Taken November 24/09 Okotoks, Alberta, Canada, from my deck.
From Wikipedia, the free Encyclopedia:
Adiabatic warming of downward moving air produces the warm Chinook wind
One of the most striking features of the chinook is the chinook arch, which is a band of stationary stratus clouds caused by air rippling over the mountains due to orographic lifting. To those unfamiliar with the chinook, the chinook arch may look like a threatening storm cloud at times. However, they rarely produce rain or snow. They can also create stunning sunrises and sunsets.
The stunning colours seen in the chinook arch are quite common. Typically the colours will change throughout the day, starting with yellow, orange, red and pink shades in the morning as the sun comes up, grey shades in the mid day changing to pink/red colours, and then orange/yellow hues just before the sun sets.
Chinook winds, often called chinooks, commonly refers to foehn winds in the interior West of North America, where the Canadian Prairies and Great Plains meet various mountain ranges, although the original usage is in reference to wet, warm coastal winds in the Pacific Northwest.
Chinook is falsely claimed by popular mythology in Alberta and Montana and similar inland areas to mean “snow-eater” but it is really the name of a people in the region where the usage was first derived. The reference to a wind or weather system, simply “a Chinook”, originally meaning a warming wind from the ocean into the interior regions of the Pacific Northwest (the Chinook people lived near the ocean, along the lower Columbia River). A strong Chinook can make snow one foot deep almost vanish in one day. The snow partly melts and partly evaporates in the dry wind. Chinook winds have been observed to raise winter temperature, often from below −20°C (−4°F) to as high as 10°C to 20°C (50°F to 68°F) for a few hours or days, then temperatures plummet to their base levels. The greatest recorded temperature change in 24 hours was caused by Chinook winds on January 15, 1972, in Loma, Montana; the temperature rose from -48°C (-56°F) to 9°C (49°F).
How chinooks occur
The Chinook is a foehn wind, a rain shadow wind which results from the subsequent adiabatic warming of air which has dropped most of its moisture on windward slopes (orographic lift). As a consequence of the different adiabatic rates of moist and dry air, the air on the leeward slopes becomes warmer than equivalent elevations on the windward slopes.
As moist winds from the Pacific (also called Chinooks) are forced to rise over the mountains, the moisture in the air is condensed and falls out as precipitation, while the air cools at the moist adiabatic rate of 5°C/1000 m (3.5°F/1000 ft). The dried air then descends on the leeward side of the mountains, warming at the dry adiabatic rate of 10°C/1000m (5.5°F/1000 ft).
The turbulence of the high winds also can prevent the normal nocturnal temperature inversion from forming on the lee side of the slope, allowing night-time temperatures to remain elevated.
Quite often when the Pacific Northwest coast is being drenched by rain, the windward side of the Rockies is being hammered by snow (as the air loses its moisture), and the leeward side of the Rockies in Alberta is basking in a foehn chinook. The three different weather conditions are all caused by the same flow of air, hence the confusion over the use of the name “Chinook wind”.
Two common cloud patterns seen during this time are:
· A chinook arch overhead
· A bank of clouds (also referred to as a cloud wall) obscuring the mountains to the west. It appears to be an approaching storm, but does not advance any further east.
Pronunciation in BC and the Pacific Northwest
In British Columbia and parts of the Pacific Northwest, the word Chinook is pronounced with a tshi-, as in Salish. In Central Washington, Alberta, and the rest of Canada, it is pronounced with a shi-, as in French. This difference may be because it was the Métis employees of the Hudson’s Bay Company, who were familiar with the Chinook people and country, brought the name east of the Cascades and Rockies, along with their own ethnified pronunciation. Early records are clear that tshinook was the original pronunciation, before the word’s transmission east of the Rockies.
Chinooks are most prevalent over southern Alberta in Canada, especially in a belt from Pincher Creek and Crowsnest Pass through Lethbridge, which get 30 to 35 chinook days per year on average. Chinooks become less frequent further south in the United States, and are not as common north of Red Deer. But they can and do occur as far north as Grande Prairie in northwestern Alberta and Fort St. John in northeastern British Columbia, and as far south as Albuquerque, New Mexico.
In southern Alberta, most of the winter can be spent with little or no snow on the ground. In Calgary, there is snow about 59% of the time on Christmas, compared to 88% for Edmonton. In Canada, only the West Coast of British Columbia and southern Ontario have fewer white Christmases than southern Alberta.
