AVHRR image composed of radiances from channels 1, 2 and 4. This combination of shortwave channels with an infrared channel is often used because it allows the discrimination between different cloud and surface types. In cloud-free areas, land surfaces appear brown, red or slightly green and water is marked by deep blue. The colour of optically thick clouds depends on the cloud-top height and varies between yellow for relatively low clouds and white for high cloud-tops. In addition, thin cirrus clouds can be detected mainly over water surfaces when no other cloud type is present. This is in the contrast to surrounding cloud-free areas, where the signature of cirrus clouds is a faint bluish colour. Data recorded on 21 November 2006.