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Tornado Info:

A tornado is a violent windstorm characterized by a twisting, funnel-shaped cloud. The word "tornado" comes from the Spanish verb "tornar", meaning "to turn."

It is spawned by a thunderstorm and produced when cool air overrides a layer of warm air, forcing the warm air to rise rapidly. Many tornadoes are the tail end of a mesocyclone and they have a characteristic "hook echo" signature on a radar screen. The damage from a tornado is a result of the high wind velocity and wind-blown debris. Tornado winds range from a slow 40 mi/h (65 km/h) at the low end to a possible 300 mi/h (480 km/h) in the strongest storms. Tornado season in North America is generally March through August, although tornadoes can occur at any time of year. They tend to occur in the afternoons and evenings: over 80 percent of all tornadoes strike between noon and midnight.

The intensity of tornadoes is given by the Fujita - Pearson Tornado Scale (also known simply as Fujita scale). The intensity can be derived directly with high resolution Doppler radar wind speed data, or empirically derived from structural damage compared to engineering data. Also, note that intensity does not refer in any way to the size, or width, of a tornado.