Although tornadoes occur in many parts of the world, these destructive forces of nature are found most frequently in the United States east of the Rocky Mountains during the spring and summer months. Stronger west winds aloft come into contact with southeast winds near the ground. 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. The rotation goes vertical. Rising air with the thunderstorm updraft tilts the rotating air from horizontal to vertical.
Tornadoes occur as a result of the rotation.
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. Ron Przybylinski A lower cloud base in the center of the photograph identifies an area of rotation know as a rotating wall cloud. This area is often nearly rain-free. Note the rain in the background.