<P> Luminosity has been reported in the past and is probably due to misidentification of external light sources such as lightning, city lights, and power flashes from broken lines, as internal sources are now uncommonly reported and are not known to ever have been recorded . In addition to winds, tornadoes also exhibit changes in atmospheric variables such as temperature, moisture, and pressure . For example, on June 24, 2003 near Manchester, South Dakota, a probe measured a 100 mbar (hPa) (2.95 inHg) pressure decrease . The pressure dropped gradually as the vortex approached then dropped extremely rapidly to 850 mbar (hPa) (25.10 inHg) in the core of the violent tornado before rising rapidly as the vortex moved away, resulting in a V - shape pressure trace . Temperature tends to decrease and moisture content to increase in the immediate vicinity of a tornado . </P> <P> Tornadoes often develop from a class of thunderstorms known as supercells . Supercells contain mesocyclones, an area of organized rotation a few miles up in the atmosphere, usually 1--6 miles (1.6--9.7 kilometres) across . Most intense tornadoes (EF3 to EF5 on the Enhanced Fujita Scale) develop from supercells . In addition to tornadoes, very heavy rain, frequent lightning, strong wind gusts, and hail are common in such storms . </P> <P> Most tornadoes from supercells follow a recognizable life cycle . That begins when increasing rainfall drags with it an area of quickly descending air known as the rear flank downdraft (RFD). This downdraft accelerates as it approaches the ground, and drags the supercell's rotating mesocyclone towards the ground with it . </P> <P> As the mesocyclone lowers below the cloud base, it begins to take in cool, moist air from the downdraft region of the storm . The convergence of warm air in the updraft and cool air causes a rotating wall cloud to form . The RFD also focuses the mesocyclone's base, causing it to draw air from a smaller and smaller area on the ground . As the updraft intensifies, it creates an area of low pressure at the surface . This pulls the focused mesocyclone down, in the form of a visible condensation funnel . As the funnel descends, the RFD also reaches the ground, fanning outward and creating a gust front that can cause severe damage a considerable distance from the tornado . Usually, the funnel cloud begins causing damage on the ground (becoming a tornado) within a few minutes of the RFD reaching the ground . </P>

Tornadoes are a destructive force in the united states