Tornado means to turn in Latin. A tornado is a violent whirling wind, characteristically
accompanied by a funnel-shaped cloud extending down from a cumulonimbus cloud. Commonly known as
a twister or cyclone, a tornado can be a few meters to about a kilometer wide where it touches the ground,
with an average width of a few hundred meters. It can move over land for distances ranging from short
hops to many kilometers, causing great damage wherever it descends. The funnel is made visible by the
dust sucked up and by condensation of water droplets in the center of the funnel. The same condensation
process makes visible the generally weaker sea-going tornadoes, called waterspouts, that occur most
frequently in tropic waters. Most tornadoes spin counterclockwise in the northern hemisphere and
clockwise in the southern, but occasional tornadoes reverse this behavior.
The exact mechanisms that cause a tornado to form are still not fully understood, but the funnels
are always associated with violent motions in the atmosphere, including strong updrafts and the passage of
fronts. They develop within low-pressure areas of high winds; the speed of the funnel winds themselves is
often placed at more than 480 km/hr (more than 300 mph), although speeds of more than 800 km/hr (500
mph) have been estimated for extremely strong storms. Damage to property hit by a tornado results both
from these winds and from the extremely reduced pressure in the center of the funnel, which causes
structures to explode when they are not sufficiently ventilated to adjust rapidly to the pressure difference.
The pressure reduction is in keeping with Bernoulli's principle, which states that pressure is reduced as
velocity increases.
Tornadoes are most common and strongest in temperate latitudes, and in the U.S. they tend to
form most frequently in the early spring; the Atornado [email protected] shifts toward later months with increasing
latitude. The number of funnels observed each year can vary greatly in any given region.
A cyclone is an area of low atmospheric pressure surrounded by a wind system blowing, in the
northern hemisphere, in a counterclockwise direction. A corresponding high-pressure area with clockwise
winds is known as an anticyclone. In the southern hemisphere these wind directions are reversed. Cyclones
are commonly called lows and anticyclones highs. The term cyclone has often been more loosely applied to
a storm and disturbance attending such pressure systems, particularly the violent tropical hurricane and the
typhoon, which center on areas of unusually low pressure.


1. "Tornado," Microsoft (R) Encarta. Copyright (c) 1994 Microsoft Corporation. Copyright (c) 1994 Funk
& Wagnall's Corporation.

2. "Cyclone," Microsoft (R) Encarta. Copyright (c) 1994 Microsoft Corporation. Copyright (c) 1994 Funk
& Wagnall's Corporation.

3. Tornado, New Standard Encyclopedia, Standard Education Corporation, Chicago, Ill., 1992.