A chinook is a warm, dry wind that descends the eastern slope of the Rocky Mountains onto the plains of Alberta, Montana, Wyoming and Colorado, capable of raising temperatures by tens of degrees within hours and stripping a snowpack in a day. It is the North American member of the foehn family: moist Pacific air is forced up the western slope of the Rockies, cools as it rises, drops most of its moisture as precipitation on the western side, then descends the eastern slope as dry air, warming through compression at roughly 10 °C per 1,000 metres. Because the descending air is dry, it warms faster on the way down than it cooled on the way up, and it arrives on the plains far warmer than the air it started as. The effect is dramatic enough that chinooks hold both world records for temperature change: a rise of 27 °C in two minutes at Spearfish, South Dakota on 22 January 1943, and a 24-hour swing of 57 °C at Loma, Montana on 15 January 1972.
Loma, Montana, and the Fastest Warming on Record
The records are worth stating precisely, because they are the clearest demonstration of what the mechanism can do.
At Loma, Montana, between 14 and 15 January 1972, the temperature rose from −48 °C to 9 °C in twenty-four hours — a change of 57 °C, and the largest 24-hour temperature rise ever recorded anywhere. The cold figure was arctic air that had settled over the plains; the warm figure was chinook air that displaced it.
At Spearfish, South Dakota on 22 January 1943, the record is stranger. At 7:30 am the temperature was −20 °C. Two minutes later it was 7 °C — a 27 °C rise, the fastest recorded temperature change in history. Later the same morning it dropped back nearly as fast. What produced it was the boundary between the cold air and the chinook air sloshing back and forth across the town like the edge of a tide, so that a thermometer a few hundred metres away could read a different season. Shop windows in Spearfish reportedly cracked from the thermal shock, and car windscreens frosted over instantly when the cold air returned.
Alberta produces the same effect routinely if less extremely. Pincher Creek recorded a rise of 41 °C in one hour in January 1962, and Calgary can go from deep winter to shirtsleeves and back within a single day several times a season.
One caveat belongs with the Spearfish figure. Records of that vintage came from manual observation and single-station thermometers, and the two-minute reading has been questioned on instrumentation grounds ever since. What is not in doubt is the phenomenon it describes: the chinook front is often a genuinely sharp boundary, only tens of metres wide, and observers along the Rocky Mountain Front routinely report driving through it and watching a car thermometer jump twenty degrees in the space of a block.
Why Descending Air Gets Hot
The asymmetry at the heart of the foehn mechanism is the part worth understanding, because without it the air would simply arrive back at the temperature it started.
Rising air cools as it expands. As long as it stays unsaturated it cools at about 10 °C per 1,000 metres — the dry adiabatic rate. Once it saturates and cloud forms, condensation releases latent heat, which partly offsets the cooling, so saturated air cools more slowly, at roughly 5 to 6 °C per 1,000 metres. Most of a Pacific air mass crossing the Rockies spends the upper part of its ascent saturated, raining or snowing on the western slopes and losing that moisture permanently.
On the eastern side the air descends, but now it is dry, so it warms the whole way down at the full 10 °C per 1,000 metres. It cooled slowly going up and warms quickly coming down. Air that began at 10 °C at sea level on the Pacific coast, crossed a 3,000 metre range, and descended to 1,000 metres on the Alberta plains can arrive at close to 20 °C — warmer than it started, having gained nothing but the latent heat released from its own lost moisture. The full physics is set out in the guide to foehn winds, of which the chinook is the best-documented example.
The Chinook Arch and the Belt Where It Blows
Chinooks announce themselves visually. The signature is the chinook arch: a long, smooth band of cloud lying along the western horizon, with a sharp lower edge and a strip of clear sky beneath it, often lit spectacularly at sunrise or sunset. It is a foehn wall — the downwind edge of the cloud deck dumping precipitation on the mountains — seen from the dry side, and Calgarians treat its appearance as a reliable forecast.
The geography of the chinook belt is set by where the mountains are steep, the barrier is continuous, and the plains beyond are low.
- Southern Alberta is the heartland. Calgary, Lethbridge and Pincher Creek record chinook conditions on 30 to 40 days a winter, and Lethbridge is among the windiest cities in Canada as a result.
- Montana and Wyoming along the Rocky Mountain Front, with Great Falls, Livingston and Sheridan all heavily affected.
- Colorado's Front Range, where the equivalent events at Boulder are strong enough to cause structural damage, with gusts recorded above 200 km/h.
The name itself is North American. The most widely accepted account is that it comes from the Chinook peoples of the lower Columbia River, and that settlers in the interior Pacific Northwest applied it to a warm wind understood as blowing from the direction of Chinook territory. A separate and less supported story holds that it referred to a wind that thawed the snow around a particular encampment. Either way it entered wide use in the nineteenth century and displaced whatever earlier terms existed.
Downslope Windstorms and the Rotor Beneath
Not every chinook is merely warm. The strongest events are downslope windstorms, in which the air crossing the range behaves less like a breeze and more like water pouring over a weir, accelerating violently as it plunges down the lee slope.
Boulder, Colorado is the best-studied case. Sitting directly at the foot of a steep, continuous barrier, it experiences windstorms most winters in which gusts exceed 160 km/h and have on occasion topped 200 km/h, stripping roofs, felling trees and overturning trucks on the highways east of the Front Range. Research aircraft flights through these events in the 1970s produced much of what is now understood about how mountain waves break, and the Boulder windstorm remains a standard reference case in mountain meteorology.
Underneath and downwind of that plunging flow sits a rotor — a zone of air turning over on itself on a horizontal axis, with violent turbulence and abrupt reversals of wind direction. Rotors are a serious aviation hazard, capable of exceeding an aircraft's control authority at low altitude, and they are the reason airports along the Rocky Mountain Front issue specific warnings during chinook conditions. Glider pilots, conversely, seek out the smooth wave above the rotor, and the lee waves of the Front Range and southern Alberta have produced a long series of soaring altitude records.
What It Does to Snow, Livestock and People
The regional nickname is snow-eater, and it is literal. A chinook can remove 30 centimetres of snowpack in a day, and much of that snow does not melt in the ordinary sense — it sublimates, passing directly from ice to vapour into air dry enough to take it, leaving bare brown grass with no meltwater and no mud.
For nineteenth and early twentieth-century ranching on the Canadian and American plains, this was the difference between a viable industry and a failed one. Cattle left on open winter range cannot feed through deep snow, and the chinook periodically clears the grass and makes overwintering possible. Prairie ranching history is bound up with the wind, and the severe winters remembered in the record are largely the ones in which the chinooks failed to arrive.
The effect on the water year is more consequential than it looks. Snow that sublimates into chinook air is water leaving the catchment entirely rather than running off into rivers and reservoirs in spring. Repeated strong chinooks therefore reduce the effective snowpack available for summer supply across the eastern slope, which matters in a region where irrigation, municipal supply and hydro generation all depend on mountain snowmelt reaching the plains. Measuring how much water is lost this way is difficult, and it remains an active question in Alberta and Montana water management.
The costs are real too. Rapid melting followed by a hard refreeze produces ice layers in the snowpack that livestock cannot break through and that destabilise mountain snow, and avalanche risk rises sharply during and after a strong event. Gusts damage roofs and overturn high-sided vehicles. And, as with the Alpine foehn and the Argentine zonda, a persistent local belief holds that chinook conditions bring headaches, insomnia and irritability, with the same difficulty of separating a physiological effect from the accumulated disruption of a wind that howls for three days. Southern Alberta has produced enough clinical interest in the migraine association for it to have been studied directly, with mixed and much-debated findings.
