HAIL, SNOW, AND GLACIERS. 107

 

 

ern slopes, the winds thus arriving dry and va-
porless at the western.

Cause of Deserts—Deserts are caused entirely
by the absence of moisture. Their soil, though gen-
erally finely pulverized, or sand-like, does not dif-
fer, save in the absence of vegetable mould, from
that of other areas. Thus neither the nature of
its temperature, nor its soil, is the cause of the
desert of Sahara, since a vigorous vegetation ai-
ways follows the appearance of water, on the suc-
cessful boring of an artesian well. It is probably
true that deserts, once formed, tend to perpetuate
themselves, by the influence their naked surfaces
exert on the rainfall.

—00$f0-0——__

CHAP TER il:

Hail, Snow, and Glaciers.

279. Hail falls when considerable differences
of temperature exist between higher and lower
strata of air, and the moisture is suddenly con-
densed in the presence of great cold. . Generally,
several layers or bands of dark, grayish clouds
are seen. Hail falls most frequently in summer,
near the close of an excessively warm day.

Structure of the Hailstone.—If a large hailstone be
placed on a hot surface until one-half is melted, the struc-
ture can be readily examined. Concentric layers, similar
to those of an onion, will be noticed, arranged around a
central nucleus, sometimes of ice and sometimes of snow,
though generally the latter. The stones are more or less
oblately spheroidal in shape. Their general weight varies
from a few grains to several ounces, but they have been
known to weigh several pounds.

 

Fig, 94. Structure of a Hailstone,

Origin of Hail—The cause of hail is not ex-
actly understood, and several-theories have been
framed to account for it. One of these is the
Rotary Theory.

The wind is supposed to rotate as in a cyclone, only the
axis of the whirl is horizontal instead of vertical. Two
horizontal layers of cloud exist—the upper layer of snow,
the lower, of rain. The snowflakes, which form the nu-
clei of the hailstones, are caught in the whirl, and dipped
13

 

in rapid succession into the two clouds, thus receiving al-
ternate coatings of ice and snow, until] at last they are
hurled to the ground.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

    
  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 95. Rotary Theory of Hail,

Thunder and lightning are the invariable attendants of
hailstorms, and some authorities have attributed the for-
mation of the stones to successive electrical attractions
and repulsions of the snowflakes between a snow and a
rain cloud. Others have imagined a number of alternate
layers of snow and rain, and have attributed the hail-
stones to drops of rain falling through the successive
clouds.

280. Snow.—When the moisture of the air is
condensed at any temperature below 32° Fahr.,
the vapor crystallizes, and snowflakes are formed.

The snowflakes grow, as they fall, by condensing addi-
tional moisture from the air. They are larger in mild
than in cold weather.

Snow-crystals assume quite a variety of forms, but are
built up by various groupings of minute rhombohedrons
of ice. The star-shape is the most common.

 

Fig. 96, Snow-Crystals,

If the temperature of the air near the surface
is much warmer than 32° Fahr., any snow that
is formed in the upper regions will melt before
reaching the ground. Hence, in the temperate
zones, as a rule, snow falls only in winter, while
in the tropics it never falls, except near the sum-
mits of lofty mountains.

It is a mistake to suppose that the fall of snow is greater
in regions near the poles than elsewhere; for in high lati-

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