Foehn Wind - The Cause of Warm, Dry Conditions On The Lee Side

The Cause of Warm, Dry Conditions On The Lee Side

The condition exists because warm moist air rises through "orographic lifting" up and over the top of a mountain range or large mountain. Because of decreasing atmospheric pressure with increasing altitude, the air expanded and adiabatically cooled at the dry adiabatic lapse rate to the point that the air reaches its adiabatic dew point (which is not the same as its constant pressure dew point commonly reported in weather forecasts). Upon reaching the adiabatic dew point, water vapor in the air begins to condense, with the release of latent heat from condensation slowing the overall rate of adiabatic cooling of the air to the saturated adiabatic lapse rate as the air continues to rise. Condensation is also commonly followed by precipitation on the top and windward sides of the mountain. As the air descends on the leeward side, it is warmed by adiabatic compression at the dry adiabatic lapse rate. Because the air has lost much of its original water vapor content, the descending air creates an arid region on the leeward side of the mountain.