A tornado is a violent rotating column of air extending from a thunderstorm to the ground. The most violent tornadoes are capable of tremendous destruction with wind speeds of up to 400 mph. They can destroy large architecture, uproot trees and hurl cars hundreds of yards. They can also drive straw into trees. Damage paths can be in excess of one mile wide to 50 miles long. In an average year, 1000 tornadoes are reported nationwide.
Before thunderstorms develop, a change in wind direction and an increase in wind speed with increasing height creates an invisible, horizontal spinning effect in the lower atmosphere. Rising air within the updraft tilts the rotating air from horizontal to vertical. An area of rotation, 2–6 miles wide, now extends through much of the storm. Most strong and violent tornadoes form within this area of strong rotation.
|Fujita-Pearson (FPP) tornado scale|
|Category||Maximum Wind Speed (mph)||Path Length (miles)||Path Width (yards)||Damage|
|F0||40-72||less than 1.0||less than 18||Light damage to signs, chimneys; pushing-over of shallow-rooted trees and the breaking of tree branches.|
|F1||73-112||1.0-3.1||18-55||Moderate damage; autos pushed from roads, mobile homes pushed from foundations.|
|F2||113-157||3.2-9.9||56-75||Considerable damage. Roofs torn off frame houses; mobile homes demolished; boxcars pushed over; large trees snapped or uprooted; light-object missiles generated.|
|F3||158-206||10-31||176-556||Severe damage. Roofs and some walls torn off well-constructed houses; trains overturned; most trees uprooted; vehicles lifted off ground and thrown.|
|F4||207-260||32-99||0.3-0.9 mi.||Devastating damage. Well-constructed houses leveled; structure with weak foundation blown off some distance; all vehicles thrown and large missiles generated from debris.|
|F5||261-318||100-315||1.03-3.1 mi.||Catastrophic damage. Strong-frame buildings either explode or are lifted off foundations and carried considerable distance to disintegrate; automobile-sized missiles fly through the air in excess of 100 yards; small towns within the damage path are leveled.|
Tornadoes can happen at any time of the year and at any time of the day. In the southern states, peak tornado season is from April through June. Peak times for tornadoes in the northern states are during the late summer. A few southern states have a second peak time for tornado outbreaks in the fall. Tornadoes are most likely to occur between 1 p.m. and 7 p.m.