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Tornado,
violently rotating column of air extending from within a
thundercloud down to ground level. The strongest tornadoes
may sweep houses from their foundations, destroy brick
buildings, toss cars and school buses through the air, and
even lift railroad cars from their tracks. Tornadoes vary
in diameter from tens of meters to nearly 2 km (1 mi),
with an average diameter of about 50 m (160 ft). Most
tornadoes in the northern hemisphere create winds that
blow counterclockwise around a center of extremely low
atmospheric pressure. In the southern hemisphere the winds
generally blow clockwise. Peak wind speeds can range from
near 120 km/h (75 mph) to almost 500 km/h (300 mph). The
forward motion of a tornado can range from a near
standstill to almost 110 km/h (70 mph).
A tornado becomes
visible when a condensation funnel made of water vapor (a
funnel cloud) forms in extreme low pressures, or when the
tornado lofts dust, dirt, and debris upward from the
ground
. A mature tornado may
be columnar or tilted, narrow or broad—sometimes so
broad that it appears as if the parent thundercloud itself
had descended to ground level. Some tornadoes resemble a
swaying elephant's trunk. Others, especially very violent
ones, may break into several intense suction vortices—intense
swirling masses of air—each of which rotates near the
parent tornado. A suction vortex may be only a few meters
in diameter, and thus can destroy one house while leaving
a neighboring house relatively unscathed.
Scientists
study tornadoes to gain a better understanding of their
formation, behavior, and structure. Scientists who study
tornadoes have a variety of powerful research tools at
their disposal. Advances in computer technology make it
possible to simulate the thunderstorms that spawn
tornadoes using computer models running on desktop
computers. Doppler radars, which detect the rain in
clouds, allow meteorologists, scientists who study
weather, to "see" the winds inside the storms
that spawn tornadoes. Modern video camera footage and
reports from trained storm-spotters provide an
unprecedented amount of high-quality tornado
documentation. These tools all contribute greatly to the
scientific understanding of tornadoes. This information
may eventually lead to increased tornado warning times,
better guidelines for building construction (especially
schools), and improved safety tips.
Many tornadoes,
including the strongest ones, develop from a special type
of thunderstorm known as a supercell. A supercell
is a long-lived, rotating thunderstorm 10 to 16 km (6 to
10 mi) in diameter that may last several hours, travel
hundreds of miles, and produce several tornadoes.
Supercell tornadoes are often produced in sequence, so
that what appears to be a very long damage path from one
tornado may actually be the result of a new tornado that
forms in the area where the previous tornado died.
Sometimes, tornado outbreaks occur, and swarms of
supercell storms may occur. Each supercell may spawn a
tornado or a sequence of tornadoes.
This
information has brought it to you from Encarta 2002
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