How snow level is derived

Last updated: 9 August 2026

Snow settling level is derived by interpolating the 0°C crossing from the model temperature profile, then applying conservative, and selectable, corrections:

1. GFS freezing level (primary for GFS). The native GFS freezing_level_height field is used as the primary snow level source for GFS. GFS profile interpolation is also available as an optional comparison line.

2. Profile interpolation (primary for EC; optional for GFS). Temperature and geopotential height at all available pressure levels: EC uses 1000, 925, 850, 700, 600 hPa; GFS uses 1000, 975, 950, 925, 900, 850, 800, 750, 700, 600 hPa. Walking upward from the surface, the first adjacent pair where temperature crosses 0°C is found and the crossing height is linearly interpolated using actual geopotential heights in metres MSL. EC's limited pressure level availability means greater uncertainty, especially when 850 hPa temperature is above zero with 700 hPa as the next available data point (~1550m gap). GFS's available 9-level profile provides better resolution.

3. Wet-bulb correction. Correction = (100 − RH%) × 3.5 m using 850 hPa RH (preferred; ~1450m, closest to the phase-transition layer), falling back to 1000 hPa RH. 0 m at saturation; up to 350 m at very low humidity. In practice this correction is small during active frontal precipitation when the sub-cloud layer is near-saturated — it is most relevant in showery or post-frontal conditions with drier air.

4. Precip cooling. Sub-cloud evaporative cooling: −40 m (≥1 mm/hr), −80 m (≥3 mm/hr), −120 m (≥10 mm/hr).

5. Orographic forcing. Applied only for SE Australian mainland locations. When 850hPa wind is from 240–330° (NW–W–SW frontal fetch) at ≥5 m/s during precipitation (≥0.5 mm/hr), forced ascent over the ranges depresses the effective snow level on windward slopes. Linear scale: −30m at 5 m/s to −75m at ≥15 m/s. T850 floor uses the saturated adiabatic lapse rate (143m/°C) during precipitation, dry adiabatic (100m/°C) otherwise.

Thickness lines (optional). The traditional 1000–500 hPa thickness is a standard proxy for lower-atmosphere temperature: 5240 m thickness = snow to sea level, +1000 m per 120 m of additional thickness. Two additional thickness layers are also provided: 1000–700 hPa (base 2767m, slope 16.2m/m) and 1000–850 hPa (base 1278m, slope 35.5m/m), both calibrated to match the 1000–500 formula for the same atmospheric state. These use only the lower-troposphere layer most relevant to alpine snow levels. All thickness lines are shown without corrections.

850hPa Temp derived snow line (optional). Snow level estimated as 1450 + T850 × lapse_rate — the height at which a parcel at 850 hPa would reach 0°C following the adiabat downward. Uses SALR (143m/°C) during precipitation, DALR (100m/°C) otherwise. Shown as a dashed line, no corrections applied.

Data: Open-Meteo.com (ECMWF IFS 0.25° + NOAA GFS Seamless), CC BY 4.0 — non-commercial use.