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Tornadoes

A tornado is a violently rotating column of air in contact with the ground. It usually forms under a cumulonimbus (thunderstorm) cloud and is visible as a condensation funnel or by the dust and other debris incorporated into the rotation. About 1,500 tornadoes are recorded annually around the middle latitudes of Earth, and they typically cause 100 to 300 deaths.

Regional Occurrences and Frequencies

Tornadoes occur in many regions of the world but are most frequent in the middle latitudes, especially in the Central and Southern United States, but with the most loss of life and livelihood occurring in Central Bangladesh. The United States records about 1,100 tornadoes annually. This count has doubled since the 1950s, but the increase is thought to be due to better reporting and recording of tornadoes rather than an actual increase in the number of tornadoes. Canada records about 60 tornadoes annually; the United Kingdom and Australia each about 30; Germany, France, and Italy each about 20; and Argentina and Bangladesh each about 10. A substantial number of tornadoes also occur in other European countries and in India, South Africa, China, Japan, and New Zealand. Tornadoes are undercounted in many countries because small tornadoes are not observed and recorded. The number of tornadoes seems to increase in these regions as reporting efficiency and record keeping improve.

The occurrence of tornadoes is restricted to the regions where conditions are favorable for their formation. Thus, they are most common in the middle latitudes, where a contrast of air masses, vertical wind shear, instability, a strong jet stream, and strong cyclonic storms and fronts most commonly occur. Although tornadoes may occur more frequently over flat terrain, they are known to occur in hilly terrain and to cross large mountain ranges and descend into deep valleys. No terrain is “protected” from tornadoes.

Seasonal patterns of tornado frequency exist, with a spring and summer peak in the United States. The occurrence of tornadoes generally coincides with the annual peak in thunderstorm frequency. On the diurnal scale, tornadoes most often form during the afternoon and evening hours when atmospheric instability is greatest and thunderstorms are most violent.

Conditions for Formation

Tornadoes are classified into two broad categories, supercell tornadoes and nonsupercell tornadoes. A supercell tornado forms within supercell thunderstorms that contain mesocyclones in the middle levels of the atmosphere. Mesocyclones are rotating updrafts that may persist for several hours. They form in a moist, unstable environment with strong vertical wind shear. A supercell thunderstorm may produce several tornadoes, called a “tornado family,” with one tornado forming as the previous one dissipates. Rotation develops in the thunderstorm updraft due to the change of wind speed and wind direction with height above the ground, called vertical wind shear. This wind shear causes horizontal vorticity. Stronger winds aloft cause a tilting and stretching of this horizontal vorticity, and this results in a rotating cloud with vertical vorticity, the mesocyclone.

Not all supercell thunderstorms produce a tornado. The mechanisms that create rotation at ground level, that is, a tornado, are poorly understood. Large vertical vorticity must arise at the ground. This may develop when horizontal vorticity near the ground is tilted into a vertical rotation. Vertical vorticity near the ground may also arise through the transfer of vertical vorticity toward the ground by a rear-flank downdraft from the mesocyclone. The rear-flank downdraft is dry air that descends from midlevels in the atmosphere and wraps around the back of a mesocylone. The rear-flank downdraft may tilt horizontal vorticity into vertical vorticity, transfer vertical vorticity from the mesocyclone toward the ground, or enhance existing vertical vorticity near the ground, and any of these may lead to a tornado.

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