Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 123

this reason, smoking and fires (even for cooking) have traditionally been forbidden in
many villages in the Alps during a foehn. In some cases, special guards ( Föhnwächter )
were appointed to enforce the regulations. In New Zealand, foehn winds commonly en-
train glacial sediments, causing dust storms that degrade grasslands (McGowan and
Sturman 1996b). The foehn is purported to cause various psychological and physiolo-
gical reactions, including a feeling of depression, tension, irritability, muscular convul-
sions, heart palpitations, and headaches (Cooke et al. 2000). The suicide rate is said to
rise during the foehn; however, analysis of large data sets does not support this claim
(Deisenhammer 2003).
In spite of its hazards, the foehn is generally viewed with favor, since it provides res-
pite from the winter's cold and is very effective at melting snow (McGowan 2004), a fact
reflected in many local sayings from the Alps, for example: “If the foehn did not inter-
fere, neither God nor his sunshine would ever be able to melt the winter snows” “The
foehn can achieve more in two days than the sun in ten” and “The wolf is going to eat
the snow tonight” (De La Rue 1955: 36-44). In North America, the Chinook in the Great
Plains is awaited after the deep snows of winter.
The causes of the foehn are complex. One of the early explanations in the Alps was
that the warm dry wind came from the Sahara Desert. The wind was usually from the
south, so this seemed a perfectly logical conclusion, until one day somebody climbed to
the side of the mountain from which the foehn was coming and found that it was raining
there, a very unlikely effect for a Saharan wind to produce! The Austrian climatologist
Julius Hann (1903) is given credit for the correct explanation. When air is forced up a
mountain slope, it is cooled at the dry adiabatic rate, 3.05°C per 300 m (5.5°F per 1,000
ft) until the dew point is reached and condensation begins. From this point on, the air is
cooled at a lower rate (wet adiabatic rate) of approximately 1.7°C (3°F) (Fig. 3.28). On
the lee side of the summit, precipitation ceases and the air begins to descend. Under
these conditions, the air is warmed at the dry adiabatic rate, 3.05°C per 300 m (5.5°F
per 1,000 ft) for the entire length of its descent. Consequently, the air has the potential
to arrive at the valley floor on the leeward side warmer than its original temperature at
the same elevation on the windward side (Fig. 3.28). While this general model has been
widely demonstrated, many foehn winds involve site-specific processes (Barry 2008).
The foehn develops only under specific pressure conditions (McGowan and Sturman
1996b). The typical situation is a ridge of high pressure on the windward side and a
trough of low pressure on the leeward, creating a steep pressure gradient across the
mountain range. Under these conditions, the air may undergo the thermodynamic pro-
cess already described in a relatively short time. In order for there to be a true foehn,
however, the wind must be absolutely warmer than the air it replaces (Brinkman 1971).
Either side of the mountains may experience a foehn, depending upon the orientation of
the range and the development of the pressure systems. In the Alps, the south foehn af-
fects the north side of the mountains and comes from the Mediterranean, and the north
foehn comes from northern Europe and affects the south side of the Alps. Because of
its original warmth, the south foehn is much more striking and more frequent than the
north foehn, which has to undergo much greater warming to make itself felt (Defant
1951). Similarly, in western North America, most Chinook winds occur on the east side
of the mountains, because of the prevailing westerly wind and its movement over the