Cloud properties

Cloud properties addresses the problem of establishing where clouds are and what they are like: cloud cover, cloud top height, temperature and pressure, cloud type and thermodynamic phase, optical depth, and liquid and ice water path. The Global Climate Observing System (GCOS) lists Clouds as an Essential Climate Variable, states that cloud properties have profound effects on radiation and precipitation, and lists cloud cover, optical depth, liquid and ice water path, drop effective radius, and top temperature and height among its products ^[1]. Within cloud properties, effective radius and water path are treated as bulk retrieval outputs; interpretation at the scale of individual particles is the subject of aerosol and cloud microphysics.

The World Meteorological Organization (WMO) OSCAR database lists cloud cover as a requirement for global and high-resolution numerical weather prediction (NWP), nowcasting and atmospheric climate monitoring, and cloud type for global NWP, high-resolution NWP and nowcasting, with aeronautical meteorology requirements for both ^[2][3]. NOAA lists cloud top height, temperature and pressure for judging cloud growth and precipitation likelihood and for initialising clouds in NWP models, and states that its clear sky mask identifies cloud-free pixels for products such as land and sea surface temperature ^[4]. The WMO CIMO Guide names solar power forecasting for photovoltaic systems as an important application of daytime cloudiness measurement ^[5], and a peer-reviewed SEVIRI method nowcasts direct normal irradiance up to 120 minutes ahead for concentrating solar plants ^[6].

Multi-band passive imagers underlie the operational products, and Fmask is a published cloud and cloud shadow detection algorithm for Landsat and Sentinel-2 imagery ^[7]. NASA MOD06_L2 derives cloud top from infrared radiances and optical thickness and effective radius from visible and near-infrared reflected radiances ^[8]. EUMETSAT lists near-real-time Level-2 cloud products for Meteosat SEVIRI covering cloud mask, phase, cloud top pressure, optical thickness and effective radius at about 3 km and 15 minutes ^[9]. METimage, 3MI and CO2M CLIM cloud products are listed only as test data packages, not yet operational data ^[9]. Climate records built on these instruments include CLARA-A3 from AVHRR, 1979 to 2020 ^[10] and CLAAS-3 from SEVIRI, 2004 to present ^[11].

Active profiling adds vertical structure. The CALIOP 5 km cloud layer product gives layer base and top altitude and optical depth per column ^[12], and CloudSat 2B-GEOPROF-LIDAR combines radar and lidar for layer base and top heights ^[13]. The CALIPSO and CloudSat science missions have ended ^[12][14], while ESA lists EarthCARE Level 2 cloud products with releases from November 2025 ^[15].

The WMO CIMO Guide states that from space cloud amount and temperature are observed while cloud top height is inferred, and that low stratus is hard to separate from fog ^[5]. The GEWEX cloud assessment identifies thin cirrus above low cloud as the main driver of differences between datasets in high-cloud amount ^[16]. The geostationary CLAAS-3 record is regional, covering Europe, Africa, the Atlantic and parts of South America, the Middle East and the Indian Ocean ^[11].