STAU e FÖHN.
When a mass of air perpendicularly hit a vertical obstacle like mountains, the wind is forced to climb along the windward slope. During this ascension the mass of air is forced to cool and therefore its volume increases as the elevation increase, pressure it decreases.
If the cooling of the air is strong enough to reach the dewy temperature, the moist contained it condenses. Therefore, numerous clouds are formed on the windward slope and this clouds generate precipitations (rains and/or snow). This it’s the STAU, a common atmospheric phenomenon in the Alps that can occur both on the southern slopes, when from the maritime regions moisty winds arrives as well as on the northern side where the cold winds arriving from North Europe strikes the alpine region.
Once the mass of air went through the crest of the mountain, it continue it run descending the mountain on the leeward slope. The mass of air is by now shoal in how much you/he/she has lost the damp through the precipitations and for compression it increases both the pressure and the temperature. This is the FÖHN, a dry and warm wind that blows from the mountain to the valley.
In meteorology, Mountain Waves are a form of Lee Waves an atmospheric stationary wave. Mountain Waves are generated by the vertical motion that forces periodic changes in speed and direction of the air within this air current. They always occur in groups on the lee side of the terrain that triggers them.
As we have seen earlier, speaking about of the birth of the clouds, when a mass of air perpendicularly hit a vertical obstacle, it’s forced to rise upward. This vertical movement once overcome the top of the mountain produces a wave-flow of air that follows a trajectory characterized by ascending and descending motions. The Lee Wave that is so generated, stays motionless in comparison to the mountain and can produce vortexes (named Rotors), under the flow of the wave on the leeward slope of the mountain; this turbulences are very dangerous for small airplanes.
LENTICULAR CLOUDS(Altocumulus lenticularis; Lat. - abbreviation Ac len)
The mass of air that crosses the crest of the Mountain Wave, does an adiabatic process (an adiabatic process occurs without transfer of heat or mass between a thermodynamic system and its surroundings.) and generate a lens shaped well-defined cloud that doesn't produce any precipitation.
Dramatic Cap Cloud formation over Monte Bianco crest, on a cold January sunset.
Lenticular Clouds stays motionless in the sky in a leeward position respect the object that has produced the wave (in our case the top of the Monte Bianco). This happens because despite the moist particles of the cloud races very quickly, pushed by the strong wind, the points where moist condenses and evaporates doesn’t move because these two points are perfectly still comparing them to the mountain.