Tornado facts and history

Jet streams flow from west to east in the uppe...

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Some interesting historical information about tornadoes in the U.S.  I never knew that they occur only in the U.S.  See: Despite Advances, Tornado Forecasts Show Limits. NY Times, May 31.

One factor is that, for unknown reasons, 2011 has had many more tornadoes than other recent years. Another is what the historian Thomas Grazulis describes with a single word: coincidence.

Meteorologists in those days knew the basic science about tornadoes — that they are caused by masses of warm, humid air from the Gulf of Mexico rising through cold, dry air moving south from Canada. The high-altitude jet stream carries strong winds that torque the warm air, making it rotate.

These two conditions — warm air in the south and fast-moving cold winds swooping down the alley between two mountain ranges (the Rockies and the Appalachians) — exist nowhere else in the world, said Jeff Masters, director of meteorology for Weather Underground, a service based in San Francisco.

For deep details, see Tom Grazulis’s own website, the Tornado Project.