How Tornadoes Form

How Tornadoes Form

A tornado can form where winds blow at different speeds causing rotation (A) which can tighten and speed into a funnel cloud if caught in the flow of air moving into the storm (B). Rain and hail can cause the funnel to bend downward (C).
Credit: UCAR

Only about one thunderstorm in a thousand produces tornadoes. Most tornadoes form during rotating supercell thunderstorms.

How the column of air begins to rotate is not yet completely understood, but one way the rotation appears to happen is when winds at different altitudes blow at different speeds, creating wind shear. For example, a wind at 1000 feet above the surface might blow at a speed of five miles per hour and a wind above that, at 5000 feet, might blow at 25 miles per hour. A horizontal rotating column of air can form at the boundary between these two winds.

If the rotating column of air gets caught in the flow of air moving up into the storm (an updraft), the spin tightens and speeds up, much like a skater's spins faster when arms are pulled close to the body, creating a funnel cloud. The rain and hail in the thunderstorm cause the funnel to bend downward. If it touches the ground, it’s a tornado.

Most tornadoes spin in the same direction - counterclockwise when viewed from above. A few spin clockwise.  The direction that a tornado spins is probably related to the direction of rotation in the thunderstorm.