The mountain ranges of British Columbia, culminating in the Continental Divide between B.C. and Alberta, force wet Pacific air masses to rise and, in doing so, release moisture. By the time the air mass crosses the Continental Divide it has lost its moisture content and descends the eastern slopes of Alberta's Rockies and onto the foothills and prairies as a warm, dry wind, known locally as a "Chinook".
In the winter, the onset of a Chinook can cause amazing temperature changes, bringing a warm breath of spring into the coldest of winter days. The Chinook wind was referred to by First Nations people as the "Snow Eater" for its ability to erase a blanket of winter snow in a matter of days. Temperature changes during a Chinook are very dramatic, often rising more than 20 degrees Celsius in a matter of hours.
As air masses rise in elevation they decrease in temperature, and when they descend in elevation they increase in temperature. Precipitation occurs when moist air masses cool.
Large, moist air masses move in from the Pacific Ocean over British Columbia and are forced to rise over mountain ranges where precipitation (rain, snow, fog) occurs. The temperature of moist air masses decreases at a fixed rate known as the moist adiabatic rate, until it has lost all of its moisture (usually at the height of the Continental Divide). The now dry air mass descends the eastern side of the mountains and increases in temperature faster than when it climbed the eastern side of the mountains (dry adiabatic rate).
As such, the air mass reaches a higher temperature than it had on the western side of the mountains. Such flow generally creates a warm, dry wind on the foothills of the eastern slopes of the Rockies. In winter, temperatures can increase suddenly with the onset of a Chinook wind. In southern Alberta, a normal Chinook wind in winter will drive temperatures up by 20 degrees Celcius in a matter of hours. Chinooks can last hours or days and southern Alberta experiences approximately 30-35 Chinooks per year.
In 1962, a record Chinook in Pincher Creek, Alberta drove the temperature up 41 degrees Celsius in one hour. Strong winds are also associated with Chinooks, and empty semi-trailer transport trucks and empty train cars have been known to be blown over by the winds. A broad band of stationary stratus clouds extending hundreds of kilometres north to south and paralleling the mountains often appears during a Chinook and is known as a "Chinook Arch".
Chinook Facts from Waterton Lakes National Park
What is a chinook? It's a strong wind that becomes warm and very dry while rapidly descending the lee side of mountain slopes. The wind turbulence is similar to the action river water makes when it hits a rock. As the air flows down, waves are created. The high crest of the wave creates a distinctive cloud band parallel to the mountains called a chinook arch.
Where are they found? They're found where prevailing winds cross mountain ranges situated parallel to them. They're particularly strong in southern Alberta and northern Montana. Chinooks also occur in other parts of the world including Germany, Switzerland, New Zealand and Argentina.
How frequent are they? Chinooks can happen year-round, although the warming condition is more apparent in colder weather. A chinook can last less than an hour or for several days. Not all east slope winds are Chinooks.
Why do they happen? Winds carrying warm, moist air from the Pacific glide up and over the western slopes of mountains - expanding, cooling and losing moisture. Once over the Coast and Rocky Mountains, the now-dry air plummets to the foothills, warming by compression. For every 100-metre drop in elevation, temperature raises 1*C. Chinook winds blow from the south or southwest and can reach speeds over 150 km/h.
Source: Environment Canada, 1995.