Great read from National Geographic discussing tornadoes:
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.)