Gone with the wind: last spring, the most savage tornadoes in U.S. history blew away trees, cars, and houses.

SUNDAY, MAY 4, 2003: Two Iowa teens crawl into a bathtub and grail on fin dear life as a tornado flings their house 8 meters (25 feet) into three sturdy maple trees. The boys climb out unhurt--from the hole where the kitchen wall used to be. Meanwhile, residents of Pierce City, Missouri, emerge from their basements to find most of their town turned to rubble, including the storm shelter where two people were killed.

The week of May 4th marked the most devastating week of tornadoes in U.S. history. Three hundred and eighty-four slammed through 19 states, leaving thousands homeless and 45 people dead. Some twisters lasted for a half hour rather than the typical 10 minutes. And at least five storms rated a rare, violent "F4" on the tornado-rating system known as the Fujita Scale (see page 16), with winds raging from 207 to 260 miles per hour. What triggered the record-breaking barrage?

TWISTED

April to June are peak tornado months in the midwestern U.S., but nobody expected a week like the one last May. "Weather conditions kept setting up idea tornado conditions every day," says meteorologist Daphne Zaras of the National Severe Storms Lab. "But the surprising thing was that thee conditions went on for so long." Meteorologists know a lot about the basic climate "recipe" that might brew a tornado, but they still can't pinpoint the exact ingredient that sparks the savage whirlwinds.

A tornado can result when a large patch of warm, moist air collides with mass of cool, dry air. These two types of breezes clash so frequently in the Midwest, the region is nicknamed "Tornado Alley"--the twister capital o the world (see "How Tornadoes Form," below). But if tornadoes simply resulted from colliding air masses, meteorologists would be able to forecast them early enough to evacuate whole states if necessary. In reality, such advance warning is impossible. "You can't simplify tornadoes to a clash of weather systems," says Zaras. "They're a complicated mix of things."

What colliding air masses reliably produce are giant whirling thunderstorm clouds called supercells.

Meteorologists can predict these quite well, but they're still baffled why some supercells merely spawn rain and thunder while others unleash a whirling trail of destruction. "It's tricky to know when the supercells will become tornadic," Zaras says.

STORM WATCH

To understand such complex weather systems from a safe distance, scientists use a tool called Doppler radar. This technology pulses electromagnetic waves (streams of energy) into the air to scour for rain or snowstorms. How? The waves bounce off flying water and ice particles. And the radar carefully listens for the number of waves per second, or frequency, zipping back to the antenna. Precipitation moving toward the radar causes the electromagnetic waves to bounce back with higher frequency--it's called the Doppler effect. The collected data is interpreted by a computer and displayed as images on a screen. This shows scientists (1) how much rain or ice is in the air; and (2) how fast and in what direction wind is churning the particles.

Meteorologists can read the wind's twists and turns to predict thunderstorms up to 12 hours in advance; forecasting tornadoes proves trickier. So far, scientists have learned that certain patterns in the downdrafts (downward-moving winds) at the rear of storms may signal a forming twister. They've also observed that storms shaped like kidney beans, or those with particularly low clouds, have greater chances of generating twisters.

WIND ADVISORY

Right now, meteorologists usually spot tornadoes just early enough to alert the public; warning time for many of May's storms was around 20 minutes. If people act quickly, that's usually enough time to find shelter inside secure structures and away from windswept objects. "In May, there was a fatality when someone waited too long," Zaras says. A man was hit by flying debris while running to a neighbor's shelter. "Ten minutes earlier and he would have been fine."

One thing for sure: Scientists will continue to perfect methods to better predict tornadoes. That's because on average about 800 tornadoes hit the U.S. each year. Luckily, 99 percent of them rate below F3, and sprees like the one last May seldom occur. "I'm glad we don't get it like that very often," Zaras says.

HOW TORNADOES FORM

A tornado is a violently rotating column of air that is in contact with both the ground and a cloud base.

Epicenter: Most tornadoes in the U.S. occur in "Tornado Alley," the central plains stretching from North Dakota to Texas. Violent storms form when slow-moving warm air from the Gulf of Mexico collides with fast-moving cold, dry air from the north and west. Wind shear, the sudden change of wind direction and speed, can cause a column of air near the ground to start rotating. Result: possible tornado.

Cold air meets wars:

The layer between cold air above and warm air below, called the cap, sometimes starts rotating with incoming weather fronts.

