chinook wind

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Adiabatic warming of downward moving air produces the warm Chinook wind
Chinook winds, often just called chinooks, are a variety of Föhn winds[1] pattern observed in the interior West of North America, where the Canadian Prairies and Great Plains end and the mountains begin. These wind patterns are named for the country of the Chinook Native Americans, which lies in the direction these winds were realized to originate from. The same term is used on the British Columbia Coast and in the Puget Sound area to designate a warm, very wet, southwesterly (from the direction of the Chinook country) wind, which is in fact the same weather system prior to its being stripped of its moisture by the various mountain ranges in between.

In popular myth, Chinook is supposed to mean "snow eater", as a strong Chinook can make a foot of snow all but vanish inside of one day. The snow partially melts, and partially evaporates in the dry wind. However, the true origin of the name is that "Chinook Wind" in the local argot of the fur trade era meant that the wind came from the direction of the country of the Chinooks (the lower Columbia River, i.e. from across the Rocky Mountains).

Chinook winds have been observed to elevate winter temperatures, 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, at the end of which, the temperatures plummet to their base levels.

One of the most dramatic examples of the chinook winds occurred on January 15, 1972, in Loma, Montana. The temperature rose from -48°C (-56°F) to 9°C (49°F); the greatest temperature change ever recorded during a 24-hour period. x A "Chinook Wind" or simply "a Chinook", has a different meaning on the British Columbia and Pacific Northwest coasts where the term originates. At these locations west of the Rockies, a Chinook is a warm, moist wind from the southwest, likely to bring rain (or snow at higher elevations). This was the origin of the name that spread via the fur trade to the Prairies - the wind from the country of the Chinooks, i.e. from the southwest. Its pronunciation also varies by geography, with the <ch> in Chinook pronounced as in the word "church" on the Pacific Coast. On the Prairies, the <ch> digraph is generally pronounced as in French (i.e., shinook), because it was the French-speaking voyageurs of the fur companies who brought the term back across the mountains.

Chinooks in Alberta

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Where chinooks occur most frequently


Chinooks are most prevalent over southern Alberta in Canada, especially in a belt from Pincher Creek and Crowsnest Pass through Lethbridge, getting 30 to 35 chinook days per year on average. Chinooks become less frequent further south in the United States, and chinooks are not as common north of Red Deer, although 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 most recent winters (since the 1980s), chinooks and warmer weather have all but banished winter to just a few spells of sharp cold of one or two weeks, and some midwinter months have averaged high temperatures over 5°C (41°F), similar to the West Coast of British Columbia, where Canada's warmest winters are found.

In southern Alberta, most of the winter can be spent with little or no snow on the ground. Calgary can expect a white Christmas about 59% of the time, compared to 88% for Edmonton.[2]. 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). The winds gusts can be strong enough to shatter car windows through air pressure differential. On November 19, 1962, the winds there gusted to 171 km/h (107 mph) in an especially powerful chinook.

In Pincher Creek, the temperature rose by 41°C (from -19°C to +22°C) in one hour in 1962.[3] Also, trains have been known to be derailed by chinook winds there.

Calgary is also well known for getting many chinooks, as 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.

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).

Chinook arch

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 colors seen in the chinook arch are quite common. Typically the colors will change throughout the day, starting with yellow, orange, red and pink shades in the morning as the sun comes up, gray shades in the mid day changing to pink/red colors, and then orange/yellow hues just before the sun sets.




Chinook arch over Calgary, January 6, 2003

The extreme colors of a Chinook arch

Chinook arch in Calgary, Alberta, November 19, 2005

Chinook arch over Calgary, March 2007

Chinook arch over Kelowna, BC Canada, October 2, 2007


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Chinook arch in southern Alberta
Chinook arch in southern Alberta

How chinooks occur

The eastern-slope chinook phenomenon is a Föhn wind that results from the movement of high and low pressure systems over the Rocky Mountains[4][5]. As the wind moves over and through the mountains, the moisture in the air condenses and falls out as precipitation, warming the air by releasing latent heat. Then the air is warmed and dried[6] by adiabatic compression as the air descends the leeward side of the mountain range.[7]

Warm air descending the slope can also displace an existing cold, moist air mass, enhancing the temperature increase and moisture decrease observed with the Chinook.<ref name="Whiteman" />

The turbulence of the high winds also can prevent the normal nocturnal temperature inversion from forming on the lee side of the slope, allowing nighttime temperatures to remain elevated.<ref name="Whiteman" />

Quite often, when the West Coast is being hammered 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 chinook.

Two common cloud patterns seen during this time are:
  • A chinook arch overhead
and/or
  • A bank of clouds (also referred to as a cloud wall) obscuring the mountains to the west. It would appear to be an approaching storm, but it does not advance any further east.

The Manyberries chinook

Often, a chinook is preceded by a "Manyberries chinook" during the end of a cold spell. This southeast wind (named for a small village, now a hamlet, in southeastern Alberta, from where the wind seems to originate) can be fairly strong and cause bitter windchill and blowing snow. The wind will eventually swing around to the southwest and the temperature rises sharply as the real chinook arrives.

The Chinook in British Columbia

The term Chinook Wind is also used in British Columbia, but there it refers to the same winds before they are dried out by the successive ranges of mountains between the Coast and Alberta. Such winds are extremely wet and warm and come from the southwest, and are also known as the Pineapple Express since they are of subtropical origin, roughly from the area of Hawaii; in the days of the sailing ships vessels carrying fresh fruit from Hawaii reached Vancouver, Seattle and Portland in record time, hence the name Pineapple Express — which also applies because of the wind's warmth, which is a blessing in winter. The air associated with a west coast Chinook is stable; this minimizes wind gusts and often keeps winds light in sheltered areas. In exposed areas, fresh gales are frequent during a Chinook, but strong gale or storm force winds are uncommon (most of the region's stormy winds come when a fast westerly jet stream lets air masses from temperate and subarctic latitudes clash).

