Imagine a January morning in the foothills of the Rockies. The thermometer reads a bitter −4°F (−20°C) and the ground is buried under a foot of snow. Then a warm wind starts pouring down off the mountains, and within an hour the temperature has leapt above freezing. By afternoon it's shirtsleeve weather, the snowbanks are collapsing, and puddles are forming on ground that was frozen solid at breakfast. That is a Chinook wind at work.
Native peoples of the northern plains called it the "snow eater," and the name is perfect. A strong Chinook can devour a deep snowpack in a matter of hours, not through sunshine but through sheer warmth and dryness carried down the lee side of a mountain range.
A Chinook belongs to a family of warm, dry downslope winds that meteorologists call foehn winds, found on the sheltered side of mountain ranges around the world. In North America, the classic Chinook blows down the eastern slopes of the Rocky Mountains across Alberta, Montana, Colorado, and Wyoming. The word itself comes from the Chinook people of the Pacific Northwest, and it originally described a warm, moist wind off the ocean — but today "Chinook" most famously means the dramatic snow-eating wind of the high plains.
The magic is in what happens to air as it crosses a mountain range. Picture a mass of air being pushed up the windward side of the Rockies. As it rises, it cools and its moisture condenses into clouds and precipitation, dumping rain or snow on the western slopes. Crucially, when that water vapor condenses it releases latent heat into the air. By the time the now-dry air spills over the crest and races down the other side, it warms rapidly through compression — gaining roughly 5.5°F for every 1,000 feet (10°C per km) it descends.
Because the descending air is both dry and compressing fast, it arrives at the base of the mountains far warmer than the air it's replacing. That's why a Chinook can raise the temperature so violently and so fast — it's not a warm air mass drifting in from somewhere balmy; it's cold mountain air transformed into hot, dry wind on the way down.
Chinooks produce some of the most extreme short-term temperature changes ever recorded. The town of Loma, Montana holds a famous record: on January 15, 1972, the temperature rose from −54°F to 49°F (−48°C to 9°C) in just 24 hours — a swing of 103°F (57°C). Even faster, Spearfish, South Dakota jumped from −4°F to 45°F (−20°C to 7°C) in only two minutes on January 22, 1943, as a Chinook washed over the town. Calgary, Alberta, sees dozens of Chinooks each winter, giving the city its reputation for wildly changeable cold-season weather.
The same downslope-warming process powers famous winds on other continents. In the European Alps it's called the foehn, in Argentina the zonda, in California the Santa Ana winds share the dry, warm downslope character, and in the Andes and elsewhere similar winds go by dozens of local names. Wherever tall mountains meet moving air, the leeward side tends to end up warmer and drier — the fingerprint of a foehn wind.
| Feature | What a Chinook does |
|---|---|
| Temperature | Rises fast — sometimes 40°F+ (22°C+) in an hour |
| Humidity | Drops sharply; air becomes very dry |
| Snow | Melts or sublimates rapidly — the "snow eater" |
| Wind | Gusty, sometimes damaging, from the west or southwest |
| Sky | Often a clear "Chinook arch" cloud band over the mountains |
A Chinook is a warm, dry downslope wind that forms as air is wrung of its moisture crossing a mountain range, then heats up dramatically as it compresses on the way down the lee side. Along the eastern Rockies it can send temperatures soaring by dozens of degrees in minutes and strip away a deep snowpack in a day — earning its old name as the snow eater. It's a vivid reminder that mountains don't just block the weather; they manufacture their own.