A spiral of fast wind, a tornado can shatter anything in its
path, as it cuts across the open landscape. Although only a
few hundred feet in diameter, and lasting no more than an
hour or two, its thin trial of destruction can run for miles.
(Natural Disasters Readers Digest, 1996)
A tornado is a violent whirling wind, accompanied by a funnel-shaped cloud extending down from a cumulonimbus cloud. Commonly known as a twister, a tornado has an average width of a few hundred metres, but can be anywhere from a few metres to a kilometre wide where it touches the ground. It can move over land for distances ranging from short hops to many kilometres, causing great damage wherever it descends.
Tornadoes form when two masses of different temperatures and humidity meet. If the lower layers of the atmosphere are unstable, a strong upward movement of warmer air is formed. This starts to spiral as it rises, and intensifies. Only a small percentage of these systems develop into the narrow, violent funnels of tornadoes.
Tornadoes are probably the most spectacular type of storm, but are certainly one of the most violent. Fortunately, however, they are restricted in their range. Although they occur in some form on all continents, it is the U.S.A. where they most frequently cause disaster. Tornadoes usually form during cloudy, stormy weather and descend from severe thunderstorms as rapidly-spinning white funnels of cloud.
A tornado descended from a cumulonimbus cloud
Tornadoes in the U.S.A. form most frequently during the 'tornado season' in early Spring. The season shifts during later months with increasing latitude. The number of funnels observed each year can vary greatly in any given region. Texas and the southern states of the U.S.A. are visited by hundreds of tornadoes a year. Because they are so common in this area, the midwestern and south central U.S.A. has been nicknamed 'tornado alley'.
Missouri, Illinios, Indiana
Worst in New England
37 Tornadoes in one day
Warning and Evacuation worked
Alabama to Ontario
The Super-Outbreak-148 in 21 hours