Spring and summer may be the most dangerous tornado seasons in the United States, but twisters can still wreak havoc in winter. At least nine tornadoes barreled through Texas over the weekend, killing at least 11 and damaging up to 1,000 buildings and homes across Dallas and the surrounding counties.
"Tornado Alley," which includes many of the Great Plains states and parts of Texas, is the the most notorious staging ground for U.S. twisters. But in December tornadoes tend to form in the southeast and east Texas, fueled by the warm, moist air coming off of the Gulf of Mexico.
Record warm temperatures across much of the eastern U.S. have caused unusually large amounts of water to evaporate into the air, giving recent storms more moisture—and greater potency—than usual. (See why December temperatures have been so freakishly warm.)
Scientists don’t completely understand how tornadoes like this weekend’s form. And meteorologists struggle with forecasting tornadoes, since they’re short-lived, finicky, and relatively tiny compared with other atmospheric phenomena.
Here's what we do know: A tornado is a violently rotating column of air that extends between the Earth's surface and a cloud.
The most intense tornadoes spawn from supercells, massive thunderstorms with rotating hearts called mesocyclones. For a supercell to form, the skies "need the ingredients for a regular thunderstorm," says Harold Brooks, a research meteorologist with the National Oceanic and Atmospheric Administration's (NOAA) National Severe Storms Laboratory (NSSL) in Norman, Oklahoma.
Those ingredients include warm moisture near the surface and relatively cold, dry air above. But supercells require winds that increase in strength and change direction with height, generating horizontal tubes of rotating air parallel to the earth’s surface.
Warm air below a horizontal vortex rises “like a hot-air balloon," says Brooks, and bends it into an arch. One of the two vertical vortices forming the arch then peters out. Shearing winds tilt the top of the surviving vortex, causing cool rain to fall away from the warm surface air fueling the newly born mesocyclone, letting the supercell purr like a finely tuned engine. (See an animation of how tornadoes form.)
Powerful Tornadoes are Rare
Most tornadoes last for fewer than ten minutes and travel only a few miles before they dissipate. But large tornadoes can last up to 30 minutes and can have wind speeds of more than 300 miles (483 kilometers) per hour, which can rip buildings off their foundations.
Powerful tornadoes count for only about one percent of all U.S. tornadoes, but they cause the majority of the country’s 60 annual tornado fatalities, mostly from debris. (See "Interactive: Forces of Nature.")
What defines powerful? In many countries, including the United States, tornadoes are measured with the Fujita scale or the updated Enhanced Fujita Scale. An F0 or EF0 tornado damages trees but substantial structures are left unharmed; a tornado in the strongest category—F5 or EF5 —blows away buildings.
Scientists are still measuring this weekend’s Texas tornadoes, but the National Weather Service has confirmed that the tornado that slammed into Garland, Texas, on Saturday was an EF4, with winds up to 180 miles (290 kilometers) per hour.
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