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.

Sudden and dramatic drops in barometric pressure are what produce the extremely high winds in tornadoes and hurricanes. If the barometric pressure suddenly drops from, say, a normal reading of 1,000 millibars to 900 millibars, a very powerful wind will result. Meteorologists have long suspected that a tornado causes such a dramatic drop, but Samaras's probe was the first solid proof that this can happen.

It's only been within the past few decades that scientists have started accumulating data about tornadoes, said meteorologist Greg Forbes, severe weather expert for the Weather Channel. Forbes thinks that at least 80 percent of the tornadoes that occurred before 1950 were not recorded. "Some weak tornadoes may still go undetected and unreported today. … We are fairly certain that the number of 'missing' tornadoes in the database increases as we go farther back in history," Forbes said.

Monster Twisters

Tornado reports have increased from about 600 per year in the late 1950s to about 1,200 today, said meteorologist Harold Brooks of NOAA's National Severe Storms Laboratory in Oklahoma. The vast majority of those tornadoes don't do a great deal of damage. But about 5 percent of these twisters are violent enough to level buildings and sometimes kill people.

Every once in a while, an unusually violent tornado will pounce on an unsuspecting town and kill dozens—and sometimes hundreds—of people.

It happened on May 6, 1840, when a killer tornado touched down in eastern Louisiana and roared up the Mississippi River into Natchez, Mississippi, around 1 p.m. The storm sank riverboats, smashed houses, and flung debris for miles. At least 317 people—and probably many more—were killed. "Never, never, never, was there such desolation and ruin," The Free Trader, a Natchez newspaper, reported after the storm.

On March 18, 1925, a monster tornado dropped out of the sky near Ellington, Missouri, and began a devastating 219-mile (352-kilometer) trek across three states. Before this horrific twister finally dissipated several hours later, it had killed 695 people in Missouri, Illinois, and Indiana.

Sometimes, extremely powerful thunderstorms spawn multiple tornadoes, dropping them like bombs as the parent weather system moves over the countryside. The worst outbreak of tornadoes on record occurred April 3 and 4, 1974, when a large system of thunderstorms rumbling across the Midwest spun off 148 twisters. More than 300 people were killed as tornadoes struck 13 states from Michigan to North Carolina.

The death toll could have been higher had it not been for Dick Gilbert, a helicopter pilot and traffic reporter for WHAS Radio in Louisville, Kentucky. Gilbert was aloft when a tornado with winds exceeding 207 miles an hour (333 kilometers an hour)— classified as F4 on the Fujita Scale (see sidebar)—struck Louisville shortly after 4 p.m. on April 3. Gilbert tracked the twister from his helicopter as it ripped through the city. His warnings allowed hundreds of people to get out of the way.

Meteorologists today want to give that same kind of warning sooner and over a larger area. That's one of the motivations behind tornado research. "What is the match that's lit to get a tornado going?" said NOAA's Edwards. "What happens the instant they form?"

Answering those questions might help meteorologists provide that advance warning and save lives.

For more on tornadoes and storm chasing, see related stories and links below.

© 1996-2008 National Geographic Society. All rights reserved.