THOSE of you lucky enough from time to time to have been able to contemplate life's mysteries while gently strolling through little villages in Switzerland, may have noticed here and there that a notice saying "Das Rauchen ist bei Fohnwetter im Freien verboten". It tells you that smoking out of doors is not permitted when the fohn, or foehn, is blowing, and refers to a hot, dry wind which is a frequent visitor to many Alpine valleys in the wintertime. The word is believed to have its origins in the Latin word favonius, the "west wind", since to the Romans the most striking characteristic of favonius was, not its direction, but its warmth.
Prior to the onset of the foehn, the weather may have been unusually cold, and the snow covered valley filled with damp and stagnant air. Then comes a sudden change the wind begins first in spasmodic gusts and later in a strong and steady current. It brings a palpable rise in temperature, sometimes by as much as 20 in a few hours, and a very noticeable drop in the humidity. Indeed the air can become so dry that wooden houses are liable to catch fire from stray sparks and whole villages have been known to have been destroyed hence the prominent enjoinders to reduce the risk.
It is the mountains that create the foehn. If moist air is forced upwards by rising ground, its moisture is often extracted in the form of rain on the windward slopes. The stream of air which then flows down the leeward side is relatively dry, and for complex reasons related to its dryness, its temperature increases very rapidly on the descent, to be experienced as a warm, dry breeze by the down wind population.
It is remarkable, however, that some residents of the Alpine valleys known precisely when a foehn is imminent. For many of them it is preceded by a range of symptoms known collectively as "foehn disease", whose sufferers experience headache, nausea and sleeplessness, together with irritability, depression and a general feeling of debility. Medical examination reveals an increased pulse rate and a fall in blood pressure.
Nobody knows exactly what causes foehn disease. For many years it was believed to be associated with a characteristic oscillation in atmospheric pressure which often precedes the warm wind a variation of several hectopascals over a few minutes. But such pressure changes are now regularly experienced in lifts and aeroplanes with no ill effects, even in the case of persons with a history of foehn disease. The ailment, therefore, remains one of the great unsolved mysteries of bio meteorology.