Tornadoes are caused by the air becoming abnormally heated over certain
areas. Likewise, caused by a difference in pressure. Tornadoes are local
whirlwinds of great energy, generally formed within thunder storms. They
are most easily distinguished by a funnel-shaped cloud that hangs from
the bottom of the larger thunder cloud mass above it. The funnel is
formed around a violent ascending mass of whirling winds; its diameter
sometimes reaching several hundred feet, being larger above than below,
the winds themselves covering a greater space.
[Illustration:
Fig. 12
]
The whirling funnel advances generally to the east or northeast at a
rate of twenty to forty miles an hour, accompanied by a deafening noise,
destroying everything in its path. The path is usually less than a
quarter of a mile in width.
The winds in the vortex (the apparent cavity or vacuum formed in the
center of the whirling winds) of the tornado attain an incredible
violence, and due to this fact houses are shattered, trees uprooted, and
human lives lost, besides other devastation of property and animal life.
It is, therefore, the vorticular whirl that causes the destruction
produced by tornadoes.
Tornadoes are more frequent in the southern states than anywhere else in
the country, and occur in the warmer months.
The velocity of the whirling winds in a tornado increase towards the
center, and it is because of this that the point of danger is only a
small distance from the funnel cloud. The direction of the whirling
motion is from right to left. From the appearance of the funnel formed
in a tornado, it looks as though the currents were descending from the
cloud to the earth, when in reality the currents are ascending. The
ascending current draws on the warm and moist air near the surface of
the earth for its supply, and this inrush of air in a spiral form into
the low pressure core made by the higher whirl constitutes the
destructive blast of the tornado.
Tornadoes approach rapidly, and it is therefore almost impossible for
those who happen to be in their path to escape their violence.
A tornado at sea is termed a water spout.