“Over open ground, substantial vertical wind velocity gradients commonly exist due to friction between the moving air and the ground. Wind speed profiles are strongly dependent on the time of day, weather conditions and the nature of the surface. The wind speed, in the absence of turbulence, typically varies logarithmically up to a height of 30 to 100 meters, then negligibly thereafter. As a result of this velocity gradient (and the resulting change in sound velocity which it causes), a sound wave propagating in the direction of the wind will be bent downward. In the upwind direction the sound speed decreases with altitude, sound waves are directed upward, away from the ground, forming a “shadow zone” into which no direct sound penetrates (Figure 3). This process is called refraction, whereby the path of sound waves curves in the direction of the lower sound velocity. The radius of curvature of the sound path is inversely proportional to the velocity gradient. Sound always refracts toward the lower sound speed.”1
1. John S. Lamancusa, Noise Control (Pennsylvania: Pennsylvania State University, 2000), np.