Foehn wind 3) Mechanical mixing: When river water passes over rocks, turbulence is generated in the form of rapids, and white water reveals the turbulent mixing of the water with the air above. Similarly, as air passes over mountains, turbulence occurs and the atmosphere is mixed in the vertical. This mixing generally leads to a downward warming and upward moistening of the cross-mountain airflow, and consequently to warmer, drier foehn winds in the valleys downwind. 4) Radiative warming: Dry foehn conditions are responsible for the occurrence of rain shadows in the lee of mountains, where clear, sunny conditions prevail. This often leads
Foehn wind it expands and cools due to the decrease in pressure with height. Since colder air can hold less water vapour, moisture condenses to form clouds and precipitates as rain or snow above the mountain's upwind slopes. The change of state from vapour to liquid water is accompanied by heating, and the subsequent removal of moisture as precipitation renders this heat gain irreversible, leading to the warm, dry foehn conditions in the mountain's lee. This mechanism has become a popular textbook example of atmospheric thermodynamics and it lends itself to attractive diagrams. However the common occurrence of 'dry' foehn events, where