A “Fern Wall” Behind The Foothills Sets Of A Row Of Geese On Lake Wanaka. Photo by Lenny “Lensworth” Frieling

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. 

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Lenny “Lensworth” Frieling

Lenny Frieling Pen Of Justice

• Multi-published and widely syndicated blogger and author.
• Most recently published by Amazon, his first book, "Lensworth"a book of his prize-winning photos.
• University lectures at University. of Colorado, Boulder, Denver University Law School, Univ. of New Mexico, Las Vegas NM, and many other schools at all levels. Numerous lectures for the NORML Legal Committee
• Former Judge
• Media work, including starring in episodes of Fox’s Power of Attorney, well in excess of many hundreds media interviews, appearances, articles, and podcasts, including co-hosting Time For Hemp for two years.
• Life Member, NORML Legal Committee, Distinguished Counsel Circle.
• Photographer of the Year, AboutBoulder 2023
• First Chair and Originator of the Colorado Bar Association’s Cannabis Law Committee, a National first.
• Previous Chair, Boulder Criminal Defense Bar (8 years)
• Twice chair Executive Counsel, Colorado Bar Association Criminal Law Section
• Life Member, Colorado Criminal Defense Bar
• Board Member Emeritus, Colorado NORML, and prior chair during legalization, as well as pre and post legalization
• Chair, Colorado NORML, 7 years including during the successful effort to legalize recreational pot in Colorado
• Senior Counsel Emeritus to the Boulder Law firm Dolan + Zimmerman LLP : (720)-610-0951
• Board member, Author, and Editor for Criminal Law Articles for the Colorado Lawyer, primary publication of the Colorado Bar Assoc. 7 Years, in addition to having 2 Colorado Lawyer cover photos, and numerous articles for the Colorado Lawyer monthly publication.
• http://www.Lfrieling.com