raceday.ski continuously compares the calculated snow temperature with real measurements: over 130 IMIS stations of the WSL Institute for Snow and Avalanche Research SLF measure the snow surface temperature in the Swiss Alps. Every clean measurement-model pair flows into the evaluation below — automatically and without preselection. How the calculation works is explained on the methodology page.
Measurement operation starts with the winter season
In summer the IMIS stations are snow-free — the plausibility check discards surface temperature readings without snow, so deliberately no comparison pairs are created. As soon as the stations are snow-covered again, raceday.ski automatically collects pairs of calculated and measured snow temperature; this page then fills itself — without manual selection, without massaging the numbers.
The metrics and their definitions are already fixed (below) — so it is clear in advance what the model will be measured against.
Metrics — how they are calculated
- Error = calculated − measured snow surface temperature (°C). The comparison uses the pure physics calculation of the 3-layer model — before residual correction and before the Kalman station adjustment. Deliberately so: the station correction that is additionally active in the product learns from the very same measurements; including it would flatter the accuracy.
- Bias = mean of the errors (systematic deviation; negative = the model runs too cold). MAE = mean absolute error. RMSE = root mean square error (penalises outliers more strongly).
- Pairing: only calculations for the current day whose start time is at most 2 hours away from the measurement time; each 30-minute measurement enters at most once per station and time-of-day window. Ground truth is the nearest IMIS station in each case (max. 15 km distance, max. 500 m altitude difference — the spatial distance is part of the reported error).
- Dimensions: time of day in four 6-h windows (local time Europe/Zurich, identical to the correction windows of the Kalman filter); weather situation from the cloud cover at calculation time (clear < 25 %, variable 25–75 %, overcast > 75 %).
- Data window: rolling over the last 90 days (retention of the raw pairs); additionally the last 7 days separately.
Measurement data: WSL Institute for Snow and Avalanche Research SLF (www.slf.ch), licence CC BY 4.0. In Austria, additionally surface-temperature measurements from the Tyrol Avalanche Warning Service — Datenquelle: Land Tirol - data.tirol.gv.at (CC BY 4.0). On Swiss and Austrian ski resort pages, raceday.ski shows the live comparison with the nearest station.
To the wax advisor — or to the methodology: how the 3-layer model calculates and where its limits are.