The surface layer is the part of the boundary layer where the frictional forces of the earth’s surfaces are felt by the atmosphere. By definition, those forces must approach zero at the top of the surface layer and must reach their maximal close to the earth’s surface. Between these two extremes, wind velocities in the surface layer usually decrease logarithmically with decreasing height above the earth’s surface (Figure 7.1). The steepness of the curve and how far above the ground winds will be slowed by surface friction depends in part on the roughness of the surface terrain. Air flowing over rough surfaces such as those containing boulders, trees, or hills will experience more friction than air flowing over a smooth surface (for example, a frozen lake). As a result, the surface layer above a rough surface will reach higher into the atmosphere because air currents further aloft will be slowed by the greater frictional forces exerted by the rougher surface.