Cloud Base & Freezing Level

Cloud Base

The cloud base is calculated per model using the Hennig formula:

$z_{\text{base}} = (T - T_d) \times 125 + z_{\text{DEM}}$

Where $(T - T_d)$ is the spread between temperature and dew point and $z_{\text{DEM}}$ is the DEM elevation of the API grid point. The factor 125 m/°C corresponds to the dry adiabatic gradient.

Inversion guard: When spread < 2.5 °C (fog/stratus), the cloud base is set to a minimum of 200 m above ground level.

The ensemble value is the weighted average of all active models.

Freezing Level

Five models provide the freezing level: ICON EU and ICON D2 (via snowfall_height, wet-bulb temperature), GFS, ECMWF and AIFS (via pressure-level interpolation). AROME is excluded (no pressure-level data). Details: Snow Level.

When the freezing level is below the location elevation, the row is highlighted in red – snow down to the valley.