Topography

The structure and orientation of terrain features will often influence and even control air motion and mechanical turbulence in the lower atmosphere. Small hills and trees may alter wind speed and direction while also enhancing mechanical turbulence. Larger terrain features such a mountains, hills and valleys may be a governing factor in the control of wind speed and wind direction. For example, wind may be diverted around large hills and mountains and channeled through valleys. Terrain features may also cause important diurnal variations of wind. A common cause for air pollution episodes in a valley which contains numerous pollutant sources, is the occurrence of a persistent temperature inversions.