A tornado is a violently 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. They are often referred to as twisters or
cyclones. Tornadoes come in many shapes and sizes, but they 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 about 250 feet (76 m)
across, and travel a few miles (several kilometers) before dissipating. The
most extreme tornadoes can attain wind speeds of more than 300 miles per hour
(483 km/h), stretch more than two miles (3.2 km) across, and stay on the ground
for dozens of miles (more than 100 km).
The vast majority of tornadoes occur in the Tornado Alley
region of the United States, although they can occur nearly anywhere in North
America, including in New York.
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 or debris balls, as well as by the efforts of storm
spotters. There are several 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.
Terms to Know
Tornado Watch - Conditions are conducive to the development
of tornadoes in and close to the watch area. A Tornado Watch includes the
large hail and damaging wind threats, as well as the possibility of multiple
tornadoes. Typical watches cover about 25,000 square miles, or about half the
size of Iowa. Click here to view a video clip on What is a Watch?
Tornado Warning - A tornado has been sighted by spotters or
indicated on radar and is occurring or imminent in the warning area.