Luke Howard’s three genera of clouds reflect the mechanisms of their formation. Cloud forms when moist air is cooled, generally as a result of rising through the atmosphere, becoming saturated. When an extensive mass of air rises uniformly, the result is “stratus” or layer cloud, with few breaks and a generally vague and diffuse structure. In other situations, convection breaks out, with buoyant plumes of warm air rising rapidly through a generally clear and slowly descending environment. Convection is characterised by rapidly developing “cumulus” clouds, cauliflower shaped masses of vapour, with generally sharp edges and dramatic shapes. Cirrus cloud is made up of fibres or strands of cloud, and occurs when the air is so cold that water condenses as ice crystals rather than as liquid water droplets. Cirrus clouds generally form high in the atmosphere, 6-12 km above the ground, while stratus and cumulus clouds generally have bases only 1 or 2 km above the ground.
By and large, it is the cumulus clouds which particularly appeal to Constable. In the sequence of pencil sketches illustrated here, all but three or four represent cumulus clouds in varying degrees of development. No doubt, it was the bold shapes, the dramatic lighting effects and the sense of movement which appealed to this Romantic painter.