What Triggers Tornadoes? New Season May Hold Answers

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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.

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