Abstract
An operative method has been developed for determining cloud amounts by digital data in 0.4–1.1 μm spectral band from METEOSAT. The main endeavour is the estimation of cloud amounts within satellite pixels. It is supposed that totally clear and totally cloudy skies may be separated using a lower and an upper threshold value of brightness. In areas of the brightness interval between the two thresholds the cloud amounts are defined by variance analysis (for subsets of 4×4 pixels). The brightness values are corrected according to variation of solar zenith angle. The effect of the atmosphere is considered as uniform in the whole investigated area. The cloud amounts are evaluated for areas of 16×16 pixels (about 50×70 km in Central Europe). In a case study 71 per cent of the differences between satellite and surface observations were within ± 1 okta. The agreement was especially large (92 per cent) when ground observer reported total coverage.
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