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ShareJune 1, 2005 — Tornadoes can form when hurricanes make landfall, as their winds at ground level slow down, while the winds near the top keep their momentum. Data from 2004 show this tendency seems to have increased. Residents of hurricane-stricken areas should prepare for the eventuality of tornadoes.
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Hurricane season officially started in June, and now forecasters say once a hurricane reaches land, it could form tornadoes -- thousands of miles away.
"This is where I felt the floor shudder," Tena Selby says. Selby's floor wasn't shaking from an earthquake. It was a tornado ripping through her backyard. "When I peaked around the corner and looked outside, I didn't see any grass," she says. "All I saw was debris, limbs, and the trees had fallen out back."
Spiraling tornadoes are usually caused by violent thunderstorms. But now meteorologists say another kind of weather system can stir up tornadoes...
"These are tornadoes that get their strength and power from the hurricane in which they're embedded," says Joe Schaefer, a meteorologist at NOAA Storm Prediction Center in Norman, Okla.
Once a hurricane makes landfall, the storm often weakens -- slowing down winds near the surface, while the winds near the top keep their momentum. This can lead to tornadoes, even thousands of miles away.
Meteorologist Greg Forbes, a severe weather expert at The Weather Channel in Marietta, Ga., says, "It takes real strong shifting winds at low levels, the lowest 3,000 feet or so, to be the source for these hurricane-related tornadoes."
Researchers often don't realize a tornado has formed until they survey the hurricane's damage and recognize the tell-tale signs of a tornado.
"A hurricane, if you go look at the damage, is just a big area where there's relatively uniform damage. When a tornado strikes, you get very intense, very narrow lines of damage," Schaefer says.
Spotting tornadoes from hurricanes could keep forecasters busy this season. Selby feels fortunate she survived her close encounter. "It didn't hurt any of the houses in the neighborhood," she says. "We were so fortunate."
The year 2004 brought the most tornadoes on record with 1,817, and more than 330 of those were from hurricanes. Hurricane Ivan produced the most tornadoes ever in September 2004 with 123 spawned tornadoes.
BACKGROUND: Scientists have discovered that hurricanes can actually encourage the formation of tornadoes. When meteorologists studied hurricane-related damage, they found some damage that didn't fit the pattern, and discovered it was actually damage resulting from hurricane-related tornadoes.
HOW TORNADOES FORM: Air is a gas and water is a liquid, but in the realm of science, both fall into the category of fluids. When a fluid's flow is disturbed somehow, it causes turbulence. For instance, branches sticking into and under the water can disrupt the flow of a stream, forming tiny eddies or whirlpools. The same thing happens when you move your hand quickly through water. Technically, these are known as vortices. The water moves in a circular motion around a central point, and this causes a depression or cavity to form in the center, which draws flowing objects towards that center. Think of water spinning down the bathroom drain.
These sorts of swirling vortices can also form in air. As a thunderstorm develops, if the wind speeds up and changes direction, this can cause a horizontal spinning effect in the lower atmosphere. As air rises, pulled upwards by the developing thunderstorm, it tilts the horizontal rotation into a vertical rotation. A tornado is simply a violently rotating column of air that extends from a thunderstorm in the atmosphere to the ground. The pressure inside can be 10 percent lower than the surrounding air, and this causes that air to rush towards the low-pressure center from all directions. As it streams inward, the air spirals upward around the core until it merges with the airflow of the thunderstorm that gave rise to the tornado.
TORNADO SAFETY TIPS:
- Move to a pre-designated shelter, such as a basement
- Stay away from windows
- Get out of automobiles; don't try to outrun a tornado
- Abandon your mobile home; mobil homes offer little protection from tornadoes
- An underpass is not safe: debris can fly underneath it and be deadly. Instead, head for a ditch
The American Meteorological Society and the American Geophysical Union contributed to the information contained in the TV version of this report.
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