Föhn

The föhn (also spelled foehn in the international meteorological literature that uses German Umlaut-free transliteration) is the most celebrated of all the warm, dry winds that descend the leeward slopes of mountain ranges. It occurs when moist air from the Mediterranean side of the Alps is driven northward by pressure systems, rises over the Alpine crest, loses its moisture as precipitation on the Italian side, and descends the northern slopes into the valleys of Switzerland, Austria, and Bavaria as dramatically warmer, drier air. The word Föhn comes from Old High German phōnno, which derived from Latin favonius — the Roman name for the mild west wind that announced the arrival of spring. The Swiss-German Alps word was already in use by the medieval period to describe specifically the warm mountain wind; it was adopted into scientific German and then into international meteorology as the generic term for any warm, dry, leeward mountain wind worldwide.

The föhn's characteristic weather sequence is familiar to residents of Alpine valleys: a long south wind that brings increasingly heavy cloud and rain to the Alpine peaks, a period of violent wind through the mountain passes, and then the sudden dramatic arrival of the warm, transparent air in the northern valleys, with visibility stretching to extraordinary distances, the sky an almost painful blue, and the temperature climbing rapidly. The same air that was depositing snow on the Gotthard Pass arrives in Lucerne or Innsbruck as a summer afternoon in February. The snow on rooftops disappears not by melting from the surface but by sublimation — the föhn's extremely low relative humidity evaporates snow directly into water vapor. Entire meters of mountain snow can vanish in twenty-four hours of föhn.

The föhn's psychological effects have been documented since ancient times and formally recognized in Swiss and Austrian legal tradition. The Roman scholar Pliny the Elder noted the troubling effects of warm Alpine winds on human behavior. Medieval Swiss chronicles record the destruction wrought by föhn-driven fires in Alpine towns — the combination of extreme wind speed and very low humidity makes fire impossible to control, and numerous Swiss and Austrian villages were destroyed by föhn fires in the Middle Ages. More recently, a body of medical and psychiatric literature has examined 'föhn sickness' — documented increases in migraine, insomnia, irritability, and depression in föhn-prone Alpine regions during föhn events. Swiss courts historically admitted föhn conditions as a mitigating factor in certain criminal proceedings, and a study of Munich traffic accident rates found statistically significant increases during föhn days.

The word föhn became the meteorological term for the entire class of warm, dry, leeward mountain winds worldwide, regardless of which mountains or which direction. The chinook of the Canadian Rockies, the Santa Ana of California, the berg wind of South Africa, the Canterbury Northwester of New Zealand — all are classified as föhn-type winds in meteorological literature, with the German word providing the generic category name. This is a significant linguistic achievement: a word derived from the Latin name for the spring west wind, filtered through Old High German into Swiss valley usage, became the international scientific term for a global category of meteorological phenomenon. The föhn effect — the adiabatic warming and drying of air on the leeward side of a mountain range — is one of the foundational concepts of physical meteorology.