A destructive, rotating storm under a funnel-shaped cloud which advances over the land along a narrow path. The tangential speed of the whirling air may exceed 100 m s⁻¹, the core may perhaps be 200 m across, the duration of the storm about 20 minutes. Within a tornado, the central pressure is around 100 mb below that of the exterior; this may cause buildings to explode. This type of storm is very common in ‘tornado alley’, extending from northern Texas through Oklahoma, Kansas, and Missouri, with as many as 300 tornadoes a year. The exact mechanism of its formation is not fully understood, but tornadoes are associated with intense local heating coupled with the meeting of warm, moist air from the Gulf of Mexico and cold air from the basin and range area of the western United States. See H. B. Bluestein (2006).

http://www.spc.noaa.gov/faq/tornado/ef-scale.html The enhanced Fujita scale, which uses 28 tornado damage indicators.