A severe thunderstorm may spawn a tornado, a
violently rotating column of air which descends from a thunderstorm
cloud system. The winds of a tornado are the most violent winds that
occur on the Earth, reaching speeds of up to 300 mph. It extends down
from a mass of dark clouds as a whirling funnel and moves over land in a
narrow path. On the average, tornadoes move about 30 miles an hour,
however, some move very slowly while other speed along at 60 miles an
hour or more. Tornadoes generally rotate counterclockwise in the
northern hemisphere, and clockwise in the southern hemisphere.
Tornadoes usually last less than an hour,
traveling distances of about 20 miles. Tornadoes occur throughout the
world, but mostly in the central United States in what is called
"Tornado Alley" during the spring and summer.