Winters in Calgary are
occasionally moderated by
chinooks, sudden warming winds that
periodically sweep down the
eastern flanks of the Rockies.
Often heralded by a steely cloud
band spreading from the mountains
over the city, a chinook can raise
the temperature by as much as 10°C
in a couple of hours and evaporate
a foot of snow in a day. Chinooks
are the result of a phenomenon
that occurs on leeward slopes of
mountains all over the world, but
nowhere more dramatically than in
the plains of southwestern
Alberta. The effect has to do with
the way prevailing westerly winds
are forced to rise over the
Rockies, expanding and cooling on
the way up and compressing and
warming up again on the way back
down. On the way up the cooling
air, laden with Pacific moisture,
becomes saturated (ie clouds form)
and drops rain and snow on the
windward (western) side of the
mountains. All this condensation
releases latent heat, causing the
rising air to cool more slowly
than usual; but on the leeward
descent the air, now relieved of
much of its moisture, warms up at
the normal rate. By the time it
reaches Calgary it's both drier
and warmer than it was to start
with.
The name comes from the people
that traditionally inhabited the
area around the mouth of the
Columbia River in Washington and
Oregon, from where the winds seem
to originate; the Chinook people
also give us the name of the
largest species of Pacific salmon.
