In some parts of the Earth extensive tracts exist where rain is never
known to fall, and if at all only at intervals, and then in small
quantities. The rainless districts of the New World include the flat
territories of northern Chili and Peru, some parts of Mexico, and some
parts of California. In the Old World an extensive rainless band extends
from the western shores of Africa to the central regions of Asia,
including the Great Sahara Desert, Egypt, part of Arabia, and the Desert
of Gobi. Countries so circumstanced, unless like Egypt rendered fertile
by the irrigation of a great river, constitute the most arid and
desolate regions of the earth.
The quantity of rain which falls in any region depends greatly on local
causes, such as the variations of the surface, the prevailing winds or
the proximity of the ocean. Rain is usually more copiously deposited in
mountains and well-wooded islands than in any other description of
surface.
In tropical regions the rains follow the sun, i. e., when the sun is
north of the equator, the rains prevail in the northern tropic, and when
south of that line in the southern tropic. This forms the rainy and dry
seasons to which countries so situated are subject. This does not,
however, apply to the whole intertropical regions, for in a zone
extending from the fifth to the tenth parallels on each side of the
equator there are two rainy and two dry seasons.
In the narrow belt called the variables, between the regions of the
north and south trade-winds, rain is almost incessant, accompanied by
thunder and lightning. In many parts of the intertropical regions during
the rainy season the rain pours down in such torrents that a larger
quantity falls in a few hours than in a whole month in temperate North
America.
[Illustration: =TRAVELERS GROUPED ON THE SANDS OF THE SAHARA, TERRORIZED
BY AN APPROACHING SIMOON=
The dreaded Simoon of the desert is a whirlwind of terrific force that
raises great gyrating clouds of sand, and sweeps forward with
suffocating effect upon both man and beasts. It frequently darkens the
sky at midday, and sometimes lightning accompanies it caused by the
friction of the sand and air, though no rain falls. The Simoon seldom
lasts more than twenty minutes.]