The Föhn: a Dry, Hot Wind

The surface of the earth creates wind effects of various kinds. One of the most important is the föhn, a dry, hot wind in the lee of a mountain

The föhn, also called foehn by those who haven't figured out how to make an umlaut on their computers, can be found in the lee of most mountain ranges. The word is German and originally referred to European winds on the north side of the Alps. The word is now used in other parts of the world, but these winds are often known locally by colorful names like Chinook, Garmoosh, Zonda, and Hnukapeyr.

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The Cause of the Föhn

The föhn, unlike many atmospheric phenomena, has a simple explanation. Moist air hitting a mountain range is obliged to rise; rising air cools; cool air cannot carry as much moisture as warm. The result is that moisture in the rising air condenses, releasing latent heat. Thus as the air ascends the mountain, the cooling with height is mitigated by the warming of condensation. The condensed water vapor falls as rain or snow.

When the air descends the mountain, it has left its moisture behind, along with its latent heat; now it warms at a faster rate than that at which it cooled. At the bottom of the mountain, the air will be hot and dry.

There are fancy names for the rate at which air cools as it ascends and warms as it descends: if the air is not saturated, it warms or cools at the "dry adiabatic lapse rate"---about 5 degrees Fahrenheit per 1000 feet; if the air is saturated and rising, it cools at the "wet adiabatic lapse rate" of approximately 3 degrees per 1000 feet. So if saturated air meets a mountain that is 10,000 feet high, the air will be 20 degrees warmer when it reaches the bottom of the lee side.

Where Does a Föhn Occur?

Föhn winds occur on every continent, including Antarctica. They can raise the temperature to searing levels. The Santa Ana wind of southern California is the most pronounced form of the föhn in the United States.

Meteorological Effects of the Föhn

Aside from rain on the windward side of the mountain and hot, dry air on the lee side, a föhn often leaves as a calling card a peculiar kind of cloud. The lenticular cloud is formed from leftover moisture at the top of the mountain, and is a round, flat cloud, almost like a pancake.

Effects of the Föhn on People

Föhn winds have been associated with various illnesses, but the links have never been proven to the satisfaction of rigorous scientists. There have been studies that show an increase in depression and suicide rates, but all kinds of weather that deviates from normal can put people in a bad mood.

One result of the föhn that is not disputed is its ability to melt snow---in fact it has the nickname snow-eater in some places. The reasons are obvious: the air is warm; the air is dry. In fact the air is often so dry that the snow disappears by the process known as "sublimation," in which ice turns directly into water vapor without going through the liquid stage.

The Santa Ana winds near Los Angeles can create an extreme fire hazard. The effects of the subsiding air can be exacerbated by the funneling of fierce winds through local canyons. The combination of the hot, dry winds with unwise forest management and construction in vulnerable areas has put homes and lives at risk.

Reference:

http://polarmet.osu.edu/PolarMet/PMGFulldocs/speirs_steinhoff_jc_2010.pdf