From WindsportAtlanta.com: Wiki
The effect of an object upwind obstructing or creating turbulence in the wind flow.
- Other sail craft
- Other boats
- Shoreline objects including trees and buildings
- Geographical features such as hills or mountains
- Swell and waves in the water. While this might also be included as surface friction, the effect, nonetheless, is the same. For example, this makes it more difficult to punch through a surf zone as the waves disturb and block the wind, reducing your power at times when you need it most. As well, it can make waterstarting in a large swell more difficult as the wind is blocked or reduced when you are in a trough.
As a rule of thumb, effects of local, upwind objects can extend a distance of ten to fifteen times the height of the obstruction.