What Triggers Tornadoes? New Season May Hold Answers

Willie Drye
for National Geographic News
March 15, 2004

On any given day during the United States tornado season, which begins later this month and peaks in May, there's a good chance that a twister will touch down somewhere in the country. Despite being so common, however, these dangerous and sometimes deadly storms remain shrouded in mystery.

Meteorologists know that tornadoes are spawned by thunderstorms, but they don't know when or if conditions within a storm will produce a twister.

"Most storms don't produce tornadoes," said Roger Edwards, a meteorologist with the National Oceanic and Atmospheric Administration's Storm Prediction Center in Norman, Oklahoma. "There are a lot of steps that have to take place, a lot of dominoes that have to fall in the atmosphere before a thunderstorm puts a tornado down."

Tornadoes occur in other parts of the world and all across the United States. But some unique weather influences regularly produce these storms in the U.S. Tornado Alley. The region runs south from the Dakotas to the Gulf Coast and is bordered on the west by the Rocky Mountains, and on the east by the Appalachian Mountains.

In this area dry air coming off the Rockies meets warm, moist air from the Gulf of Mexico and cold Arctic air sweeping down from the north. This mingling of hot and cold, dry and moist provides the raw energy for thunderstorms and tornadoes.

Gathering information about tornadoes is like trying to jot down notes about the habits of a savage beast seconds before it attacks.

Unlike a hurricane—a tornado's big, long-lived and usually slow-moving meteorological cousin—a twister is small, brief, and fast moving. That means tornado researchers have to chase likely-looking thunderstorms hoping a tornado might form. When a storm does spit out a twister, researchers have to gather data literally on the fly, keeping one eye on the nearby tornado's funnel and the other on the nearest escape route.

Tornado Probes

Some recent innovations in tornado chasing have added important data to meteorologists' knowledge. National Oceanic and Atmospheric Administration (NOAA) researchers have used vans packed with instruments to chase tornadoes and collect data.

Research scientist Tim Samaras of Denver, Colorado, helped design a probe containing sensors that collects information about a tornado. His development work was largely funded by the U.S. Department of Commerce. A grant from the National Geographic Society Committee for Research and Exploration helps pay for the probe's use in the field. The probe—known as a turtle—is tough enough to withstand a direct hit from a tornado.

In June 2003 Samaras and his partner approached to within a couple hundred yards (about 180 meters) of a twister near Manchester, South Dakota. They dropped the probe and fled as the storm was almost upon them. "It sounded like a high-powered waterfall combined with a jet engine," Samaras said of his brush with the tornado.

The tornado's vortex roared right over the probe and gave up a few of its secrets as it passed. The turtle recorded a barometric pressure drop of 100 millibars within the vortex.

Continued on Next Page >>


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