Violent Twisters Don’t Always Respect Seasons, or Geography

Tornado season normally lasts just three months: April, May and June, but the off-season twisters that leveled parts of Arkansas and Tennessee on Thursday were not unusual. Given the right conditions, twisters can form any time of the year.

Tornadoes produce the most violent winds on Earth, packing more energy per square inch than an atomic bomb. They can swirl at speeds as high as 300 mph, reach heights above 60,000 feet and spin with enough force to jam a piece of straw into the side of a barn.

Improved technology and increased interest by amateurs, spurred by the Weather Channel and the Internet, have led to a sharp increase in the number of reported tornadoes in the United States, according to Dr. Joe Schaefer, director of the national Storm Prediction Center in Norman, Okla.: 1,254 were reported in 1998, compared with 201 in 1950. “There aren’t more tornadoes, there are just more reports of them,” he said.

Tornadoes are formed when warm and cool air collide, buffeted by winds from the southwest and east. The opposing winds cause the warm air to rotate as it rises. The air coils in increasingly tighter spirals, eventually spinning off into a tornado.

The majority form in Tornado Alley, a corridor that stretches from Texas to Iowa. This area is prime tornado country because moist air from the Gulf of Mexico, blocked by the Rockies, pools there until cooler air blows down from the north, setting up ideal twister conditions.

Mobile home parks are particularly vulnerable because their foundations are temporary. In 1998, nearly 50% of the nation’s 129 tornado deaths occurred at trailer parks, according to Storm Prediction Center statistics.

Most tornadoes travel for about a mile. The longest track on record came on March 18, 1925, when the Great Tri-State Tornado traveled 219 miles through Missouri, Illinois and Indiana.