As illustrated on this drawing, the turbulence is more pronounced behind the obstacle than in front of it. Moreover, the turbulence zone can extend to some three time the height of the obstacle. Therefore, it is preferable to avoid major obstacles close to wind turbines, particularly if they are upwind in the major wind direction.

Obstacles will then decrease the wind speed downstream from the obstacle, creating a shelter behind the obstacle. The decrease in wind speed largely depends on the porosity of the obstacle. As example, a tree during winter has a much greater porosity than a building, and therefore the shelter effect is weaker behind the tree than behind the building. The same tree will have a weaker porosity during summer, when its foliage will be denser. The porosity can actually be defined as the open area of the object divided by its total area facing the wind.
The slowdown effect of an obstacle on the wind also increases with its height and length. This effect is finally larger close to the obstacle, and close to the ground.

It is then very important than people planning to install a wind turbine take into account every obstacle close to the turbine, particularly in the upstream direction (typically 1 kilometre in the main wind direction).

Wind Shade

This graph presents the impact on wind speeds of a typical rough obstacle. Wind speeds decrease substantially behind the obstacle. The wind shade is represented by the different shades of grey. The blue numbers in boxes indicate the wind speed in percent of the wind speed without the obstacle. In this example, the obstacle is a 20 metres tall and 60 metres wide building placed at a distance of 300 m from a wind turbine whose hub is at a 50 meters height. We can see that at the top of the yellow tower, the wind speed has lost 3 percent. This loss means a loss of nearly 10 percent in energy (1-0.973=8.73%).