In addition to total cloud cover, topo^alpin shows cloud cover broken down into three height layers: low, mid and high.
Layers
| Layer | Altitude | Typical cloud types |
|---|---|---|
| Low | Surface – 2 km | Stratus, stratocumulus, fog |
| Mid | 2 – 6 km | Altostratus, altocumulus |
| High | > 6 km | Cirrus, cirrostratus |
Aggregation rules
Not all models define layer boundaries the same way. ICON (DWD) uses sigma levels (pressure-based model layers), while ECMWF and GFS use fixed pressure levels. These definitions diverge systematically for the low and high cloud layers:
| Layer | Models used for ensemble | Reason |
|---|---|---|
| Low | ICON EU + ICON D2 only | Sigma levels diverge strongly from pressure levels |
| Mid | All active models | Consistent definition across all models |
| High | ICON EU + ICON D2 only | Sigma levels diverge strongly from pressure levels |
Within the selected model group, the weighted average is computed (same model weights as the ensemble).
Application
- Low clouds: Visibility, fog, stratus — relevant for valley approaches
- Mid clouds: General cloud tendency, precipitation likelihood
- High clouds: Cirrus as a front precursor, little impact on visibility