Why do some thunderstorms spawn deadly columns of wind while others simply leave us drenched?
Anatomy of a Tornado
From storm to major event: how havoc happens
1. Warm, moist air at the surface rises rapidly, creating an updraft.
2. Falling rain evaporates, cooling the air around it.
3. The wall cloud rotates as it’s hit with winds from opposite directions.
4. As the rotation intensifies, a visible funnel drops out of the clouds.
5. A prominent overshooting top forms when the updraft is very strong.
6. Powerful updrafts give hail time to form.
7. A dust shroud is kicked up by the tornado’s strong winds at ground level.
8. Central downdrafts appear in some tornadoes.
Next: The role of global warming »
The role of global warming
Everyone from the insurance industry to Al Gore worries that global warming may be causing more tornado activity. But there’s no baseline for comparison. That’s because we have no accurate record of tornadoes before the 1950s, back when it was possible for these brief, freakish funnels of air to blow over then-unpopulated areas without notice.
Tornado warnings have improved over the years; forecasters can now issue warnings about 18 minutes before touchdown for 75 percent of twisters. As for longer-term forecasts, the science isn’t there yet, despite what you hear on the nightly news. “Television forecasters pander to the public’s curiosity about extreme weather,” says Michael H. Glantz, a professor at the University of Colorado at Boulder. Their goal, of course, is to be first with a forecast of trouble ahead. But beware those who insist on linking El Niño (the cyclical surface warming of the eastern Pacific Ocean) to tornado likelihood. The slight correlation is “not enough to be directly useful in forecasting,” says Joe Schaefer, director of the Storm Prediction Center in Norman, Oklahoma.<
The rising toll
The number of deaths per tornado is greater in the South than in Tornado Alley (the Great Plains and part of the Midwest)—a disparity that can’t be explained by storm frequency or severity. Experts say things like terrain, population density, and the number of mobile homes play a role. But the biggest factor might be timing: “In the Plains,” explains Schaefer, “we have a much more clearly defined period of tornado activity, generally from March to late June. In the Southeast, tornadoes can happen all year long, so there may be less vigilance.” Research also shows that the South may have a climatological propensity for nighttime storms, which catch people unawares—asleep in bed. Last year’s two killer tornado outbreaks in North Carolina landed between 11:30 p.m. and 3:30 a.m.
Next: How we can be better prepared »
Strengthening mobile homes
Mortality rates for people struck in mobile homes account for approximately half of all deaths by tornadoes. After a tornado killed 18 people in an Evansville, Indiana, mobile home park, the state passed a first-of-its-kind law requiring that these manufactured homes come equipped with weather radios and alarms. Supporters claim that radios are equivalent to smoke detectors. But opponents argue that a smoke detector warns of a more specific risk: “How would you like it,” one critic asked, “if your fire detector went off whenever there was a fire in your county?”
Building stronger homes
Tornadoes and thunderstorms cost insurers $10.5 billion last year. Several major companies now offer reduced premiums for homes that offer better protection. Carl Schneider, an insurance agent in Mobile, Alabama, lives in a home made of reinforced concrete with steel framing and says he’s on a mission to get others to build smarter. “In the past, insurance companies didn’t provide an incentive for people to build resistant homes, because they didn’t recognize differences in construction type.” What’s key, says Larry J. Tanner, of the Wind Science and Engineering Research Center at Texas Tech University, is to make sure your house is bolted, not just nailed, to its foundation and to properly secure the roof with metal clips. He also recommends wind-resistant windows and garage doors, since homes often collapse after these weak spots are blown out. But keep in mind, he says, that if you’re hit by a house-wrecking tornado (just 3 percent of them are), the only truly safe spot will be the concrete shelter you worked into the blueprints. In less severe twisters, you may be safe in a closet, a windowless bathroom, or in the space below a staircase.
Sirens have long been the standard of tornado readiness. High-risk Oklahoma City, for instance, swears by its $4.5 million computer-controlled network of 167 sirens that sound for three minutes after a warning is received from the National Weather Service. But the use of sirens varies widely. Some officials say sirens don’t tell enough about the location or severity of the twister and that people inside their homes may not hear them.
What works best to save lives?
A combination of warnings, says Harold Brooks, research meteorologist at the National Severe Storms Laboratory in Norman, Oklahoma. In his state, sirens are supported by local radio and TV broadcasts. And in Orange County, Florida, a new service sends storm warnings to people’s cell phones and e-mail accounts.