The air stops rising at high levels and spreads sideways to form a characteristic anvil-shaped storm cloud. For a supercell to develop, the winds at higher elevations must differ markedly from those at lower levels in speed, direction, or both. Such a large difference in wind speed or direction is called wind shear. Wind shear makes the column of rising air begin to rotate, forming a broad, horizontal tube of swirling air. As the storm continues, this tube turns on its end, producing a rotating column of air called a mesocyclone.
Studies show that most supercells containing mesocyclones eventually produce tornadoes.
A low, dark, heavy cloud called a wall cloud forms underneath the mesocyclone. Tornado funnels develop out of the wall cloud. The first sign of an approaching tornado may be light rain, followed by heavier rain, then rain mixed with hail. The hailstones may grow to the size of golf balls or even baseballs. After the hail ends, a tornado may strike.