Causes Foehn wind

the warm moist air northern italy blocked on windward side, loses of water vapour content, , descends on french plateau , valley of mont-cenis range in maurienne valley

explanations of foehn warming , drying effect in popular literature or on web single out 1 causal mechanism (#1 - condensation , precipitation - in below), there in fact 4 known causes (illustrated in schematic @ top right of page). these mechanisms act together, contributions varying depending on size , shape of mountain barrier , on meteorological conditions, example upstream wind speed, temperature , humidity.

1) condensation , precipitation: when air forced upwards on elevated terrain, expands , cools due decrease in pressure height. since colder air can hold less water vapour, moisture condenses form clouds , precipitates rain or snow above mountain s upwind slopes. change of state vapour liquid water accompanied heating, , subsequent removal of moisture precipitation renders heat gain irreversible, leading warm, dry foehn conditions in mountain s lee. mechanism has become popular textbook example of atmospheric thermodynamics , lends attractive diagrams. common occurrence of dry foehn events, there no precipitation, implies there must other mechanisms.

rotor cloud revealing overturning , turbulence above lee slopes of antarctic peninsula during westerly foehn event.

2) isentropic draw-down (the draw-down of warmer, drier air aloft): when approaching winds insufficiently strong propel low-level air , on mountain barrier, airflow said blocked mountain , air higher near mountain-top level able pass on , down lee slopes foehn winds. these higher source regions provide foehn air becomes warmer , drier on leeside after compressed descent due increase in pressure towards surface.

3) mechanical mixing: when river water passes on rocks, turbulence generated in form of rapids, , white water reveals turbulent mixing of water air above. similarly, air passes on mountains, turbulence occurs , atmosphere mixed in vertical. mixing leads downward warming , upward moistening of cross-mountain airflow, , consequently warmer, drier foehn winds in valleys downwind.

4) radiative warming:

dry foehn conditions responsible occurrence of rain shadows in lee of mountains, clear, sunny conditions prevail. leads greater daytime radiative (solar) warming under foehn conditions. type of warming particularly important in cold regions snow or ice melt concern and/or avalanches risk.