A tornado (often referred to as a twister) is a violent, dangerous, rotating
column of air that is in contact with both the surface of the earth and a
cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. Tornadoes come in many shapes and sizes, but are typically in the form of a visible
condensation funnel, whose narrow end touches the earth and is often encircled
by a cloud of debris and dust. Most tornadoes have wind speeds less than
110 miles per hour (177 km/h), are approximately 250 feet (80 m) across, and
travel a few miles (several kilometers) before dissipating. The most extreme can
attain wind speeds of more than 300 mph (480 km/h), stretch more than two miles (3 km) across, and stay on the ground for dozens of miles (more than
Various types of tornadoes include the landspout, multiple vortex tornado, and
waterspout. Waterspouts are characterized by a spiraling funnel-shaped wind
current, connecting to a large cumulus or cumulonimbus cloud. They are
generally classified as non-supercellular tornadoes that develop over bodies of
water. These spiraling columns of air frequently develop in tropical areas
close to the equator, and are less common at high latitudes. Other tornado-
like phenomena that exist in nature include the gustnado, dust devil, fire whirls,
and steam devil.
Tornadoes have been observed on every continent except Antarctica. However,
the vast majority of tornadoes in the world occur in the Tornado Alley region of
the United States, although they can occur nearly anywhere in North America.
They also occasionally occur in south-central and eastern Asia, the Philippines,
northern and east-central South America, Southern Africa, northwestern and
southeast Europe, western and southeastern Australia, and New Zealand.
Tornadoes can be detected before or as they occur through the use of Pulse-
Doppler radar by recognizing patterns in velocity and reflectivity data, such as
hook echoes, as well as by the efforts of storm spotters.
There are several different scales for rating the strength of tornadoes. The
Fujita scale rates tornadoes by damage caused, and has been replaced in some
countries by the updated Enhanced Fujita Scale. An F0 or EF0 tornado, the
weakest category, damages trees, but not substantial structures. An F5 or EF5
tornado, the strongest category, rips buildings off their foundations and can
deform large skyscrapers. The similar TORRO scale ranges from a T0 for
extremely weak tornadoes to T11 for the most powerful known tornadoes.
Doppler radar data, photogrammetry, and ground swirl patterns (cycloidal
marks) may also be analyzed to determine intensity and assign a rating.
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