104. PHYSICAL GEOGRAPHY.

 

the drops fall through the cloud they grow larger
by the addition of other drops which unite with
them. Raindrops, therefore, are larger when the
clouds are thicker. They are, in general, larger
in the tropics than in the polar regions, and dur-
ing the day than at night.

To produce rain, it is necessary that the tem-
perature of a large mass of air be reduced con-
siderably below its dew point. There are several
ways in which this cooling may be effected:

(1.) By a change of latitude. A warm, moist-
ure-laden wind may blow into a cold region. The
equatorial currents of air deposit their moisture

in the temperate and polar zones on account of
the chilling experienced as they recede from the
equator.

(2.) By a change of altitude. By an ascending
current of air, which carries the moisture of the
lower strata into the upper regions, where the
cold there existing, together with that produced
by the rapid expansion of both air and vapor
under the diminished pressure, condenses the moist-
ure of the air. It is mainly in this manner that
the rains of the tropical regions are caused.

The rain in mountainous districts has a similar
cause. A moist wind, reaching a mountain-range,
is forced by the wind back of it to ascend the
slopes. Contact with the cold, upper slopes causes
condensation of the vapor as rain.

(3.) The mingling of masses of cold and warm
air. By this means heavy clouds and a moderate
rainfall may be produced; but the precipitation
can never be considerable, because the cooler air
will be warmed by the mixing, and, therefore,
will have its capacity for moisture increased in-
stead of diminished.

272. Distribution of the Rainfall—The dis-
tribution of rain may be considered both as re-
gards its periodicity and its quantity. The distri-
bution of the rain is dependent upon the direction
of the winds. Each wind zone has a character-
istic rainfall.

The following simple principles determine the
rainfall in any particular wind zone:

(1.) The equatorial currents are rain-bearing,
because they are moist, and while on their way
to the poles, their temperature and consequent
capacity for moisture, is constantly decreasing.

(2.) The polar currents are dry, because they
are constantly increasing in temperature as they
approach the equator; hence they take in, rather
than give out, moisture.

 

 

When they have reached the zones of the trade winds,
the polar currents may bring abundant rains, provided
they have previously crossed an ocean. They then dis-
charge the moisture with which they are saturated, either
by an ascending current, or by blowing against the ele-
vations of the continent.

273. Periodical Rain Zones.

The Zone of Calms.—In the zone of calms it
rains nearly every day. In the early morning
the sky is cloudless; but near the middle of the
day, as the heat increases, the ascending currents,
rising higher, begin to condense their moisture ;
cumuli clouds form, and, increasing rapidly, soon
cover the sky, when torrents of rain descend, ac-
companied by thunder and lightning. After a
few hours the rain ceases, and the sky again be-
comes clear. In this zone it seldom rains at
night.

274, The Zone of the Trades.—Since the trades
are generally dry winds, it is only when their tem-
perature is considerably decreased that they can
cause rain. In the zone of the trades, except in
mountainous districts and on the windward coasts
of a continent, the rainfall occurs during the
greatest heat of the season, when the sun is di-
rectly overhead and the ascending currents are
powerful. Hence, it rains during a few months
in summer, when immense quantities of water
fall; the remainder of the year is dry. Copious
dews, however, occur at. night.

The precipitation is not continuous throughout the en-
tire summer. Since the rain only falls when the sun is
nearly overhead, a brief interval of dry weather occurs
in regions near the equator, thus dividing the season into
two parts: one, during the passage of the sun over the
zenith; the other, on his return to the zenith from the
adjacent tropic. Near the limits of the zone, however, the
two seasons are merged into one.

Over the ocean, during most of the year, there
is no rain in the zone cf the trades, although the
actual humidity of the air is quite high.

Between latitude 24° and 30°, in both the Northern and
Southern Hemispheres, there are regions of comparatively
scanty rains. Here the summers are not hot enough to
cause rain by the ascending currents, but are sufficiently
hot to prevent the equatorial current from bringing much

rain. Here also the return branch of the equatorial cur-
rent becomes drier on its return to the equator.

275. The Monsoon Region of the Indian Ocean.-~
During the prevalence of the winter monsoon, the north-
east winds bathe the eastern shores of Hindostan in copious
rains, while the western shores, shielded by the ranges of
the Ghauts, are dry. During the summer monsoon, the
south-west winds bathe the western shores and the south-
ern slopes of the Himalayas in heavy rains, while the
eastern shores are dry. This monsoon also brings rains to
the western coasts of the peninsula of Indo-China.

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