Download Chinook winds

The Chinook
Author: Keith C. Heidorn
Published on: March 1, 2003
They flow off the mountain ridges, rushing winds that are very hot and very dry.
Along the eastern slope of the Rocky Mountains, they have been called "Snow
Eaters" but today are more commonly known by their native American name:
Chinook.
Such hot and dry winds descending mountain slopes are found around the world.
Since they were first studied in the Alps, they are generically known by their local
Alpine name: the Fohn winds (or Foehn, or more correctly Föhn winds). Fohn winds
flow off mountain ridges, usually in the lee of the prevailing wind direction. In Libya,
their local name is ghibli, while in Java they're called the koembang. From off the
Andes, they blow as the puelche, or on the Argentinian pampas as the zonda. In
North America, they have had several local names including the Santa Ana winds of
California, but most of us know them as the chinook of the western Prairies and
Plains regions.
The fohn-wind family consists of winds that are warm and dry because their air has
been warmed by compression as they flow over the mountain ranges and then down
the leeward slopes. To describe to you how they typically form, I'll use the Canadian
(Alberta) chinook as my example.
From a global perspective, the prevailing wind regime, the Prevailing Westerlies —
or just the Westerlies — blow across southern Canada from the Pacific Ocean to the
Atlantic shore. We first pick them up approaching the Pacific coastline where in
crossing the northern Pacific waters during the winter, they have picked up heat and
moisture from the underlying ocean surface.
When these relatively warm and very moist winds make landfall, they encounter the
coastal mountain ranges of British Columbia, they are forced to rise over the
mountain barrier on their trek eastward. As the flowing air mass rises toward the
summit ridges, it cools until it eventually reaches its saturation temperature. At this
time, its water vapour burden condenses into liquid water, first forming clouds, then
precipitation. Much of that liquid, or frozen, water will eventually fall as prodigious
amounts of rain or snow on the Coastal Ranges, watering the lush temperature
rainforests for which the region is famous.
Although the airmass cools by expansion as it rises over the mountains, it gains
back a great deal of heat when its water vapour converts to liquid water — the latent
heat of condensation which amounts to about 2.5 kilojoules or 597 calories per gram
of water. (Additional heat — the latent heat of fusion — is released should the liquid
water freeze to ice within the airmass.) By the time the airmass has traversed all of
British Columbia's ranges, most of its water content has been lost through
precipitation; however, a good portion of that released latent heat still remains in the
airmass.
When the airflow descends from the high ridges of the last mountain barrier, the
Rockies, onto Alberta's high plains, it warms through the compression of the air, like
the air in a bicycle pump when the plunger is pushed down. The airmass warms by
about 9.8 Celsius degrees per 1000 metres of descent (5.4 Fahrenheit degrees per
1000 feet).
Since many of the ridge lines in the Rockies are 3000 metres (10,000 feet) above
sea-level and the Alberta plain is around 1000 metres (3300 ft), the air will warm by
nearly 20 Celsius degrees (36 Fahrenheit degrees) in its descent. The air parcel is
also very dry since it lost most of its initial moisture content crossing the mountains
while gaining very little new moisture during its journey. This warm descending air is
the chinook.
The most impressive chinook winds blowing off the Rockies can reach speeds of
between 65 and 95 km/h (40-60 mph) with gusts exceeding 160 km/h (100 mph).
When blowing at those speeds, the chinook can tip railcars off the tracks and blow
semi-trailer units off the road.
Impressive as the chinook is as a wind, the temperature changes it brings can be
astonishing, often as much as 20-25 Celsius degrees (36-45 F degrees) in an hour.
The greatest chinook temperature jump ever recorded occurred on January 22,
1943, when a chinook shot the temperature in Spearfish, South Dakota, from a
chilling minus 4oF (-20oC) at 7:30 AM to 47oF (8.3oC) just two minutes later! And, in
Pincher Creek, Alberta, a chinook jacked the temperature 21 Celsius degrees (37.8
F degrees) in four minutes on January 6, 1966.
No wonder, the chinook has the reputation its name "Snow Eater" engenders. The
deadly-to-snow combination of high temperature and dry air rushing by at high
speeds can literally remove a foot (30 cm) of snow in a few hours. And it may not
just melt the snow but evaporate it as well, often all in one single process called
sublimation, without leaving a liquid pool behind. The very dry air soaks up the liquid
like a sponge, replenishing some of the water vapour lost in the mountain traverses.
When, on February 25, 1986, a chinook descended on Lethbridge, Alberta with
winds gusting to 166 km/h (104 mph), it fully removed a snow pack of 107 cm (42
inches) in depth in eight hours. Lethbridge was left with substantial wind damage
and new lakes standing in the surrounding fields and pastures.
Chinook winds may last from only a few hours to a few days, sometimes even
persisting for several weeks. They can be welcome visitors during the winter, giving
a respite to residents of the cold Prairies. But in other seasons, the searing dry
winds can dessicate vegetation, raise soil into dust storms, and rapidly increase the
danger of grass and forest fires.
For many living under the chinook influence, its winds bring debilitating physical
effects ranging from sleeplessness and sleeplessness to anxiety and severe
migraine headaches.
Recent studies also suggest that Chinook winds rolling off the mountains can
damage aircraft at cruising altitudes by generating turbulence powerful enough to rip
an engine from a jet.
The fire menace of fohn-type winds brings much worry in many areas particularly in
the summer and autumn seasons. In the Canadian Prairies, concerns heighten over
grass fire potential during dry periods. Similar worries arise each year between
October and February for residents of Southern California. During this period, when
the Santa Ana winds — a chinook cousin — blow, the dry chaparral country
becomes a tinder box needing only a spark to touch off devastating wildfires.
I'll leave you with a small table with some of the poetic names given locally to fohn
winds around the world.
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Autan from France
Bohorok from Sumatra
Chinook from North American Rockies
Föhn from European Alpine regions
Ghibli from Libya
Koembang from Java
Maloja from Switzerland
Puelche from the Andes
Reshabar from the Caucasus Mountains
Santa Ana from Southern California
Warm Braw from New Guinea
Yama Oroshi from Japan
Zonda from Argentina