On a local scale, the tornado is the most intense of all atmospheric circulations. Its vortex usually rotates cyclonically (on rare occasions anticyclonically rotating tornadoes have been observed) with wind speeds as low as 30 m s-1 (67 mph) to as high as 135 m s-1 (300 mph), and is generally < 2 km (1.25 mi) in diameter. Tornado intensity is often estimated on the basis of wind damage using the Enhanced Fujita Scale; however, this estimate can be refined using other measurements, especially in the absence of damage indicators. Some tornadoes may also contain secondary vortices (also referred to as suction vortices, subvortices, multiple, and satellite vortices).Tornadoes have been observed on all continents except Antarctica but are most common in the United States, where the average number of reported tornadoes is roughly 1000 per year, with the majority of them on the central plains and in the southeastern states (see Tornado Alley). They can occur throughout the year at any time of day. In the central plains of the United States they are most frequent in spring during the late afternoon.(See also Fujita Scale.)
term edited 8Oct2013
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