Cumulus cloud is the opposite of stratus in several ways, being isolated and vertical rather than horizontal and extensive, and it forms more rapidly. The flat base of cumulus at the condensation level differs completely from its cauliflower-like sides, where turrets thrust upwards and then unfold to spill sideways and down. Buoyant parcels of air within the cloud bulge outward, then entrain air from the environment so that the surface of each parcel expands and becomes cooler by mixing with the enveloped air and by evaporation of cloud droplets into it. This leads to negative buoyancy (i.e. air heavier than the surroundings) and consequently a downdraught within the cloud, maybe continuing to the surface.
There is a range of size, from small fair-weather cumulus to cumulo-nimbus clouds. Cumuliform clouds can grow to a height of 17 km near the equator. All cumulus has a well-defined appearance, with plenty of surface detail where they are composed of water
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