1. Air Rotates Because of Wind Shear.
How the column of air begins to rotate is not completely understood by scientists, but one way the rotation appears to happen is when winds at two different altitudes blow at two different speeds creating wind shear. For example, a wind at 1000 feet above the surface might blow at 5mph and a wind at 5000 feet might blow at 25mph. This causes a horizontal rotating column of air.

2. Faster Spin Makes a Funnel Cloud.
If this column gets caught in a supercell updraft, the updraft tightens the spin and it speeds up (much like a skater's spins faster when arms are pulled close to the body. A funnel cloud is created.

3. The Funnel Rotates and Touches Down.
The rain and hail in the thunderstorm cause the funnel to touch down creating a tornado.

HOW ARE TORNADOES FORM???