If you live downwind of a major mountain range (say, in Calgary, Alberta, or Ticino, Switzerland, or on the Canterbury Plains of New Zealand’s South Island), chances are you have encountered many a skyline like this:
Source: Oldfaw, http://upload.wikimedia.org/wikipedia/en/5/58/Helm_bar_mallerstang.JPG. Used under the GNU Free Documentation License Version 1.2.
Or perhaps even this:
Source: Surrealplaces, http://commons.wikimedia.org/wiki/File:Chinook-arch-03.jpg. Licensed under the Creative Commons Attribution-Share Alike 3.0 Unported licence.
And if you have, chances are your jacket came off rather quickly if you were wearing one. That abrupt arch of cloud is indicative of a foehn wind (called a chinook in North America, nor’wester in New Zealand, or Puelche in Chile, or helm winds in England, just to name a few). Foehns are strong, dry, warm winds that rush down-slope from the rain-shadow side of mountains onto the plains or valley below. When foehns (and their accompanying wind gusts) hit, things warm up quickly; temperatures can rise dozens of degrees in just a few hours. When they occur in winter, people who were just living through -15°C in the morning can find themselves wearing T-shirts in the afternoon. Assuming there’s little snow on the ground, foehns can be a very pleasant surprise in the middle of winter, whether they last for just an hour or two or for a few days. Two extreme examples: on 22 January 1943 in Spearfish, South Dakota, the temperature went from -20°C to 8.3°C in just two minutes. In 1966, Pincher Creek, Alberta experienced a rise of 21°C in four minutes (unsurprisingly, it’s also one of North America’s hotspots for wind power generation).
The foehn wind process.
Foehns typically occur when warm, moist, high-pressure air originates from sea level only to run into a mountain chain. As the air ascends in elevation trying to cross the mountains, it cools (the dry adiabatic lapse rate is around 1°C/100m). As the air cools, the water vapour held within condenses into cloud and eventually results in precipitation, releasing heat (the wet adiabatic lapse rate is usually between 0.5 and 0.65°C/100m). Having released its moisture content, the wind crosses over to the leeward, lower-pressure side of the mountain, increasing in speed and in temperature, warming again now at the dry adiabatic lapse rate. This results in large wind gusts temperatures at an elevation on the leeward side being higher than temperatures on the windward side at the same elevation. When the wind reaches the base of the mountains, it spreads out rapidly, producing the foehn. Residents of southern California will be unsurprised to hear that similar processes result in the infamous Santa Ana winds.
One of the many nicknames for foehn winds is ‘snow eater’, and for good reason: the warm, dry winds quickly melt winter snow and ice, often resulting in flooding, and certainly in a lot of slush (which is a treacherous pain to deal with the next day when the temperature plummets back to normal and the slush freezes, leaving chunky sheets of ice all over roads, sidewalks and lawns; you can imagine the carnage on streets and highways when this occurs). As pleasant as a sudden burst of warm weather can be for many people; foehns are bad news for businesses such as ski resorts, and can kill vegetation by prematurely triggering photosynthetic processes in the middle of winter when the ground is still frozen. Foehns also can trigger seasonally-affected ailments such as migraine headaches. In summer, foehns can help accelerate forest fires and create instant droughts as they suck moisture out of the air and ground.
Below, a time-lapse video of a chinook arch:
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WeatherFreaks (n.d.). Foehn Wind. WeatherFreaks.net. Available at http://weatherfreaks.net/foehn-wind. Accessed 15 Novermber 2010.