Various cloud-detection schemes are applied to 1.1 km Advanced Very High Resolution Radiometer (AVHRR) day- and night-time data to determine an optimum automated scheme for deriving cloud-free radiances over both land and sea. A combination of the spatial coherence method at infrared wavelengths (11 #x3BC;m) and dynamic visible threshold methods proved to be the most effective scheme for day-time use. Uniform thin cirrus (i.e. reflectance less than 15 per cent) was difficult to detect with all methods. Problems were also encountered over regions with a changing underlying surface type (e.g. coastal areas) where the automated scheme was not as effective as over uniform surfaces. At night a combination of the spatial coherence method and a scheme based on the differences in brightness temperatures between the three infrared channels at 37, II and 12 #x3BC;m wavelength was successfully used. Results obtained by applying these algorithms to AVHRR data are presented and the different problems encountered with each algorithm are discussed.
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