Following on from the general Introduction To Winds, Foehn, or Fohn Winds (the name comes from a German local wind) are among the most interesting. They occur when a warm moist wind encounters a mountain range, cools as it rises above it, and warms as itdescends the other side.
Sounds simple? Well there's a little more to a Foehn it than that.
As it rises it cools and the water vapor it carries begins to condense, resulting in heavy rain or snow on the windward slopes. When this happens, some heat is added to the system, called latent heat of condensation.
(This is a term usually important only to meteorologists, some physicists and senior high school students until they have completed their science exams. It is easiest to understand bycomparing it to its opposite number, latent heat of vaporisation.If we want water to boil, when liquid is converted to vapor, wehave to add heat. It only seems fair that when water vapor returnsto a liquid state, that heat should be returned to the system - and it is, as latent heat of condensation. The numbers that go with this tell us that moist rising air cools at around 0.5-0.65°Cper 100 metres in elevation (2.7-3.5°F/1000 ft), but dry air warms by 1°C per 100 m (5.4°F/1000 ft) as it descends.And that's the end of the physics section).
In the classic Foehn wind scenario, the rising wind has lost mostof its moisture content by the time it crosses the crest of the range.
Now it starts to descend, and as the air pressure increases with decreasing height it begins to warm up. In the normal course of events, dry air will lose heat as it rises, and gain it all back as it descends, provided the starting and finishing heights arethe same. But because of the addition of heat to the moist wind as rain and snow condense out - due to our complex friend latent heat of condensation, its temperature as it starts to descend ishigher than we might expect.
So by the time it reaches the lowlands on the far side of the range, it is warmer than when it started out, and because Foehn winds are most common in winter, the effect of a strong warm wind is immediately noticeable, often spectacularly so.
Examples of Foehn Winds
Foehn winds occur anywhere warm moist winds rise above a mountain
barrier. The name itself comes from the European Alps, but
other examples are the Zonda of Argentina, Puelche
of Chile, the Helmin England, and the Canterbury
Northwester in New Zealand.
But the most best known and most spectacular Foehn type winds
occur in North America, and include the Santa Ana,
Diablo, Mono or Sundowner of California,
with their sometimes disastrous effects during the wildfire season.
But the grand daddy of them all is the Chinook.
Best developed in Canada and the northern Great Plains of the USA,
the Chinook is a strong, warm wind which can change the local
temperature virtually in seconds. Temperature rises of 50 to
60°F (27-33°C) are often recorded over less than a day, with
the often quoted record being a rise of 49°F , or 27°C in
two minutes at Spearfish, South Dakota.
What this means is a spectacular change in temperatures from
well below zero (both scales) to relatively comfortable. Snow
and ice disappear, both through melting and through direct
evaporation, until eventually the wind cools off, leaving
towns further to the east unaffected.
Other features of the Chinook and its relatives elsewhere are
strong winds, which can reach the lower end of the hurricane
scale near gaps in the range where wind speeds are at a maximum,
and some spectacular cloud formations.
Most noticeable of these is the Foehn Wall or Chinook Arch,
an elongated cloud which forms over the range crest where the
wind has reached its maximum elevation and lowest temperature,
and the last of its available water vapor has condensed. As
it starts to descend condensation stops, leaving a sharply
defined cloud boundary.
Cloud may also rest on the crest of the range, looking like
an approaching storm, but actually remaining stationary.
Sometimes the wind will develop waves, particularly if it
is passing over foothill ranges. In the right conditions,
stationery clouds will develop at the crest of the waves,
where limited condensation occurs,with clear air over the
troughs. The clouds which form here are smoothly sculptured
by the wind, and look somewhat like flying saucers. They
are called lenticular clouds, and it is a little eery to
be standing in strong Foehn winds, watching strangely shaped
clouds which don't seem to move.
The overall Chinook wind system is very easy to follow as
it begins on the Pacific coast and crosses the ranges -
so much so that in the Vancouver area the Chinook is known
as a moist wind that may bring heavy rain and snow to the
hills and ranges behind the city. Same wind, different results.
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