Foehn Effect (steps)


(1) Relatively stable, mild (e.g 20°C) but moist air starts at sea-level and has to cross the 3000m-mountain range situated parallel to it. The air is now forced to ascend and to glide up and over the windward mountain slopes. While initially decompressing and thus cooling at the dry adiabatic lapse rate of 1°C/100m, the air will eventually become saturated and the water vapour will condensate. Thus clouds will form and rain is setting in. The height where this happens is called condensation-level (at 600m a.s.l. in our example).

(2) Condensation releases heat and the further lifted air will cool somewhat slower now, lets say at a typical rate between 0.5 and 0.65°C/100m. This is called the wet or saturated adiabatic lapse rate. Once over the the mountain crest, the air will also have reached it's lowest temperature - somewhat around 1°C in our example.

(3) A so-called lee wave forms downwind of the obstacle. The downward motion of the initial wave forces the now-dry air to plummet from relatively high levels to the foothills, now warming by compression at the dry adiabatic lapse rate. Thus for every 100m drop in elevation, temperature raises 1°C again. The wind speed increases and the vigourous foehn storm can easily reach gale force.

(4) Air temperature on the lee-side is now significantly higher than the same air on the upslope side of the mountains.