Glossary
- Albedo
- Share of incoming sunshine a surface reflects. Fresh snow about 0.85, old wet snow 0.5, bare
ground about 0.2.
- Analysis
- The model state driven by observed past weather (ERA5), up to the forecast start.
- Anomaly (normalised)
- (value − median) / (q95 − q05) for this unit and date. Within ±0.5 is normal.
- Clearness index
- Ratio of surface sunshine to top-of-atmosphere sunshine. Used to split direct and diffuse
radiation.
- Climatology
- Percentiles (q05, q50, q95) of each variable per terrain unit and day of season, from the model
re-run over past years.
- DA (data assimilation)
- Correcting the model with observations, e.g. MODIS snow cover.
- DEM
- Digital elevation model.
- Deterministic forecast
- A single forecast run, as opposed to an ensemble.
- Downscaling
- Turning coarse weather into weather at the scale of the terrain.
- ENS / ensemble
- The set of ~50 ECMWF forecasts started from perturbed conditions, used to estimate uncertainty.
- ERA5
- ECMWF's global reanalysis of past weather, ~31 km, hourly, from 1940.
- fSCA
- Fractional snow-covered area: the share of an area covered by snow (0–1).
- FSM
- Factorial Snow Model (Essery, 2015), the energy-balance snow model behind the maps.
- GFSM
- Glacier extension of FSM, with an ice layer under glacier units.
- IFS
- ECMWF's Integrated Forecasting System, the global forecast model.
- MODIS
- NASA satellite sensor that maps snow cover daily at 500 m.
- P10 / P50 / P90
- 10th, 50th and 90th percentile across ensemble members: low case, median, high case.
- Pressure levels
- Fixed pressure surfaces (e.g. 700 hPa ≈ 3000 m) on which the weather model provides
temperature, humidity and wind.
- Runoff
- Water leaving the bottom of the snowpack or bare surface: melt plus rain.
- Sky-view factor (SVF)
- Share of the sky hemisphere visible from a point (1 on a plain, lower in a gorge).
- Snow line
- Elevation above which the ground is mostly snow-covered.
- SWE
- Snow water equivalent: the depth of water obtained by melting the snowpack, in mm.
- Terrain unit (cluster)
- A group of pixels with similar elevation, slope, aspect, sky view and location, modelled as
one. About 10 000 per board.
- TopoPyScale 2 (TPS2)
- The open downscaling engine behind SnowMapper.
- TopoSUB / TopoSCALE
- The published methods for terrain clustering and for downscaling, on which TPS2 is based.
- Water year (WY)
- The snow season, named after the year it ends. In most boards it starts on 1 September.
- Wet-bulb temperature
- The temperature a wet surface (or falling snowflake) cools to by evaporation. It decides
rain vs snow.
- Zero-degree level (freezing level)
- Elevation where the air temperature crosses 0 °C.