Tornadoes are caused by powerful thunderstorms that have the right mix of unstable air and changing winds with height. These conditions make rising air spin and can focus that spin into a tight, fast-moving column that reaches the ground.
Key ingredients
Tornadoes almost always form from severe thunderstorms in air that is warm, moist, and unstable near the ground with cooler, drier air above, which encourages strong updrafts. Another critical ingredient is wind shear, meaning wind speed and/or direction change significantly with height, which creates horizontal spin in the atmosphere that storms can tilt upright.
Role of supercells
The most destructive tornadoes usually come from supercell thunderstorms, which are storms with a long-lived, rotating updraft called a mesocyclone. In these storms, the rotating updraft and surrounding downdrafts concentrate and stretch the rotation, increasing its speed until a visible funnel and then a tornado can develop.
How the funnel reaches the ground
Inside the storm, rising warm, humid air and descending cool air interact, creating sharp temperature and pressure contrasts around the rotating core. As the rotation is stretched and pressure near the ground drops, a condensation funnel can extend downward; once this rotating column reaches the surface, it is a tornado.
Other situations that can produce tornadoes
Not all tornadoes come from classic Great Plains supercells; some form from smaller storms, squall lines, or along boundaries such as cold fronts and dry lines where air masses sharply meet. Tropical storms and hurricanes can also spawn tornadoes, typically in the outer rainbands on the right-front side of the storm as it moves inland.
