Maximum Sustained Wind - Variation

Variation

Friction between the atmosphere and the Earth's surface causes a 20% reduction in the wind at the surface of the Earth. Surface roughness also leads to significant variation of wind speeds. Over land, winds maximize at hill or mountain crests, while sheltering leads to lower wind speeds in valleys and lee slopes. Compared to over water, maximum sustained winds over land average 8% lower. More specifically, over a city or rough terrain, the wind gradient effect could cause a reduction of 40% to 50% of the geostrophic wind speed aloft; while over open water or ice, the reduction is between 10% and 30%.