CUMULUS (CU)

Widely known as "fair weather clouds," cumulus clouds are one of the three major cloud types that comprise the ten main groups or genera. Cumulus (a heap, pile, or mound) or convective clouds are defined as many individual clouds detached and separated by clear skies. The clouds are dense, developing vertically into domes or towers that resemble cauliflower. The sunlit upper portion is brilliant white, and the bases are dark and gray. The outlines of the clouds are sharp when they consist of water droplets and fuzzier or diffuse when they consist of ice crystals. Besides the genera cumulus clouds, there are four other genera that are hybrids of cumulus clouds and differ from the most generic kind of cumulus cloud: cirrocumulus, altocumulus, stratocumulus, and cumulonimbus. In addition to the more general cumulus formation, when cumulus clouds have only slight vertical ascent and are usually confined to the lowest 2 kilometers of the atmosphere they are referred to as cumulus humilis; when cumulus clouds reach moderate vertical extent (generally between 2 to 6 kilometers above the ground) and exhibit small protuberances they are termed cumulus mediocris; finally, when cumulus clouds extend high up into the atmosphere (generally 6 kilometers or more above the ground) and their bulging parts resemble cauliflower they are called cumulus congestus. [See also cirrocumulus; altocumulus; stratocumulus; and cumulonimbus.]