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Tornadoes occur in the United States more often than
anywhere else in the world. The U.S. averages about 100 tornadoes per year.
Tornadoes are most likely to occur over the southern and central Plains, southern
Mississippi Valley and upper Midwest. Texas leads the nation in the number of
tornodoes, and in the number of injuries and fatalities associated with tornadoes. |
The tornado season tends to begin in the Gulf Coast states during the late winter and early spring, and migrates northward to the Great Lakes and Canadian border by late June and July. Many of the tornadoes that affect the central and eastern Gulf Coast during the late fall and early spring originate over the northern Gulf of Mexico as intense waterspouts.
Most tornadoes form within an particularly intense weather system known as a supercell. Supercell thunderstorms occur when the warm air pushes up through an overlying, stable layer and continues upward into a zone of cool, dry air. The resulting instabilities produce powerful vortex motions that in turn create tornadoes. Within the most powerful tornadoes, wind speeds can approach 300 miles per hour. Air rushing in to fill the low-pressure void left by the tornado creates additional fierce, damaging winds.
F-Scale |
Windspeed (mph) |
|
F-0 |
40-72 mph |
chimney damage, tree branches broken |
F-1 |
73-112 mph |
mobile homes pushed off foundation or overturned |
F-2 |
113-157 mph |
considerable damage, mobile homes demolished, trees
uprooted |
F-3 |
158-205 mph |
roofs and walls torn down, trains overturned, cars
thrown |
F-4 |
207-260 mph |
well-constructed walls leveled |
F-5 |
261-318 mph |
homes lifted off foundation and carried considerable
distances, autos thrown as far as 100 meters |
Tornadoes are not the leading cause of severe storm fatalities. The greatest average yearly threat is posed by floods and flash floods, followed by lightning and tornadoes.
| No of Deaths | In mobile homes | No of Injuries | Estimated damage | |
| 1985 | 94 | 30% | ||
| 1986 | 15 | 47% | ||
| 1987 | 59 | 41% | ||
| 1989 | 50 | 24% | 2490 | $1088M |
| 1990 | 53 | 13% | 1150 | $668M |
| 1991 | 39 | 51% | 854 | $798M |
| 1992 | 39 | 51% | 1300 | $764M |
| 1993 | 33 | 39% | 3990 | $368M |
| 1994 | 69 | 38% | 1067 | $518M |
| 1995 | 30 | 647 | $408M |
