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Tornadoes
Nature Bulletin No. 349-A September 13, 1969
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation
TORNADOES
A tornado is the smallest but most vicious and destructive of all
storms. Tropical cyclones, called hurricanes in the Atlantic region and
typhoons in the Western Pacific, are mild in comparison. A hurricane
may be from 50 to 1000 miles across, travel thousands of miles during
its life -- and cause enormous damage, but the fierce winds spiraling
toward its center, or "eye", rarely exceed 150 miles per hour. No
instrument has been able to measure the violently spinning winds near
the center of a tornado cloud but they may reach velocities estimated to
be in excess of 550 mph. It shrieks, howls and roars as its "foot" races
across country, sometimes in hopscotch fashion, like an unpredictable
evil demon.
A tornado is a whirling mass of air that is started -- not at the ground
level as in the case of a "dust devil" -- but by powerful cross currents
in the storm cloud level some thousands of feet above. Such turbulent
uplifts occur most frequently in the spring months when cold masses
of air from the north encounter warm masses from the south on sunny
days when the air aloft is markedly colder than the earth-warmed air.
The funnel-shaped cloud so formed may extend only slightly below the
greenish black "mother cloud" and never touch the earth but, too often,
the rotating air acts on the air below it and sets up a spinning column
that extends down, down, down until it reaches the ground.
Sometimes the tapering column appears to dangle like an elephant's
trunk; sometimes it hangs in a loose arch, like a great thick rope.
Those whirling winds gyrate so violently -- always counterclockwise in
the northern hemisphere -- that they greatly reduce the air pressure in
the center. This abrupt reduction causes the air to expand and cool,
forming the visible cloud. It is this vacuum-like center which sucks up
roofs, causes the walls of buildings to burst outward and creates a
boiling cloud of dust and debris. Objects as heavy as an anvil, an
automobile or even a locomotive, may be carried away.
The annual number varies from 65 in 1919, to 262 in 1949. They have
been reported during every month but are rare in fall and winter. May
is the peak month, especially in the Great Plains and Midwestern
states. Most tornadoes of early spring occur in the Gulf coastal states,
and in autumn in the Atlantic coastal states are hardest hit. Except in
the south, where the majority occur between midnight and noon, most
tornadoes strike between 3 and 7 p.m.
The average width of a tornado's path is about 1200 feet, varying from
9 feet to 8 miles. It may touch the ground only briefly, or it may persist
for several hours and its path of destruction be very wide. They usually
move overland at a speed of about 40 miles per hour but sometimes
one may stand stationary for several minutes, meanwhile whirling at
tremendous speed, and then go on, or turn off in a new direction, or
even make a wide looping swing. It may come from any direction but
about seven-eighths of them come from the west -- usually the
southwest. The terrible Tri-State Tornado of March 18,1925, traveled
at 68 mph for 219 miles across Missouri, southern Illinois and
Indiana, killing 606 people, and injuring 2000 others, in Illinois alone.
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Update: June 2012
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