Since a wind turbine generates electricity from the energy in the wind, the wind leaving the turbine must have a lower energy content than the wind arriving in front of the turbine.

This follows directly from the fact that energy can neither be created nor consumed. If this sounds confusing, take a look at the definition of energy in the Reference Manual.

A wind turbine will always cast a wind shade in the downwind direction.

In fact, there will be a wake behind the turbine, i.e. a long trail of wind which is quite turbulent and slowed down, when compared to the wind arriving in front of the turbine. (The expression wake is obviously derived from the wake behind a ship).

You can actually see the wake trailing behind a wind turbine, if you add smoke to the air passing through the turbine, as was done in the picture. (This particular turbine was designed to rotate in a counterclockwise direction which is somewhat unusual for modern wind turbines).

Wind turbines in parks are usually spaced at least three rotor diameters from one another in order to avoid too much turbulence around the turbines downstream. In the prevailing wind direction turbines are usually spaced even farther apart, as explained on the next page.