Snow Level

The snow level is one of the most important parameters for alpine tour planning. topo^alpin uses two different methods depending on the weather model.

snowfall_height (ICON EU & ICON D2)

The DWD models ICON EU and ICON D2 directly provide the snowfall_height variable — the altitude below which precipitation falls as rain. This quantity is based on the wet-bulb temperature and is physically more accurate than dry-bulb interpolation:

• Accounts for evaporative cooling as snowflakes fall
• Correctly handles isothermal layers (0 °C level ≠ snow level)
• Computed directly from the model grid, no post-processing needed

When snowfall_height is available, it replaces pressure-level interpolation as the primary snow level for ICON and ICON D2.

Pressure-level interpolation (GFS, ECMWF, AIFS)

For models without snowfall_height, the freezing level is interpolated from temperatures at the 850 hPa and 700 hPa pressure levels:

$z_{0°} = z_{850} + \frac{T_{850}}{T_{850} - T_{700}} \times (z_{700} - z_{850})$

This method uses the dry-bulb temperature and can overestimate the actual snow level by 100–300 m, especially in humid conditions.

Ensemble calculation

The snow level shown in the meteogram is the weighted average of active models:

AROME provides neither pressure-level data nor snowfall_height and is therefore excluded from the snow level calculation.