Commonly called a “fern wall,” the bank of clouds behind the foothills is actually a “Foehn” wall. The name Foehn (German: Föhn, pronounced [ˈføːn]) arose in the Alpine region. Originating from Latin (ventus) favonius, a mild west wind of which Favonius was the Roman personification When winds blow over elevated terrain, air forced upwards expands and cools due to the decrease in pressure with height. Since colder air can hold less water vapor, moisture condenses to form clouds and precipitates as rain or snow on the mountain’s upwind slopes. The change of state from vapor to liquid water releases latent heat energy which heats the air, partially countering the cooling that occurs as the air rises. The subsequent removal of moisture as precipitation renders this heat gain by the air irreversible, leading to the warm, dry, Foehn conditions as the air descends in the mountain’s lee. This mechanism has become a popular textbook example of atmospheric thermodynamics. However, the common occurrence of ‘dry’ Foehn events, where there is no precipitation, implies there must be other mechanisms.
I learned the term from a glider pilot. I’ve found that glider pilots have a special affinity to and knowledge of weather and wind movements.
In Boulder, the sight of the Foehn wall suggests an incoming snow event in the next day or two. Here, the line of geese is the crowning touch to the composition of the picture. Foehn Walls are generally seen in Boulder only in the winter as a harbinger of a snow event. This appears to be a generally uncommon use of the word, but I’ve seen it too many times to believe it is anything but the predictor of snow…