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Crash course: Colliding cold and warm air causes warm air to rise in an updraft. The updraft cools, and water vapor within forms storm clouds. The updraft tilts the rotating air column.

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Supercell thunderstorm. A mesocyclone, or large mass of rotating air up to six miles wide, develops in the midst of large storm clouds. This is a supercell thunderstorm. High-altitude winds flit thunderclouds to create s characteristic anvil shape.

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Tornado touchdown: Scientists think downdrafts of cooled air yank the swirling mesocyclone toward the ground. At the same time, updrafts stretch the whirlwind vertically, speeding it up and tightening it into a narrow and violent tornado.

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THE FUJITA TORNADO DAMAGE SCALE

        F0                   F1                 F2
WIND
(MPH)  <73                 73-112             113-157

DAMAGE   Light            Moderate          Considerable

Damage to             Roof surfaces torn   Roofs blown off
chimneys, antennas,   off; some trees      some houses;
and windows;          uprooted; moving     mobile homes
snapped tree          cars overturned      destroyed; large
branches                                   trees uprooted

        F3                 F4                 F5
WIND
(MPH)  158-206           207-260            261-318

DAMAGE  Severe         Devastating        Incredible

Exterior walls torn   Houses leveled;   Large houses
off houses; cars      cars thrown       leveled and swept
lifted off the        some distance;    away, severe
ground; forests       small buildings   damage to steel and
flattened             blown away        concrete buildings

Did You Know?

* The term tornado is used only if a rotating wind column makes contact with the ground, if it doesn't, it's called a funnel cloud. Waterspouts are tornadoes that form over water. They are generally weaker than those that occur on land and are commonly found in the U.S. along the Gulf Coast.

* Most tornadoes last for only 5 to 10 minutes, but some have been known to rage for more than an hour.

* The wind in almost all tornadoes in the Northern Hemisphere spins counterclockwise, or cyclonically. The opposite is true in the Southern Hemisphere.

Cross-Curricular Connection

Language Arts: Sixteenth-century navigators used the word "tornado" to describe sudden storms accompanied by gusty winds. Have students research historical accounts of tornadoes and incorporate their research into a brief first-person fictional report in which they describe the sighting and progress of a tornado and its impact on their lives.

Critical Thinking: Have students brainstorm what is necessary to stay safe before, during, and after a tornado. Then share safety tips with them from the Federal Emergency Management Agency: www.fema.gov/hazards/tomadoes/tornadof.shtm/

Log on to the USA Today Web site on tornadoes, which includes photographs and videos: www.usatoday.com/weather/resources/basics/twist0.htm

For an informational and fun read about twisters, visit the Tornado Project Online at www.tornadoproject.com/

To learn more about tornadoes and tornado safety, see the Weather Channel Web site of the NOAA Web site:

The Weather Channel: www.weather.com/newscenter/specialreports/tornado/index.html NOAA: www.noaa.gov/tornadoes.html/

Visit meteorologist Daphne Zara's Web site at National Severe Storms Laboratory www.nssl.noaa.gov/

Directions: Answer the following questions in complete sentences.

1. What is a tornado?

2. What is the Fujita Scale?

3. Where do most tornadoes exist in the U.S.? Why?

4. What do scientists use to study weather systems from a safe distance?

BONUS: How does the technology in question 4 work?

Gone With the Wind

1. A tornado is a violently rotating column of air that is in contact with both the ground and a cloud base.

2. The Fujita Scale is a system that rates tornado severity.

3. Tornadoes in the U.S. mostly occur in "Tornado Alley," an area in the central plains stretching from North Dakota to Texas. One reason: It's where cold, dry air from the north and west frequently collides with warm, moist air from the Gulf of Mexico. This generates violent storms.

4. Scientists use Droppler radar to study weather from a safe distance.

Bonus: Doppler radar pulses electromagnetic waves into the air. The waves bounce off water and ice particles in the air. The radar then carefully "listens" for the number of waves per second, or frequency, zipping back to the antenna. Precipitation moving toward the radar causes the electromagnetic waves to bounce back with higher frequency--this is called the Doppler effect. The collected data ale interpreted by a computer and displayed on the screen. This shows the amount of rain or ice in the air, as well as the direction and speed in which wind is moving the particles.