96 PHYSICAL GEOGRAPHY.

 

The Khamsin blows at irregular intervals over
Egypt from the south; but when established,
generally continues for fifty days. It is intensely
hot and dry, like the simoom, and is loaded with
fine sand. :

252. Mountain Winds.—During the day the
elevated slopes of mountains heat the air over
them hotter than at corresponding elevations over
the valleys. Currents, therefore, ascend the val-
leys toward the mountains during the day. During
the night, however, the air near the summits be-
comes colder than that near the base. Currents,
therefore, descend the valleys from the mountains
during the night.

—.09300——
CHAPTER bv;

Storms.

2538. Storms are violent disturbances of the
ordinary equilibrium of the atmosphere by wind,
rain, snow, hail, or thunder and lightning.

During storms the wind varies in velocity from
that of a scarcely perceptible breeze to upwards
of 200 miles per hour.

VELOCITY AND POWER OF WINDS.

 

Velocity of Wind in

Miles, per hour. Common Names of Winds.
5 3

1 Hardly Perceptible Breeze,

_ 4605 Gentle Wind.
10 to 15 Pleasant Brisk Gale.
20 to 25 Very Brisk.
30 to 35 High Wind.
40 Very High.
50 Storm.
60 Great Storm.
80 Hurricane.
100 Violent Hurricane.
80 to 200 Tornado.

 

254. Cyclones are storms of considerable ex-
. tent, in which the velocity of the wind is much
greater than usual, and the air moves in eddies or
whirls, somewhat similar to whirlwinds, but of
vastly greater power and diameter.

In all such storms the wind revolves around a
calm centre; over the calm centre the barometer
is low, but on the sides, and especially on that side
toward which the storm is moving, it is high.

Besides the rotary motion of the wind, there is
also a progressive motion, which causes the storm

to advance bodily, moving rapidly in a parabolic

path. The general term Cyclone has been ap-
plied to these storms on account of their rotary
motion. They have also various local names.

Cyclones originate in the tropical regions, but
frequently extend far into the temperate zones,

 

Fig, 87, A Storm at Sea,

255. Regions of Cyclones.—The following are
the most noted regions:

The West Indies, where they are generally .

called hurricanes.

The China Seas, where they are known as
typhoons.

The Indian Ocean.

In each of these regions the storms occur about
the time of the change of the regular winds, and
have their origin in marked differences of tem-
perature; thus in the Indian Ocean and the China
Seas, they generally occur at the change of the mon-
soon, after the great heat of summer. They are at-
tended with the condensation of moisture and in-
tense electrical disturbance.

256. Cause of Cyclones.—Cyclones originate in

an area of low barometer caused by the ascending.

current of air that follows the overheating of any
region. As the air rushes in from all sides it is
deflected by the earth’s rotation, and assumes a
rotary or whirling motion around the heated area.
The centrifugal force generated by this rotation
causes the barometric pressure of the area to be-
come lower and the area to grow larger. Mean-
while the inflowing air, ascending, is chilled by
the cold of elevation and by expansion sufficiently
to condense its vapor rapidly. The heat energy,
previously latent in the vapor, is now disengaged,
and causes the air to mount higher and condense
still more of its vapor. It is to the energy thus
rapidly liberated by the condensation of the vapor
that the violence of the cyclone is due. Cyclones,
therefore, acquire extraordinary violence only
when an abundance of vapor is present in the
air.