Links: Sentinel-2: Cloud Probability in Earth Engine's Public Data Catalog Sentinel-2: Cloud Probability in Earth Engine STAC viewed with STAC Browser The S2 cloud probability is created with the sentinel2-cloud-detector library (using LightGBM). All bands are upsampled using bilinear interpolation to 10m resolution before the gradient boost base algorithm is applied. The resulting 0..1 floating point probability is scaled to 0..100 and stored as a UINT8. Areas missing any or all of the bands are masked out. Higher values are more likely to be clouds or highly reflective surfaces (e.g. roof tops or snow). Sentinel-2 is a wide-swath, high-resolution, multi-spectral imaging mission supporting Copernicus Land Monitoring studies, including the monitoring of vegetation, soil and water cover, as well as observation of inland waterways and coastal areas. The Level-2 data can be found in the collection COPERNICUS/S2_SR. The Level-1B data can be found in the collection COPERNICUS/S2. Additional metadata is available on assets in those collections. See this tutorial explaining how to apply the cloud mask.

For the license on the actual data (rather than the catalog entry), see https://scihub.copernicus.eu/twiki/pub/SciHubWebPortal/TermsConditions/Sentinel_Data_Terms_and_Conditions.pdf