Chinook winds: how a warm, dry wind changes the snow

A Chinook is a warm, dry wind that pours down the lee side of a mountain range, most famously the east face of the Rockies. One working test in an hourly forecast, and the one the rebuilt SkiBuds app will use when it launches, is a temperature jump of more than 10 °F (5.6 °C) in about three hours, humidity falling below 30%, and a wind over 20 mph (32 km/h) blowing down off the range. For skiers that means snow that softens fast, loses its fresh-snow quality faster than the thermometer suggests, and sets up hard once the wind stops.

One Montana thermometer shows how violent, and how local, a Chinook can be

On the morning of January 14, 1972, a National Weather Service cooperative observer at Loma, Montana, read 54 below zero. Loma sits in a valley at 2,580 feet, downwind of the Continental Divide about 50 miles northeast of Great Falls. That evening, Chinook conditions took over. "Shortly after midnight I woke to the sound of a howling wind," the observer said. "I dressed and read the temperature and could not believe it read 34 degrees above." By 8 a.m. on the 15th it was 49. The 103-degree swing in 24 hours is the national record, documented in a paper by National Weather Service and National Climatic Data Center scientists.

The same paper shows how local the effect was. Southwest winds of 30 to 40 mph poured into Loma's valley. Great Falls, in the same event, rose a still-startling 62 degrees, from 29 below to 33 above, and the authors note that no other official station "was ideally situated to feel the full impact of chinook conditions as the Loma COOP site." Most Chinooks are far smaller than Loma's. All of them share its lesson: how hard one hits depends on exactly where you stand, so the forecast that matters is your own mountain's.

Sinking air warms about 5.5 °F for every 1,000 feet it drops

The National Weather Service defines a Chinook as the Rocky Mountain name for a foehn, a warm, dry wind in the lee of a high range. The University of British Columbia's weather course for sailing, flying and snow sports describes two ways to make one.

In the first, a shallow pool of cold air is dammed against the range, and only the warmer air above it crosses the crest. That air gets pulled down the lee slope and warms by compression the whole way. It happens in clear air, which, UBC notes, is why there aren't many photos of it. In the second, humid air climbs the windward side, cools, and drops snow or rain there. Condensation releases heat, so the rising air cools more slowly than dry air would (in warm, saturated air, as little as about 2 °F per 1,000 feet). Sinking down the far side, the now-dry air warms at the full dry rate of about 5.5 °F per 1,000 feet, so it reaches the lee valleys warmer and drier than it started.

Either way, compression does most of the work. Air that sinks 7,000 feet, roughly the drop from the Continental Divide west of Boulder to the town, warms about 38 °F on the way down. When air like that reaches snow-covered ground, UBC's course notes, it can evaporate the snow, which is why a Chinook is nicknamed a "snow eater".

Chinooks hit the east side of the Divide, where the west wind is sinking

The physics puts Chinooks on the downwind side of a range, and in the Rockies that's the east face, from Alberta down through Montana to Colorado's Front Range. In Alberta the Canadian Encyclopedia puts the Chinook belt almost entirely in the south and centre of the province, with about one winter day in three a Chinook day in the southwest. The NWS glossary calls Chinooks a fairly common winter event across the mountainous West and parts of Alaska.

In Colorado, the Divide decides a lot. Eldora and Loveland sit on its east side, downwind of a westerly. Winter Park and Arapahoe Basin sit just west of it, where a west wind is still climbing and that side is more likely to be getting snow, so a Chinook in Boulder says little about conditions there. The same goes for a drive from Calgary to Lake Louise or Banff Sunshine. The Colorado mountain guide and the resort directory have each mountain's own forecast.

Four checks pick a Chinook out of an hourly forecast

The SkiBuds forecast engine looks for four things, and all four have to hold across four consecutive hourly readings, about three hours:

• Rapid warming. Temperature rises by more than 10 °F (5.6 °C).
• Drying. Relative humidity drops by at least 10 points and falls below 30%.
• Sustained wind. Wind stays above 20 mph (32 km/h) in every hour, not just in gusts.
• Downslope direction. Wind comes from the west, west-northwest, west-southwest, northwest or southwest, down off the range.

