Effect of Friction

Abstract

Friction reduces wind velocity and therefore weakens the apparent acceleration due to the Coriolis effect. Because the pressure gradient is not directly affected by the presence of friction, the pressure gradient acceleration will exceed the weakened Coriolis effect in an environment in which friction is non-negligible. The result is that regardless of hemisphere, around any enclosed area of high pressure at the surface (anticyclone), air flows outward at some angle less than 90 degrees across the isobars (i.e., is ageostrophic), and around any enclosed area of low pressure (cyclone), air flows inward at some angle less than 90 degrees across the isobars. The angle of flow across the isobars increasingly approaches 90 degrees as friction increases. Likewise, aloft, where friction is negligible, the counter-clockwise circulation (in the Northern Hemisphere) around cyclones does NOT flow much inward, but instead goes in a more circular counter-clockwise fashion. The ageostrophic nature of the near-surface flow ultimately destroys the highs and lows. After enough air moves out of the high-pressure center, it is no longer a high-pressure center, and after enough air moves into the low-pressure center, it is no longer a low-pressure center. Therefore, anticyclones and cyclones appear and disappear more quickly at the surface, where friction is strongest, than aloft. These anticyclones and cyclones support distinctive characteristic weather conditions.