The Foehn Cloud

Figure 3: Altocumulus lenticularis over the Swiss Alps, © Ahdigital, CC BY-SA 3.0

The Foehn wind is usually accompanied by characteristic lee clouds, the lenticularis clouds (Altocumulus lenticularis). They are stationary clouds formed at the wave crest of a vertically oscillating air current after it has passed a mountain chain. They are typically found at altitudes of 2000 to 8000 meters. Lenticularis clouds can be composed of several layers on top of each other which make them impressive to look at. This is the result of varying humidity in the vertical profile.

Lenticularis clouds are often associated with severe turbulence, for which reason they are avoided by powered aircraft pilots. Glider pilots instead actively seek them out to exploit their updrafts.