Method
How Cirque works
Cirque estimates natural snow from real weather, not from resort marketing. This page sets out what goes in, how it becomes the snow you see on the globe, how well it matches measured snow depth, and where it falls short.
Eleven seasons of weather
For each of 44 resorts we take daily snowfall and mean temperature from January 2014 to September 2025, from the ERA5 reanalysis via Open-Meteo. ERA5 is the European Centre for Medium-Range Weather Forecasts’ reconstruction of the global atmosphere, hour by hour, since 1940.
Weather is taken at three heights for every resort: the village, mid-mountain and the summit, with temperature corrected to each altitude.
From weather to snow on the ground
A simple snowpack model runs day by day. Snow builds up as it falls and melts on days above freezing, at 4.5 mm of water per degree per day, settling to about 300 kg per cubic metre. The pack starts from nothing each season, on 1 October in the northern hemisphere and 1 April in the southern.
A day counts as snow-sure when the modelled depth is 30 cm or more, roughly what an ungroomed piste needs to ski. Every figure on Cirque is summarised over eleven seasons in half-month windows, so you can see how often the snow was there on your dates, and how bad the worst year was.
Painting it onto the mountains
On the globe, modelled depth is spread across real terrain by elevation, blending between nearby resorts. Far from any modelled resort, a simpler snowline that varies with latitude and season takes over. North-facing slopes hold up to 18% more snow than sunny ones in the northern hemisphere, and the reverse in the south.
The pink line is the snowline: where natural snow runs out at 30 cm. Runs and lifts come from OpenSkiMap. Each run section carries its own elevation, so sections below the snowline for your dates are dashed.
Checking it against measured snow
To test the model we compared it with four SNOTEL stations run by the US Department of Agriculture, which measure snow depth every day inside or beside ski areas we model. For each day from November to April over eleven seasons, we set the model’s depth at the station’s exact height against what the station measured.
The model gets the shape of the season right, with day-to-day correlation of 0.86 to 0.94, and it agrees with the stations on whether there is a skiable 30 cm on 81% to 92% of days. But it is conservative: depths come out 20% to 60% shallower than measured, most of all at Alta, one of the snowiest places in North America. ERA5 smooths mountains into broad slopes, so it misses the extra snow that ridges and high basins catch.
Treat Cirque’s depths as a floor, not a forecast. Where it calls a date snow-sure, the measurements almost always agree; where it calls one marginal, the real mountain may well be better.
What it doesn’t know
Cirque models natural snow only. It knows nothing about snowmaking, which many resorts rely on for their lower runs, and nothing about grooming, wind scouring or the weight of skiers. Glaciers are treated as ordinary terrain.
Australian resorts in particular depend on snowmaking, so their natural-snow figures look worse than the skiing usually is. Current conditions are a forecast from Open-Meteo, not a resort snow report. None of this says anything about avalanche danger: always check the local bulletin before leaving the pistes.
Sources
Weather history and live forecasts: Open-Meteo, using ERA5 from the Copernicus Climate Change Service. Snow-depth measurements: USDA Natural Resources Conservation Service SNOTEL network. Ski areas, runs and lifts: OpenSkiMap, from OpenStreetMap contributors under the Open Database License. Terrain: AWS Terrain Tiles. Basemap: OpenFreeMap and OpenMapTiles.