The edge of a sea of clouds follows the contour lines. On each satellite image, we find the low clouds
(bright in the visible and at 1.6 µm, where snow is dark), then read the terrain height
where the ground emerges from the cloud. With VIIRS (375 m), we use the cloud fraction of each edge pixel and the
20 m elevation model inside the pixel. At night, Meteosat finds low clouds in the infrared: their droplets
emit less at 3.8 µm than at 10.5 µm (1 km resolution).
The base map is a summer Sentinel-2 image (22 July 2026), without seasonal snow. Current snow comes
from Meteosat: bright in the visible but dark at 2.2 µm, it stands out from clouds. For each point, we keep
the latest cloud-free observation, taken near solar noon; the snow of a 500 m pixel is placed on the highest
part of the terrain it covers.
To check that it really is a sea of clouds, we make sure on the image that the terrain well above the
estimated top is clear, and how much of the valleys the cloud layer fills (a partial layer is forming or clearing).
If clouds catch on the peaks, the cloud layer is breaking up or has broken up, and the height no longer means anything. In parallel,
the AROME profile tells whether the air mass is favourable: stable air or a temperature inversion, little wind, no rain.
Sources: Meteosat MTG (EUMETSAT, visible and infrared), NOAA VIIRS (Suomi-NPP, NOAA-20, NOAA-21) and Sentinel-2 L2A via AWS Open Data,
Copernicus DEM GLO-30, AROME Météo-France via Open-Meteo, place names © OpenStreetMap contributors (in French).