Copied from SKYVIEW WEATHER
http://www.skyviewweather.com/learning/tornado-season/
(Begin quote) A tornadic thunderstorm can form where moist, warm air gets trapped beneath warm, dry air under a stable layer of cold, dry air. This air sandwich is called an inversion.
If the cap is disturbed by a front or movement in the upper atmosphere, the warm, moist air can punch through the stable air above it. The condensing of water vapor releases . . . (End quote)
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