Tornadoes

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.  (See Tornado Numbers, Deaths, Injuries and Damage).

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.

Fujita - Pearson Tornado Scale

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.

U.S. Tornado Impact Statistics

	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

MORE TORNADO INFORMATION

• FEMA Tornado Safety page.
• NOAA-Tornados Page
• Weather Channel tornado page.
• Fujita Tornado Scale from the Tornado Project.
• National Climatic Data Center
• Storm [Tornado] Chaser Home Page
• Storm Chasing - Yahoo!
• Tornado Project Online
• Tornado Myths discussed by Tom Grazulis.
• A page dedicated to the 50th Anniversary of Tornado Forecasting.
• State by state listings of tornadoes from the Tornado Project.
• Weather Resource, The - over 320 links organized into 12 categories!

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Last modified: 05/08/03