The environmental wind field is also very important in determining what type of a thunderstorm could develop. Severe weather usually occurs when the change in horizontal winds (wind shear) is significant. This includes the change in wind direction (directional shear) or speed (speed shear).

Speed Shear
In speed shear, the wind increases in speed from the surface to the upper levels, as shown in this diagram by the arrows. This vertical shear creates horizontal rotation which can best be visualized by placing a paddle wheel in the environment. Besides the rotation, the change in wind speed with height lets the updraft separate from the downdraft, allowing the storm to survive longer and even become stronger.

Directional Shear
Directional shear refers to a change in the direction of the wind with height. Notice in this image there is both speed and directional shear as both the angle and the length of the wind vectors are changing with height. Two viewpoints are shown in the following figure. The length of the arrows represents the wind speed. The arrows point in the direction that the wind is blowing and are located at different heights in the column of air shown. Both speed and directional changes occur from one level to another.

Illustration of both directional and speed shear
Image by Bramer

Modelers run experiments with storms developing in different amounts of wind shear. Differing amounts of wind shear can determine whether a storm remains small or becomes a supercell.