Man's physical universe

xanabras

260 PHYSICAL LIMITATIONS HAVE BEEN OVERCOME

tornado — the dreaded "twister" of the prairies — always revolves

in a counterclockwise direction in the northern hemisphere and is

formed only under special meteorological circumstances. The whirl

Fig. 81. Royal palm

pierced by a pine board,

ten feet by three inches by

one inch, in the hurricane

of September 13, 1928.

(Courtesy of the U. S.

Weather Bureau.)

originates at the general cloud level and bores

downward toward the ground; the vortex is

visible as a writhing funnel-shaped cloud. It

is the most violent of all storms; the wind

velocity has been estimated, from its effects,

to have reached 500 miles per hour in some

cases. The rapid whirling leads to a considerable

decrease in

the atmospheric pressure at

the center; and as the center passes over

buildings, they sometimes burst outward because

of the sudden decrease in external pressure.

The powerful upward-spiraling winds

often lift heavy objects high into the atmosphere

and carry them for long distances. The

force of such a wind can be judged by the

picture in which

a plank is shown

to have been driven

through a tree.

Tornadoes are

small, ranging from

a few feet to a halfmile

in diameter,

and generally travel

from the southwest

to the northeast.

One should therefore

run to the northwest to avoid an oncoming

tube.

Waterspouts are tornadoes or strong whirlwinds

that occur over water areas.

The West Indian hurricanes and the

Chinese typhoons are atmospheric whirls on

a larger scale but less intense than tornadoes,

having a diameter of from 50 to

500 miles; they are usually more destructive

Fig. 82. Water spout on

LakeZug, Switzerland, June

19, 1905. (Courtesy of the

U. S. Weather Bureau.)

because they are much more widespread and long-enduring. These

tropical hurricanes travel at an average rate of about 12 miles an hour,

but the winds within the storm approach a velocity of 100 to 150 miles

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