Before thunderstorms develop, a change in wind direction and an increase in wind speed with increasing height creates an invisible, horizontal spinning effect in the lower atmosphere. Such as southeast winds at the surface and stronger westerly winds aloft in the image above...causing the spinning effect.

Rising air within the thunderstorm updraft tilts the rotating air from horizontal to vertical.

An area of rotation, 2-6 miles wide, now extends through much of the storm. Most strong and violent tornadoes form within this area of strong rotation.

A lower cloud base in the center of the above photograph identifies an area of rotation knowns as a rotating wall cloud. This area is often nearly rain-free. Note the rain in the background.

Moments later...at Woodward, Oklahoma...a strong tornado develops in this area. Softball-size hail and damaging "straight-line" winds also occurred with this storm.