1. A method of detecting cloud features, the method comprising: obtaining ( 402 ) image data ( 401 ); identifying ( 404 ) regions of the image data corresponding to a below-horizon region and an above-horizon region; classifying ( 404 ) one or more parts of the above-horizon region as a cloud feature; determining an identification model specifying a visual appearance of some or all of the parts of the above-horizon region that have been classified as a cloud feature; and using the determined identification model, classifying, as cloud features, those regions of the of the below-horizon region whose visual appearance is as specified by the identification model.