PRECIPITATION OF MOISTURE.

105

 

276. Non-Periodical Rain Zones.

The Zones of the Variable Winds.—In these
zones rain may occur at any season of the year,
and at any hour of the day or night. Here it
is the equatorial currents which bring the rain.
These regions are sometimes called the zones of
perennial rains, or of constant precipitation. In
the greater part of these zones, the equatorial
currents are more frequent in summer than in
winter. The rainfali is, therefore, greatest dur-
ing summer.

Rainfall in the Zone of the Polar Winds—In

these zones the winters are dry, because the dry,
cold polar currents then prevail; but during the
summer the equatorial currents sometimes pre-
vail, and bring with them dense clouds and fogs,
accompanied by drizzling rains. The snows occur
mainly in spring and autumn.
‘~ 277, Quantity of Rain—The quantity of rain
which falls in a given time on any area is deter-
mined by means of an instrument called the rain-
gauge or pluviometer.

The rain-gauge is generally constructed in the form of
a cylindrical vessel with a horizontal base, surmounted
by a funnel-shaped top. A vertical glass tube communi-
cates with the bottom of the vessel from the outside,
and allows the water to mount in it to the same height
as that in the inside. The rain-gauge is placed in an ex-
posed position, where it is free from eddies or whirls.
If, during any given time, the water in the instrument
is one inch deep, then during that time the rainfall over
the area equals one inch. In speaking of the rainfall of
a country, the moisture which may fall as snow is always
included.

An inch of rain over a surface a square yard
in area equals in weight 46 pounds: on the sur-
face of an acre, it is nearly equal in weight to
100 tons.

The annual rainfall is distributed, as regards
quantity, as follows:

Irrespective of the elevations of the surface,
more rain falls in the tropies than in the temperate
regions, and more in the temperate than in the
polar regions. The quantity thus decreases with
moderate regularity from the equator toward the

poles. This is caused by a similar decrease in the
quantities of heat and evaporation.
While the amount of rain that falls decreases from the

equator to the poles, the number of cloudy or rainy days
increases, being greater at the poles than at the equator.

More rain falls on the coasts of a continent than
in the interior, since near the ocean the winds are
moister. That coast of a continent which first
receives the prevailing. wind has the greatest
rainfall.

More rain falls in the Northern Hemisphere than
in the Southern. This is due to the greater extent
of the land-area of the Northern Hemisphere.

Mountains receive a heavier rainfall than the
plains below, because the moist winds, in order
to cross the mountains, are forced to ascend their
slopes and thus pass into a colder region of the
atmosphere. Therefore, the sources of rivers are
generally found in mountainous districts. Moun-
tains are among the most important causes of rain.

When the mountains are high, the winds may
reach the opposite slopes dry and vaporless. The
tropical Andes of South America afford an excel-
lent example of this.

Plateaus, though higher than plains, receive, as
a rule, less rain, because they are generally sur-
rounded by mountain chains, which rob the winds
of their moisture. Moreover, the air over a pla-
teau is warmer than at a corresponding height in
the atmosphere, and therefore dissolves, rather
than condenses, the moisture.

The rainfall of the New World, both in the
tropical and temperate regions, ts greater than
that of the Old; thus, in the tropics of the New
World, 115 inches of rain fall yearly, while the
same portions of the Old World receive but 77
inches.. In the temperate zones in America the
annual rainfall is 39 inches, while in Europe it
is but 84 inches.

The mean annual rainfall at Philadelphia, according to
Prof. Kirkpatrick, is 46.93 inches. The figures are based
on observations during 16 consecutive years.

The preceding principles find ample illustration in the
following tables:

TABLE OF ANNUAL RAINFALL (H. K, Johnston).
Rainfall in the Tropics.

OLD WORLD.
Inches.

Hindostan, mean of t!
Sierra Leone, Guinea....
Macao, China

NEW WORLD.

San Luis de Mararhio, Brazil

Cayenne, Guiana
Paramaribo, Guiana,
Grenada, Lesser Antilles.. .
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