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A tornado is a violently rotating column of air that originates within a thunderstorm and is in contact with the ground. Tornadoes often only last a few minutes, but it is possible for tornadoes to last over an hour and travel many miles. Around 1,000 tornadoes occur in the United States each year. More tornadoes strike the central United States than any other place in the world, so this region has earned the nickname "tornado alley." The peak of tornado season occurs between April and June.

Tornadoes typically form in association with supercell thunderstorms. A supercell thunderstorm is a special type of thunderstorm that can persist for several hours due to its organized internal structure. Supercell thunderstorms are characterized by a single, rotating updraft. They form in regions of strong vertical wind shear. Vertical wind shear is the change in wind speed and/or direction with height. Directional wind shear refers to the change of wind direction with height and speed shear, the change of wind speed with height. Vertical wind shear induces a "rolling" effect in the atmosphere, similar to the diagram below. This rolling effect becomes important when a thunderstorm forms because it is tilted upward and causes a thunderstorm to rotate.

Directional Wind Shear

Speed Shear

In the diagram to the left, a rotating column of air, which was produced by the speed shear, is lifted vertically by the updraft of a developing thunderstorm. This initially induces two different rotations within the supercell; a cyclonic (counter-clockwise) rotation and an anti-cyclonic (clockwise) rotation. Directional wind shear amplifies the cyclonic rotation (left side of image) and diminishes the anticyclonic rotation (right side of image). Once this occurs, only the cyclonic rotation remains. The rotating updraft of a supercell is referred to as a mesocyclone.

Due to the counterclockwise rotation of the mesocyclone, supercells often take on a "hook" appearance when viewed on radar. This is because the rain produced by the thunderstorm is wrapped around the rotating updraft. The figure to the right is a schematic of the precipitation associated with a supercell and the area encircled in red is the location of the rotating updraft. The center of the rotating updraft is near the point where a tornado is likely to form.

The term "funnel cloud" is used to describe a region of strong rotation where the circulation has not reached the ground yet. The funnel becomes visible when water vapor begins to condense into liquid droplets. One sign that the circulation has reached the ground and has become a tornado is that dust and debris on the ground will begin to rotate. The size and/or shape of the tornado is not always a measure of its strength, although very large tornadoes are almost always quite destructive. Tornadoes will gradually lose strength and take on a rope-like appearance.

Funnel Cloud
Dissipating Tornado

Tornado Classification

Tornadoes are classified according to the damage they cause, which is related to their wind speed. The original scale, called the Fujita (F) Scale, was developed by Dr. Fujita in the 1960s. A tornado's wind speeds are estimated based on the damage caused by the storm, which is assessed after-the-fact. There are some apparent problems with the F-Scale. One problem is that it is subjective. A different assessment may be made based on who is assessing the damage. Another problem is the structural integrity of buildings may vary. One last problem is that the damage assessment is completed after-the-fact. This can be a problem because the damage site might be altered before it is assessed. Due to these potential problems with the Fujita Scale, a new scale, the Enhanced Fujita (EF) Scale was implemented on February 1, 2007. This new scale uses Degree of Damage Indicators, in order to get a more realistic estimate of a tornado's winds.

More information about the EF-Scale can be found at the Storm Prediction Center.

Enhanced Fujita (EF) Scale
Scale Category Wind Speed Possible Damage
EF-0 Weak 65-85 mph Light: tree branches broken, sign boards damaged
EF-1 Weak 86-110 mph Moderate: trees snapped, mobile homes pushed off foundations or overturned, windows broken
EF-2 Strong 111-135 mph Significant: large trees snapped or uprooted, weak structures destroyed
EF-3 Strong 136-165 mph Severe: some roofs torn off framed houses, trees leveled
EF-4 Violent 166-200 mph Devastating: roofs and some walls torn off well constructed houses, car thrown or overturned
EF-5 Violent >200 mph Incredible: houses may be lifted off foundation, structures the size of automobiles can be thrown over 100 meters, steel-reinforced buildings highly damaged

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