Early spring is prime season for storms. As the weather warms, huge air masses become unstable and charge about, sometimes clashing with the force of opposing armies. Deadly tornadoes, thunderstorms, and snow that last week hit the East are but the latest example of the forces this phenomenon can unleash.
* In April 1982, a multipurpose storm front dumped 11 inches of snow on Michigan, knocked down power lines along the Atlantic seaboard, and set 86 tornadoes loose on the Southeast.
* In March 1980, a blizzard dumped 2 1/2 feet of snow on East Coast states.
* On April 10, 1979, a tornado devastated part of Wichita Falls, Texas.
* On an infamous Easter weekend in 1974, more than 100 twisters devastated Xenia, Ohio, and other Midwest towns.
''Tornadoes and severe thunderstorms require warm, moist, unstable air masses. These are most likely to occur in the spring,'' says Ken Bergman, a meteorologist in the Climate Analysis Center of the National Weather Service (NWS).
Take the most recent spring storm system as an example. While unusual in its ferocity, the storm ''was not all that unusual'' in its form, says Mr. Bergman.
It all started, he says, when strong winds in the upper atmosphere started sucking air up from the ground. This created a low pressure center (of unsual strength, as it turned out). Footloose air masses from north and south were drawn toward the low pressure center, collided, and started swirling like a cyclone.
As happens in springtime, the two air masses were of extremely different character. The one from the north was winter-cold; its opponent was warm and unusually moist.
The warm air mass rose to the top, and its moisture condensed into rain and snow - releasing a tremendous amount of latent heat. This intensified the winds and struggle between the two masses.
The result was wind, driving rain and snow, and a tornado storm the National Weather Service rated the worst to strike the Southeast in 60 years.
The tornadoes that swept the Carolinas last Wednesday are believed responsible for at least 59 deaths, hundreds of injuries, and more than $100 million in damage. Hard-hit counties in both states have been declared disaster areas, making them eligible for federal relief money. When the storm system hit the Northeast, driving wind, rain, and snow claimed another eight lives. It left hundreds of thousands without electricity and churned up higher-than-normal tides that damaged some coastal areas.
The storm's deadliness ''had to do with the population density of the area (the storm) went through'' as much as its unusual severity, says Mac McLaughlin, chief meteorologist at NWS Central Region headquarters in Kansas City, Mo.
Similar tornado outbreaks occur one or two times a year in the ''tornado alley'' of the Great Plains. They usually strike relatively unpopulated areas, however.
''Normally, in the East, the air has too much moisture'' to spin into such tornadoes, Mr. McLaughlin explains.