To the sky studies he added notes, often on the back of the sketches, of the prevailing weather conditions, direction of light, and time of day, believing that the sky was "the key note, the standard of scale, and the chief organ of sentiment" in a landscape painting. In this habit he is known to have been influenced by the pioneering work of the meteorologist Luke Howard on the classification of clouds. "I have done a good deal of skying", Constable wrote to Fisher on 23 October 1821.