Wednesday, May 5, 1999; Page A23

While the mechanics of all tornadoes are not completely understood, a thunderstorm is always the first step. If atmospheric conditions are right, the thunderstorm can spin out one or more tornadoes.

THE 'F' SCALE Tornado severity is ranked on a scale created by the late Tetsuya Theodore Fujita, University of Chicago. Wind speed and severity of tornadoes is estimated by the damage caused. F-0: Gale tornado. Winds of 40-72 mph; damage to some chimneys and some branches broken. F-1: Moderate tornado. Winds 73-112 mph; damage to roofs, mobile homes pulled off foundations, attached garages destroyed. F-2: Significant tornado. Winds 113-157 mph; roofs torn off frame houses, mobile homes destroyed, large trees uprooted. F-3: Severe tornado. Winds 158-206 mph; roofs torn off all houses, trains overturned, most trees uprooted. F-4: Devastating tornado. Winds 207-260 mph; well-constructed homes destroyed; some thrown long distances. F-5: Incredible tornado. Winds 261-318 mph; strong houses lifted off foundations and disintegrated, cars thrown more than 100 meters, steel-reinforced concrete buildings badly damaged. F-6: Winds greater than 318 mph; possible but never documented.

Thunderstorms usually contain updrafts, large rising swells of warm, moist and unstable air that rotate as they move upward. As the warm updraft punches through an overlying layer of stable air and continues upward into a cooler and drier air mass, the resulting instability can produce the powerful vortex motions that define tornadoes.

Air rushing in to fill the low-pressure vacuum left by the tornado creates additional fierce -- and potentially damaging -- winds, although sometimes an updraft gives rise to a tornado and sometimes it does not. Scientists are still unable to predict exactly when and where a tornado will appear, which makes it all the more difficult to issue warnings.

Joseph Golden, senior meteorologist for the National Oceanic and Atmospheric Administration, said Monday night's tornado outbreak in Oklahoma and Kansas occurred as several sets of these atmospheric conditions "all came together in the worst possible way."

While scientists from the National Severe Storms Laboratory in Norman, Okla., are still studying the conditions leading up to the tornadoes, Golden said there appeared to have been a collision of moist, unstable air from the Gulf of Mexico and colder air in the jet stream above, causing severe disturbances and a low pressure area at the ground. As surrounding air moved in to fill the void, a spinning effect evolved as air was pulled upward into a chimney. Normally, the air moving in toward the chimney turns counterclockwise and then rises in a twisting column that grows increasingly violent.

The strongest tornadoes often are near the edge of the updraft, not far from where air is descending from the thunderstorm. Thus, falling rain or hail can pull down air to form downdrafts, which explains why a burst of heavy rain or hail often occurs right before a tornado.

NOAA officials said that in the southern High Plains, where Monday's twisters struck, thunderstorms often form as warm air near the ground flows "up slope" toward higher terrain. If other favorable conditions exist, these thunderstorms can produce tornadoes.