We generally think of mountains as places of cold, ice, and snow thanks to their great height, and in general, they are indeed locations where Arctic temperatures persist summer and winter. We would also expect them to be the source of cold winds -- but, thanks to the nature of air and the way it heats and cools, mountains can also produce warm, dry, powerful downslope winds that can raise the temperature by 50 degrees Fahrenheit or more, melting snow and buffeting mountain-climbers dangerously. These winds, known by a range of different names worldwide -- from "snow eater" to "Chinook" to "Helm winds," but the name which is most widely accepted to describe all of these winds is the German name "Foehn."
Foehn winds occur on the downwind, 'rain shadow' side of mountains in many different places in the world. They are important weather phenomena both on a local scale -- where they can melt snow suddenly, swelling streams, as well as endangering mountain-climbers -- and on a larger scale when a major mountain range is involved, as with the central European alps. They are also capable of rapidly spreading wildfires in the areas they blow over, since they are strong and very dry, and cause a drop in humidity as well as a rise temperature.
Wherever air flows up one slope of a mountain or mountain range, and flows down the other, a Foehn can occur -- a strong, hot wind that can raise the temperature from winter to summer in a few hours, melting snow, weakening ice, and warming those who dwell in the shadow of the peaks.
The mechanics of a Foehn wind
Foehn winds are caused by the same forces that make a 'rain shadow' on the downwind side of mountains -- the cooling of air as it rises, climbing one side of the mountains, and the warming of the same air as it slides down the further side of the mountains, sinking and gaining heat through compression.
As an air mass approaches a range of mountains like the Rockies or the Alps, it is forced upwards by the rising ground. As the air rises, it cools, because it becomes less dense as the pressure of atmosphere atop it lessens. Gases cool as they become less dense, because their molecules get farther apart and cannot transmit or retain heat as efficiently. The rate of cooling is about 30 degrees Fahrenheit for every mile that the air ascends, so if the air rises two miles above sea level as it crosses a mountain range (since its rise will be higher than the physical height of the mountains, as it passes over their summits), then it will cool 60 degrees Fahrenheit.
This process of cooling also 'wrings out' the moisture from the air, since cool air can hold far less moisture than warm air. The moisture falls on the windward side of the mountain range as rain or snow -- hence the deep snows of the Sierra Nevada, or the 'rain forest' of the Pacific Northwest. The dry air mass that results can rain very little -- or not at all -- on the land beyond the mountains, so many ranges create 'rain shadows' which may be dry country (like Wyoming) or desert (like Arizona).
The air that passes over mountains may 'fall' down the other side, depending on the topography. Rushing downslope under its own weight, picking up speed as it goes, this air heats rapidly as it descends and compresses. Thus, it becomes a Foehn -- a warm or hot wind from the mountains, created by the force of gravity and the fact that air heats as it becomes denser. The Foehn can raise temperatures by 50 F or more, and lower humidity by up to 20%, radically altering a day from cool and moist to warm and dry.
The effects of Foehns
Hurtling down the mountainsides, raising temperatures abruptly in the space of just a few hours, Foehns have dramatic effects on the local climate and weather. They can cause dangerous conditions in several different ways. They melt snow and ice quickly, potentially causing streams to grow stronger or even flood. They can also trigger avalanches, both by the direct pressure of wind and the loosening of snow due to partial melting. Foehns also have beneficial local effects, such as moderating the climate and preventing the too-rapid spread of glaciers into inhabited land.
On a larger climatic scale, Foehns may actually increase the warmth of central Europe and other regions where they occur. Germany is much warmer than areas of the United States at similar latitudes, and this may be due to the numerous Foehns that blow down the eastern flanks of the Alps. These dramatic effects show how significant Foehn winds are -- they also have a part in shaping the world's whole climate.