TORNADO
A violently rotating column of air in contact with and extending between a convective
cloud and the surface of the earth. It is the most destructive of all storm-scale
atmospheric phenomena. They can occur anywhere in the world given the right conditions,
but are most frequent in the United States in an area bounded by the Rockies on the west
and the Appalachians in the east.
The months with the greatest number of tornadoes overall are April, May and June, but
tornadoes can and do occur during any
month of the year. Tornado seasons vary in different parts of the United States.
In the Southeast, the peak season for tornadoes is February through April. In the northern
Plains, tornadoes are most likely to
develop from June through August.
Generally, tornado frequency is high in the South in late winter and early spring; and in
the Plains, Midwest and Ohio Valley from early spring through summer.
The reason for this is that low-level heat, moisture, and instability necessary for
tornado formation are usually confined to southern regions early in the year, and only
reach northern sections with regularity late in the spring.
Also, the jet stream (associated with the upper-level weather systems that can contribute
to thunderstorm and tornado formation) migrates northward from winter to summer.
A secondary season of tornado activity has been observed across the South in late autumn
as the jet stream, located well to the north in the summer, migrates back southward to its
typical wintertime position.
Tornadoes can occur at any time of day, but are more likely to occur from 2:00 PM to 7:00
PM, with the maximum number forming around 5:00 PM. In Tornado Alley, very few tornadoes
occur in the morning, while in the Southeast, a significant percentage occur during the
night and morning hours.
Tornadoes can vary greatly in terms of their length, width, direction and speed. The
median path length of a tornado is just
under one mile, while its path width averages fifty yards, half the length of a football
field.