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Chinook winds, are föhn 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.
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 föhn Chinook. The three different weather conditions are all caused by the same flow of air across the Rockies, hence the confusion over the use of the name "Chinook wind".
The Föhn Effect
A föhn wind is a rain shadow wind which results from the subsequent adiabatic warming of air which has dropped most of its moisture on windward slopes due to orographic lift. As a consequence of the different adiabatic rates of moist and dry air, the air on the leeward slopes becomes warmer than at 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, and the air cools at the saturated-adiabatic rate of 1.5°C / 1000 ft. The dried air then descends on the leeward side of the mountains, warming at the dry-adiabatic rate of 3°C / 1000 ft.
The turbulence of the high winds also can prevent the usual nocturnal temperature inversion from forming on the lee side of the slope, allowing night-time temperatures to remain elevated.
A strong föhn wind can make snow 30 cm deep almost vanish in one day. The snow partly melts and partly sublimates in the dry wind. Chinook winds have been observed to raise winter temperature, often from below −20°C to as high as 10–20°C, 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 15 January 1972, in Loma, Montana; the temperature rose from −48°C to 9°C.
Chinooks are most prevalent over southern Alberta in Canada, especially in a belt from Pincher Creek and Crowsnest Pass through Lethbridge, which get 30–35 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 annually as far north as High Level in northwestern Alberta and Fort St. John in northeastern British Columbia, and as far south as Las Vegas, Nevada, and occasionally to Carlsbad, in eastern New Mexico.
In southwestern Alberta, Chinook winds can gust in excess of hurricane force 65kts.
Interaction between Chinook and Arctic Air Mass
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 temperatures while those in a desert region, just 45 nm down the road, enjoy 10°C temperatures. 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.
Impact on Aviation Operations
Airports located in areas that frequently see Chinook winds can experience significant changes in temperature within a short space of time. On the 22nd of November 2017, there was a temperature difference of 16°C between Calgary International Airport and Springbank Airport only 15nm away. Such potentially rapid changes in temperature mean that performance calculations should be reviewed just prior to take off to ensure they are based on the actual temperature. Likewise, the outside temperatures should be monitored on approach and care taken to apply the correct temperature error correction to approach altitudes. Rapid thawing and freezing of surface water can have an impact on surface friction and braking distance, with obvious ramifications for airport surface movement.
Two common cloud patterns seen during this time are a chinook arch overhead, and 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.
The Chinook arch, is a föhn cloud in the form of a band of stationary stratus clouds caused by air rippling over the mountains due to orographic lifting. To those unfamiliar with it, 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 at midday changing to pink / red colours, and then orange / yellow hues just before the sun sets.