Typically a weatherman in Vancouver might say "the Chinook is going to last for another five days, so expect heavy rain for the next week. The mountains [i.e. for skiing] will be rainy to the alpine, so expect lots of slush on the slopes." But when a Chinook comes in when an Arctic air mass is holding steady over the Coast, the tropical damp brought in suddenly cools and falls, penetrating the frozen air and coming down in volumes of powder, sometimes to sea level. Snowfalls and the cold spells that spawned them only last a few days during a Chinook, as the weather blows in from the southwest and rains out the snow, which melts quickly and is gone within a week.

The effects on the Interior of the province when a Chinook is in effect are the reverse. In a rainy spell, most of the heavy moisture will be soaked out by the ramparts of mountains before the air mass reaches the Canyon and the Thompson River-Okanagan area. The effects are similar to those of an Alberta Chinook, though not to the same extreme, in part because the Okanagan is relatively warmer than the Prairies, and because of the additional number of precipitation-catching mountain ranges in between Kelowna and Calgary. And when the Chinook brings snow on the Coast during a period of coastal cold, bright but chilly weather in the Interior will give way to a slushy melting of snow because of the warm spell more than because of rain.

The word is in common usage among local fishermen and people in communities along the British Columbia Coast. The term is also used in the Puget Sound area of Washington. It is important to note that Chinook is not pronounced as it is east of the Rockies - shinook - but is in the original coastal pronunciation tshinook.

An outflow wind is more or less the opposite of BC/Pacific Northwest Chinook. These are called a squamish in certain areas, and in Alaska are called a williwaw. They consist of cold air streams from the continental air mass pouring out of the interior plateau via certain river valleys and canyons penetrating the Coast Mountains towards the coast.

Pronunciation of Chinook in B.C. and the Pacific Northwest

In BC and parts of the Pacific Northwest the word Chinook is pronounced with a tshi-; in Central Washington, Alberta, and across Canada from there a French pronunciation of the name is preferred and considered the correct standard. The variance in pronunciation may be because it was the Métis employees of the Hudson's Bay Company who were familiar with the Chinook people and their country, and so it was they who brought the name east of the Cascades and Rockies, along with their own ethnified pronunciation.

A First Nations Myth of B.C. of the Chinook-Wind.

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, perhaps the Place Glacier near Birken Lake or the once-great Birkenhead River glacier 10,000 years ago, when most of this region was icefield. 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).

Chinooks and gardening

The frequent midwinter thaws in Great Plains chinook country are more of a bane than a blessing to gardeners. Plants can be visibly brought out of dormancy by persistent chinook winds, or have their hardiness reduced even if they appear to be remaining dormant. In either case, they become vulnerable to later cold waves. Many plants which do well at Winnipeg (where constant cold maintains dormancy all winter) are difficult in the Alberta chinook belt; examples include basswood, some apple, raspberry and saskatoon varieties, and Amur maple.

Chinooks and health

It is said that chinook winds can cause a sharp increase in the number of migraine headaches suffered by the locals and are often called "chinook headaches". It is also popularly believed they can increase irritability and sleeplessness. In mid-winter, over major centers such as Calgary, chinooks can often override cold air in the city, trapping the pollutants in the cold air and causing inversion smog. At such times, it is possible for it to be cold at street level and much warmer at the tops of the skyscrapers and in higher terrain.

Chinooks and tall tales (folklore)

There are two especially famous chinook folk tales that most people in southern Alberta are probably familiar with in some form or another 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.

Chinook Wind Records

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, SD was -4 °F (-20 °C). The chinook kicked in, and two minutes later the temperature was 45 °F (7 °C) above zero. 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.

Other winds called Chinooks

Similar phenomena are called Föhn winds by meteorologists and climatologists, but are called "chinook winds" in some places in western North America. Föhn winds, although they may be named something else, can occur in most places on the eastern side of a nearby mountain range.

One such wind occurs in the Cook Inlet region in Alaska as air moves over the Chugach Mountains between Prince William Sound and Portage Glacier. Anchorage residents often believe that the warm winds which melt snow and leave their streets slushy and muddy are a midwinter gift from Hawaii, following a common mistake that the warm winds come from the same place as the similar winds in coastal southern British Columbia, Washington, and Oregon.

Another Föhn wind occurs near Denver, where winds blowing over the Front Range have raised winter temperatures from below freezing to around 50 °F (10 °C) in just a few hours. Föhn winds called "Chinook winds" also occur in Salt Lake City and Albuquerque.

See also

References

1. ^ "Chinook". Encyclopædia Britannica. (2006). 
2. ^ Environment Canada - White Christmas probability
3. ^ [1]
4. ^ Introduction to Chinooks
5. ^ Precipitation and Sounding Diagram
6. ^ The air that descends the lee slope is warmed by compression heating at the dry adiatatic rate of 9.8° per 1000 meters. Since it was at its dew point at its highest elevation, this has the effect of raising the temperature above the dew point (i.e., drying the air).
7. ^ Whiteman, C. David (2000). Mountain Meteorology: Fundamentals and Applications. Oxford University Press. ISBN. 
Foehn wind.
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