From Glossary of Meteorology


(Abbreviated Cu.) A principal cloud type (cloud genus) in the form of individual, detached elements that are generally dense and posses sharp nonfibrous outlines.

These elements develop vertically, appearing as rising mounds, domes, or towers, the upper parts of which often resemble a cauliflower. The sunlit parts of these clouds are mostly brilliant white; their bases are relatively dark and nearly horizontal. Near the horizon the vertical development of cumulus often causes the individual clouds to appear merged. If precipitation occurs, it is usually of a showery nature. Various effects of wind, illumination, etc., may modify many of the above characteristics. Strong winds may shred the clouds, often tearing away the cumulus tops to form the species fractus. Under certain conditions cumulus clouds may be arranged in files, cloud streets, oriented approximately parallel to the wind direction. Changes in direction of illumination and in background cause modification of color and of apparent surface relief. Cumulus is composed of a great density of small water droplets, frequently supercooled. Within the cloud larger water drops are formed that may, as the cloud develops, fall from the base as rain or virga. Ice crystal formation will occur within the cloud at sufficiently low temperatures, particularly in upper portions as the cloud grows vertically. Occasionally the growth of ice crystals at the expense of water droplets will reduce the entire cloud to diffuse trails of snow. Cumulus most often forms directly in clear air as a result of convection in air of sufficiently high moisture content for a condensation level to be reached. As a result, a distant diurnal cycle of cumulus frequency is observed. Over a landmass, the cumulus maximum occurs after midday (for a horizontal extent, early afternoon; for vertical extent, somewhat later). Over a water surface, the cycle is reversed and much less obvious, with the cumulus maximum generally recognized as occurring after midnight. The vertical growth of a cumulus cell is restricted and modified by the existence and character of layers of relative static stability above the cloud base. Cumulus may also evolve from the convective transformation of stratus or stratocumulus (Cu stratomutatus or Cu stratocumulomutatus). Cumulus may be generated by altocumulus and, again, stratocumulus (Cu altocumulogenitus and Cu stratocumulogenitus).Cumulonimbus is the ultimate manifestation of the growth of cumulus; therefore, at a certain point, it is difficult to differentiate between the two. If a cloud in doubt reveals no fibrous structure, it is still cumulus; if still in doubt, cumulonimbus further differs in that it is accompanied by lightning, thunder, and sometimes hail. The elements of altocumulus are smaller and, along with those of stratocumulus, tend to be more merged than the separated units of cumulus. Cumulus has the unique ability to penetrate other preexisting cloud layers, sometimes partially dissipating, at other times apparently fusing with, the impaled layers. The cumulus, in this instance, retains its identity as long as it remains primarily vertically developed, is physically (although perhaps not visibly) separate from the other cloud, and has a tower- or dome-shaped summit.
See cloud classification, trade-wind cumulus.