Each check rules out a lookalike. Warming alone could be a sunny afternoon. Warming with falling humidity could be dry air drifting in on a light breeze, which won't strip snow the same way. The wind test shows the air is moving hard enough to keep replacing itself, and the direction test shows it's coming down the mountain rather than up it. Those directions suit mountains on the east side of a north-south range. On the west side, the same westerly wind is climbing.

The engine grades the damage by melt, drying and recovery time

Warm, dry, fast-moving air removes snow two ways. It melts it, and it sublimates it, turning ice straight into water vapor. Wind speeds up both.

Melt. The engine turns the size of the warming, the strength of the wind and the dryness of the air into a melt rate. An event that barely passes the four checks comes out around a third of an inch an hour, and the estimate is capped at 2.4 inches an hour for the most extreme events. These are formulas, not gauge readings, so use them to compare one Chinook with another.

Sublimation risk. Very dry air and strong wind together strip snow fastest:

Lowest humidity	Average wind	Sublimation risk
Below 15%	30 mph or more	Extreme
Below 22%	25 mph or more	High
Below 30%	20 mph or more	Moderate

Recovery. The engine combines the melt estimate, its confidence in the event and the event's length into a damage grade, then estimates how long the snow surface takes to come back: 18 hours for minor damage, 36 for moderate, 60 for severe and 96 for the most extreme events, lengthened for longer, higher-confidence events. The checks and estimates run inside the SkiBuds forecast engine for the rebuilt iOS and Android app, which is coming soon.

A worked example: one morning's forecast for Eldora

Say the hourly forecast for Eldora reads like this on a January morning. The numbers are made up to show the method.

Hour	Temperature	Humidity	Wind
6 a.m.	22 °F	55%	W 22 mph
7 a.m.	27 °F	45%	W 25 mph
8 a.m.	31 °F	32%	WNW 28 mph
9 a.m.	35 °F	24%	W 30 mph

Temperature rises 13 °F, which clears the 10-degree bar. Humidity falls 31 points and ends at 24%, so the drying check passes. The wind never drops to 20 mph, and every reading is west or west-northwest. All four hold, so this is a Chinook.

Now run the formulas. The melt estimate comes out at about 0.76 inch an hour, more than double the bare-minimum event. Sublimation risk is moderate: humidity is under 30% and the wind averages over 20 mph, but the air isn't dry enough for high. The engine grades the damage minor and puts recovery at about 21 hours. In practice that's a morning to ski the soft snow early, move to shaded and sheltered terrain once the warm wind takes hold, and expect a firm start the next day.

When the checks pass, ski early, find shade and expect a hard morning after

If an hourly forecast passes all four checks, ski the soft snow early, move to shaded and wind-sheltered terrain as the warm wind sets in, and plan for a firm morning after, because surfaces that soften in the wind set up hard once it stops and the temperature falls overnight. In a moderate event, expect variable surfaces and less soft snow than the morning forecast promised. In a severe one, expect major snow-quality loss and a hard refreeze, and move plans that depend on good snow back at least two and a half days, the engine's shortest recovery estimate for a severe event.

The test has limits worth knowing. The biggest is that SkiBuds hasn't checked the four thresholds or the melt and recovery formulas against a record of observed Chinooks. The code's tests confirm the rules run as written, not that they catch real events or match real melt, so read a pass as a reason to look closer and the melt and recovery numbers as rough. The test also runs on a forecast, so if the forecast misses the Chinook, the checks miss it too. Its thresholds also have hard edges: a wind that lifts the temperature 9 degrees in a 40 mph gale fails the first check, however dry it is. And Loma's lesson applies to forecasts too: a point forecast for one mountain can't speak for the next one over.

Outside resort boundaries, a Chinook can raise two of avalanche.org's red flags at once: rapid warming, and wind that moves loose snow into drifts on lee slopes. Read your local avalanche center's forecast before you go; how to read an avalanche forecast walks through one.

If you only have time for two columns, skip temperature, because any sunny afternoon can warm 10 degrees. Humidity dropping under 30% while the wind holds above 20 mph is what makes it the snow eater.