Terrain influences local winds by physically diverting and altering the normal flow. Air tends to follow
valleys. Air moving up a hillside remains on the ground and travels down the other side of the hill. As it crosses
the crest, it tends to eddy. The sharper the crest, the stronger the eddy, as shown in Figure 4.
Figure 4. Eddy or swirl effects.
Obstacles, such as buildings, large rocky structures, and groves of small trees, cause eddies which will affect
wind for a downwind distance of 15-20 times the height of the obstacle as shown in Figure 5.
Figure 5. Effects of a large building.