In Lethbridge, Chinook winds can gust in excess of hurricane force (120 km/h or 75 mph). On November 19, 1962, an especially powerful chinook there gusted to 171 km/h (107 mph).
In Pincher Creek, the temperature rose by 41°C (from -19°C to 22°C) in one hour in 1962 – trains have been known to be derailed by chinook winds there. During the winter, driving can be treacherous as the wind blows snow across roadways sometimes causing roads to vanish and snowdrifts to pile up higher than 1 meter. Empty semi trucks driving along Highway 3 and other routes in Southern Alberta have been blown over by the high gusts of wind caused by chinooks.
Calgary also gets many chinooks – the Bow Valley in the Canadian Rockies west of the city acts as a natural wind tunnel funneling the chinook winds.
In February 1992, Claresholm, Alberta hit 24°C (75°F) – one of Canada’s highest February temperatures.
Loma, Montana boasts as having the most extreme recorded temperature change in a 24-hour period. On January 15, 1972, the temperature rose from −54 °F (-48 °C) to 49 °F (9 °C), a 103 °F (57 °C) change in temperature; a dramatic example of the regional Chinook wind in action.
The Black Hills of South Dakota are home to the world’s fastest recorded rise in temperature. On January 22, 1943, at about 7:30am MST, the temperature in Spearfish, South Dakota was -4 °F (-20 °C). The chinook kicked in, and two minutes later the temperature was +45 °F (7 °C). The 49 degree (27 °C) rise in two minutes set a world record that is still on the books. By 9:00am, the temperature had risen to 54 °F (12 °C). Suddenly, the chinook died down and the temperature tumbled back to -4 °F. The 58 degree drop took only 27 minutes.
The aforementioned 107 mph / 171 km/h wind in Alberta and other local wind records west of the 100th meridian on the Great Plains of the United States and Canada, as well as instances of the record high and low temperature for a given day of the year being set on the same date are largely the result of these winds.
On rare occasions chinook winds generated on the eastern slope of the Rocky Mountains have reached or passed the Mississippi River.
Chinooks versus the Arctic air mass
In mid-winter, the chinook can seem to do battle with the Arctic air mass at times. It is not unheard of for people in Lethbridge to complain of −20°C (−4°F) temperatures while those in Cardston, just 77 kilometers (48 miles) down the road, enjoy +10°C (50°F) temperatures in shorts and T-shirts. This clash of temperatures can remain stationary, or move back and forth, in the latter case causing such fluctuations as a warm morning, a bitterly cold afternoon, and a warm evening. A curtain of fog often accompanies the clash between warm to the west and cold to the east.
It has been reported on a local TV historical program that many years ago Cardston once reported a curtain of fog remaining over Main Street for many hours. The west side of town was balmy with melting snow, while the east side of town was bitterly cold. In Calgary, recent winters have seen situations where the airport in the northeast part of the city is reporting around −20°C (−4°F) and the southwest part of the city is sitting at +7°C (45°F).
First nations myth of B.C.
Native legend of the Lil’wat subgroup of the St’at’imc tells of a girl named Chinook-Wind, who married Glacier, and moved to his country, which was in the area of today’s Birkenhead River. She pined for her warm sea-home in the southwest, and sent a message to her people. They came to her in a vision in the form of snowflakes, and told her they were coming to get her. They came in great number and quarrelled with Glacier over her, but they overwhelmed him and she went home with them in the end.
While on the one hand this tale tells a tribal family-relations story, and family/tribal history as well, it also seems to be a parable of a typical weather pattern of a southwesterly wind at first bringing snow, then rain, and also of the melting of a glacier, namely the Place Glacier near Gates Lake at Birken. Thus it also tells of a migration of people to the area, (or a war, depending on how the details of the legend might be read, with Chinook-Wind taking the part of Helen in a First Nations parallel to the Trojan War).
There are three especially famous chinook folk tales that most people in southern Alberta probably know in some form from childhood stories.
· A man rode his horse to church, only to find just the steeple sticking out of the snow. So, he tied his horse to the steeple with the other horses, and went down the snow tunnel to attend services. When everybody emerged from the church, they found that a chinook had melted all of the snow, and their horses were now all dangling from the church steeple.
· A man was riding his sleigh to town when a chinook overcame him. He kept pace with the wind, and while the horses were running belly deep in snow, the sleigh rails were running in mud up to the buckboard. The cow that was tied behind was kicking up dust.
· A man and his wife were out during a chinook. The wife was heavily dressed and the man was wearing summer clothes. When the couple had returned home, the man had frostbite, and the woman had heatstroke.