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Chinook wind

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Chinook wind

Chinook winds, or simply chinooks, are two types of prevailing warm, generally westerly winds in western North America: Coastal Chinooks and interior Chinooks. The coastal Chinooks are persistent seasonal, wet, southwesterly winds blowing in from the ocean. The interior Chinooks are occasional warm, dry föhn winds blowing down the eastern sides of interior mountain ranges. The coastal Chinooks were the original term, used along the northwest coast, and the term in the interior of North America is later and derives from the coastal term.

In the interior of North America, the Blackfoot people call these winds the "snow eater"; however, the more commonly used term "Chinook" originates from the name of the eponymous Chinook people, who lived near the ocean, along the lower Columbia River, where the term was first derived. The reference to "a Chinook" wind or weather system originally meant, to euro-American settlers along the Pacific Northwest coast, a warming wind from the ocean blowing into the interior regions of the Pacific Northwest of the North America.

A strong föhn wind can make snow one foot (30 cm) deep almost vanish in one day. The snow partly sublimates and partly melts and 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 10–20 °C (50–68 °F) for a few hours or days, then temperatures plummet to their base levels.

Its speed is generally between 16 km/h (10mph) and 60 km/h (37.5mph), gusting to 100 km/h (62.5 mph).

"Chinook" is used for coastal Chinook winds in British Columbia, and is the original use of the term, being rooted in the lore of coastal natives and immigrants, and brought to Alberta by French-speaking fur-traders. Such winds are extremely wet and warm and arrive off the western coast of North America from the southwest. These same winds have more recently been called the pineapple express, since they are of tropical origin, roughly from the area of the Pacific near Hawaii.

The air associated with a coastal 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.

When a coastal Chinook comes in when an Arctic air mass is holding steady over the coast, the tropical dampness brought in suddenly cools, penetrating the frozen air and coming down in volumes of powder snow, sometimes to sea level. Snowfalls and the cold spells that spawned them only last a few days during a Chinook; as the warm coastal Chinooks blow from the southwest, they push back east the cold Arctic air. The snow melts quickly and is gone within a week.

The effects on the Interior of British Columbia when a coastal Chinook is in effect are the reverse. In a rainy spell, most of the heavy moisture will be wrung out of the rising air as a consequence of crossing over the mountain ramparts before the air mass descends (and hence warms and dries) into the Fraser Canyon and the Thompson River-Okanagan area. The effects are similar to those of an Albertan interior Chinook, though not to the same extreme, partly because the Okanagan is relatively warmer than the Prairies, and partly because of the additional number of precipitation-catching mountain ranges between Kelowna and Calgary. When the coastal Chinook brings snow to the coast during a period of coastal cold, bright but chilly weather in the interior will give way to a slushy melting of snow, more due to the warm spell than because of rain.

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