If you wish to contribute or participate in the discussions about articles you are invited to join SKYbrary as a registered user

 Actions

Work in progress

Chinook winds

From SKYbrary Wiki

Revision as of 15:30, 4 September 2019 by Integration.Manager (talk | contribs)
Article Information
Category: Weather Weather
Content source: SKYbrary About SKYbrary
Content control: SKYbrary About SKYbrary
WX
Tag(s) Weather Phenomena

Description

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.

Fohn Effect
Fohn Effect

The Chinook is a föhn 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 / 1000 m (5.5 °F / 1000 ft).[9]

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.[9] 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 to 9 °C (−54 to 49 °F).[6]

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 120 km/h (75 mph). On 19–November 1962, an especially powerful Chinook in Lethbridge gusted to 171 km/h (106 mph).[citation needed]

In Pincher Creek, the temperature rose by 41 °C (74 °F), from −19 to 22 °C (−2 to 72 °F), in one hour in 1962.[7] Trains have been known to be derailed by Chinook winds.[citation needed] 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 a metre. Empty semitrailer 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, Alberta 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.[citation needed]

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 (−4 °F) temperatures while those in desert region, just 77 km (48 mi) down the road, enjoy 10 °C (50 °F) 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.

Chinook Arch

One of its most striking features is the Chinook arch, 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.


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, hence the confusion over the use of the name "Chinook wind".

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.