Aerial photograph
© 1999 Soren Krohn

Wind Shear and Escarpments

Do not Include the Altitude of Your Terrain in Wind Shear Calculations
The aerial photograph above shows a good site for wind turbines along a shoreline with the turbines standing on a cliff which is about 10 m (30 ft.) tall. It is a common mistake to believe that in this case one may add the height of the cliff to the height of the wind turbine tower to obtain the effective height of the wind turbine, when one is doing wind speed calculations, at least when the wind is coming from the sea.
This is patently wrong. The cliff in the front of the picture will create turbulence, and brake the wind even before it reaches the cliff. It is therefore not a good idea to move the turbines closer to the cliff. That would most likely lower energy output, and cause a lower lifetime for the turbines, due to more tear and wear from the turbulence.
If we had the choice, we would much rather have a nicely rounded hill in the direction facing the sea, rather than the escarpment you see in the picture. In case of a rounded hill, we might even experience a speed up effect, as we explain later when we get to the page on the hill effect.