most fre-
quently north-east in the northern hemisphere,
and south-east in the southern.

247. Dove’s Law of the Rotation of the Winds.—
The equatorial and polar currents usually displace each
other, and become surface winds in a regular order, first
discovered by Prof. Dove of Berlin.

In the northern hemisphere, before the polar current is
permanently established from the north-east, the wind
blows in regular order from the west, north-west, and north.
.The displacement of the polar by the equatorial currents
occurs in the opposite direction: from the east, south-east,

 

and south, before the general south-west current is perma-
nently established.

In the southern hemisphere these motions are reversed.

This rotation of the winds, together with the effects
produced on the thermometer and barometer, is indicated
in the following diagram. Since the equatorial currents
are warm, moist, and light, when they prevail the ther-
mometer rises and the barometer falls. On the establish-
ment of the polar currents, however, the thermometer
falls and the barometer rises.

NORTHERN HEMISPHERE,

SOUTHERN HEMISPHERE.

 

S. S.
Fig. 84. Rotation of the Winds (after Dove),

The “warm waves” of the zones of the variable
winds are caused by the prevalence of the equa-
torial currents. Similarly, the “cold waves” are
caused by the prevalence of the polar currents.

248. Land and Sea Breezes——During the day
the land near the coast becomes warmer than the
sea. An ascending current, therefore, rises over
the land, and a breeze, called the sea breeze, sets
in from the sea. At night the land, from its
more rapid cooling, soon becomes colder than the
water; the ascending current then rises from the

 

 

water, and a breeze, called the land breeze, sets in
from the land. The strength of these winds de-
pends upon the difference in the temperature of
the land and water; they are, therefore, best de-
fined in the tropical and extra-tropical regions,
though they may occur in higher latitudes during
the hottest parts of the year. Land and sea
breezes are periodical winds.

, 249. Monsoons are periodical winds, which dur-
ing part of the year blow with great regularity in
one direction, and during the remainder of the