Fact Sheet

Tornadoes: A guide to nature’s most violent storms

Tornadoes, which took an especially high toll on the American heartland this year, are still impossible to predict

How do tornadoes form?
Most tornadoes are born in the trailing edge of large thunderstorm systems, called “supercells,” when warm air meets a cold front. Inside that collision of air, winds blow in different directions at different altitudes, and can create a swirling vortex of air. Rapidly rising currents of warm air tilt the vortex sideways, so that it points at the ground, and provide the rotational effect that turns it into a rapidly spinning column of air called a “mesocyclone.” When the winds conspire to make that column spin faster and faster, a tornado is born. Tornadoes are rated between zero and five in intensity on the Enhanced Fujita scale, which estimates wind speed by gauging destruction on the ground. An EF-1 will overturn cars and mobile homes, rip roofs off houses, and uproot trees. An EF-5, like several that ravaged parts of Alabama in April with winds of more than 200 mph, can tear sturdy buildings from their foundations and carry them away. “I thought the whole house was just going to take off,” Sharon Blue of Pleasant Grove said. “It was like Dorothy in The Wizard of Oz. I just held my little dogs and prayed.”

Why do so many occur in the U.S.?
The Central and Southern states lie in a meteorological battleground where warm, humid air from the Gulf of Mexico collides with cool, dry air from Canada. One third to one half of all tornadoes occur each year in Tornado Alley, centered in the southern Great Plains. In Dixie Alley, which runs through the Southern states, tornadoes tend to come earlier in the spring and hit harder. But with tornadoes, there are few hard and fast rules; all 50 states have had them. They have been reported in Europe, Asia, and Australia, but the U.S. is “the tornado capital of the world,” said climatologist Bill Patzert of NASA’s Jet Propulsion Laboratory.

Was this season notably bad?
Yes. An unusually powerful jet stream pushed more frontal systems across the country’s midsection this spring, setting up more thunderstorms, which in turn spawned lots of tornadoes. More than 600 twisters were reported in April, making it the most active tornado month ever recorded; they caused 369 deaths in the nation’s deadliest tornado outbreak since 1925. There is no one explanation for that. Tornadoes in Dixie Alley stay on the ground longer and create more damage than those in the Great Plains, a recent study found, but in the end this year’s outbreak is “random,” said Bob Rauber, a tornado authority at the University of Illinois. “Sometimes you get active periods and sometimes you don’t.” This year’s tornado toll was high partly because the population of the Southern states has increased 14 percent in the last decade; mobile homes, which are very vulnerable to twisters, have proliferated there.

Is global warming a factor?
There is no evidence that it is, and to make such a claim would be “irresponsible science,” said Chuck Doswell, a meteorologist at the University of Oklahoma. We do know that the number of tornadoes reported has nearly doubled in the past 50 years, but the frequency of strong ones rated EF-2 and above has actually been decreasing. Most scientists believe that more tornadoes are being reported because more people live in tornado zones, and there are more storm spotters.

Why can’t we do a better job of predicting them?
Because the climate models and radar networks employed to forecast other weather events are useless for predicting tornadoes. Meteorologists still rely largely on human spotters, so the average tornado warning sounds only 13 minutes before touchdown—too late for orderly evacuations. The storms are so fluky that predicting them is a “crapshoot,” said Kevin Trenberth, a senior scientist at the National Center for Atmospheric Research. “A little quirky thing can set one off at one time, and another time not.” Prospects may be improving, though. Over the next two years, the National Weather Service plans to upgrade its network of 169 Doppler radars, first installed in the early 1990s. The new dual-polarization radar system is precise enough to see leaves and grass churning in a wind funnel, even during a rainstorm. They won’t help with long-range predictions, but will track tornadoes once they’re on the ground and moving.

How do you survive one?
Essentially, by getting as low to the ground as possible. Researchers who studied deaths and injuries from the powerful tornadoes that hit the Oklahoma City area on May 3, 1999, found that basements and hallways were the safest spots in a home. Stay away from windows (and don’t bother opening them) and above all avoid mobile homes, where half of all tornado deaths occur. Cars are “death traps,” said the Storm Prediction Center of NOAA. Unless you can find shelter or drive out of the path of a tornado, get out of the car and lie face down on the ground. But don’t let prudence yield to panic. For all their fury, tornadoes kill an average of 60 people a year, fewer than heat waves or floods do.

The damage done
Tornadoes are “nature’s most violent storms,” according to NOAA. They have been known to lift a railroad car off its tracks, strip asphalt from a road, and drive wood splinters into bricks. One survivor of the country’s most devastating twister, the 1925 Tristate Tornado, recalled watching a baby being blown from its mother’s arms and a cow “hurled into the village restaurant.” In one documented case, a golf course flag landed 43 miles from the course; in another a man’s jacket was found 20 miles from his home. In the wake of last month’s outbreak in Alabama, witnesses spoke of “whole neighborhoods reduced to matchsticks” in minutes. Parts of suburban Birmingham resembled Hiroshima. “The door started to shake, the glass in the windows broke, and it felt like our ears were going to explode because of the pressure change,” a survivor of April’s Tuscaloosa twister said. “We sprinted into the bathroom, and when we emerged, we couldn’t believe what we saw. Where we just were, it was gone.”


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