What exactly is a chinook

Where the warmth comes from

The chinook is the regional name for a foehn wind — the warm, dry wind found in the lee of a high mountain range. The US National Weather Service defines both terms that way, and gives a one-line cause for the warmth: adiabatic compression as the air descends the slopes. Air that is squeezed gets hotter, and air falling off a mountain range is being squeezed.

That is true and it is not the whole story, and the part usually left out is the bigger one. The Canadian Encyclopedia's entry on chinooks — written by Lawrence Nkemdirim, the University of Calgary geographer who mapped Canada's chinook belt — gives two sources of warmth and puts this one first: mild Pacific air simply replaces the arctic air that was sitting on the prairie. At Calgary's elevation that swap is worth something like 22°C on its own, before any descending or compressing is counted at all. A chinook is a change of air mass as much as it is a wind.

The descent then adds to it. Dry air warms as it falls at the dry-adiabatic rate — about 9.8°C per kilometre, which the University of British Columbia's atmospheric-science course and NWS technical material both round to roughly 10°C per kilometre.

The story most of us were taught — Pacific air dumping its rain on the western slopes, releasing heat as the vapour condenses, then pouring down the eastern side dry and warm — is real, but it is the second cause on Nkemdirim's list and he prefaces it with an if: the rain-shadow effect applies if the downslope flow follows a loss of moisture through precipitation on the windward side. Plenty of chinooks arrive without that having happened.

One more thing the textbook diagram quietly assumes is a parcel of air that descends all the way back to the altitude it started at. Alberta's chinook country sits roughly a kilometre above sea level, so a good share of that descent — and of the warming that goes with it — never happens here.

The arch

The chinook arch is a real meteorological object with a dictionary definition, not just a nice sky. The NWS glossary calls it a foehn cloud formation: a bank of altostratus east of the Rockies that heralds the approach of a chinook.

What makes it recognisable is its abrupt western edge, with a strip of clear sky between the cloud bank and the mountains. It most often precedes a strong chinook wind, which is why people who live under it read it as a forecast rather than a view.

Where they land, and where they do not

In Canada chinooks arrive most often in southern Alberta — the Pincher Creek and Crowsnest Pass area, through Lethbridge — and Calgary is the big city most associated with them. Calgary is often called the windiest major city in the country, and Environment Canada's own normals do not bear that out: St. John's averages about 22 km/h against Calgary's 14, with Regina, Winnipeg, Moncton and Hamilton also windier.

They thin out badly by the time they reach Edmonton. The mapped chinook belt covers southern and central Alberta, and the temperature signal weakens across it, from around +25°C in the southeast to about +13°C in the northwest — so the north gets some of the milder air without the abrupt onset and hard wind that make a chinook unmistakable further south. This site's own chinook detection stops at the foothills and the southern cities for that reason.

The temperature swings, carefully stated

The Alberta record most often quoted is Pincher Creek on 10 January 1962, when the temperature climbed 41°C, from −19°C to 22°C — described as the most dramatic temperature change recorded in Canada. Sources disagree on how quickly it happened — the Canada West Foundation puts the whole swing inside a single hour, while Global News presents it as the change across the day. The temperatures and the date are consistent everywhere; the stopwatch is not, so we do not put a number on it.

The world record for this kind of whiplash belongs to Spearfish, South Dakota, on 22 January 1943. The National Weather Service records a rise from −4°F to 45°F in two minutes — and then, ninety minutes later, a fall from 54°F back to −4°F in twenty-seven minutes.

The name is not a translation

You will often read that chinook is an Indigenous word meaning "snow eater". It is not. The word comes from the Chinook people of the lower Columbia River region, and "snow-eater" is a nickname for what the wind does to a snowpack, not a translation of the name.

The nickname is well earned. A chinook can strip a field bare in an afternoon, which is a mercy for cattle and a problem for anyone who wanted the snow to stay on the ski hill.

Chinook headaches are real, and smaller than the folklore

Enough Albertans blame chinooks for headaches that the question was studied properly. A University of Calgary Headache Research Clinic study published in the journal Neurology in 2000 followed 75 patients' migraine diaries against Environment Canada weather data.

It found the probability of migraine onset modestly raised on the day before a chinook and on chinook days themselves, and raised further on chinook days with winds above 38 km/h. The effect appeared in a subset of patients: most of the people studied were not sensitive to either condition. So the effect is measured and real, and it is neither universal nor as large as the conversation about it suggests.