Foehn

From AMS Glossary
Jump to: navigation, search


foehn

(Or föhn.) A warm, dry, downslope wind descending the lee side of the Alps as a result of synoptic-scale, cross-barrier flow over the mountain range.

The winds are often strong and gusty, sometimes forming downslope windstorms as a result of mountain wave activity. The air in the near-surface flow originates at or above the main crest height of the Alpine barrier, and achieves its warmth and dryness as a result of adiabatic descent. The foehn often replaces a retreating cold air mass from a polar or arctic front, producing dramatic temperature rises that reach 10°C and occasionally even 20°C or more, sometimes in a matter of minutes. This is especially true of the south foehn, which blows from northern Italy, where the air is warm, to the north of the Alps (Austria, Germany, Switzerland), where the air is cooler and could be cold arctic air as just described. The north foehn, blowing from a cooler to a warmer region, produces less dramatic temperature changes. The air in the foehn, originating from the mid troposphere, is characteristically clean. Its warm temperatures rapidly melt (or sublimate) snow, sometimes producing flooding, and the extreme dryness can lead to dangerous fire weather conditions. The Alpine foehn has been extensively studied by European scientists, and it is recognized as the type wind for similar downslope winds, resulting from cross-barrier flow, in other parts of the world. In other mountain ranges the foehn has a variety of local names, including chinook in the Rocky Mountains in North America; zonda for a westerly foehn from the Argentine Andes; ljuka in Carthinia (northwestern Croatia); halny wiatr in Poland; austru in Romania; and favogn in Switzerland. A northeasterly foehn descending the Massif Central in France and extending over the Garonne Plain is locally called aspre. A dry wind from the northwest descending the coastal hills in Majorca is named the sky sweeper. In New Zealand a foehn blowing from the New Zealand Alps onto the Canterbury Plains is the Canterbury northwester. A cross- barrier flow that produces strong winds and cooling is called a bora in many parts of the world. Many authors have attempted to classify strong wind events as foehn (or chinook) or bora, for example, for climatologies. These studies have had mixed success: Many wind events are easy to classify, but a number of events are difficult, depending on the data available (most studies attempt to use surface data) and the method used to differentiate between the two types of events.
See foehn phase, high foehn.

Defant, F. 1951. Compendium of Meteorology. 667–669.

Personal tools
Namespaces
Variants