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Tornado Season
Tue May 10, 2005
Listen in RealAudio 
With apologies to the Mamas and Papas, California dreamin' wasn't supposed to include tornadoes. Hi, I'm Bryan Yeaton, and this is The Weather Notebook.
But it's no dream: California really does get its share of twisters, and they're more likely to hit on a winter's day rather than in the warm season. That might seem odd to someone from Kansas or Texas. The big, bad tornadoes that brutalize the Great Plains tend to run wild in the spring. That's when warm, moist air from the Gulf of Mexico flows underneath upper winds that are still chilly.
Like a hot burner under a teakettle, this contrast of cool air on top of warm air helps get the atmosphere bubbling. But in California, you don't need much heat to produce a twister. The state hardly ever gets really vicious tornadoes. What it does get are a lot of little ones--a few each year on average. Australia, too, gets a lot of these weaker tornadoes, so a research group of Aussies and Americans recently took a closer look. They found that in both places, the culprit was wind shear. As the wind gets pushed around California's mountains ahead of a winter storm, it helps give thundershowers the spin they need to produce twisters, even without much heating. The winds in a California twister don't usually get much above 100 miles an hour, but that can still cause trouble.
In the last century, Los Angeles County has seen over 40 tornadoes and more than 40 injuries. So while it's snowing this winter in the Midwest, Helen Hunt may have Twister work to do back in Hollywood.
Writer Bob Henson sent us today's story. Visit The Weather Notebook on the Web, at www.weathernotebook.org. We are supported through grants from Subaru of America, and the National Science Foundation.
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