cumulus clouds are puffy and usually have well-defined boundaries. They form from the condensation or deposition of moisture in particles known as cloud nuclei present in the moist updrafts of convective plumes. The cloud particles can be composed of liquid water, supercooled water, or ice. These cloud particles are denser than air; therefore they increase the density of cumulus updrafts. However, water vapor is lighter than dry air, and therefore, except for the effects of the cloud particles, the moist updraft air is lighter than dry air at the same temperature and pressure. This effect of moisture on air density is known as virtual temperature. Scientists have shown that the virtual temperature effect is responsible for about 50 percent of the buoyancy of convective plumes that form cumulus clouds in the tropics and subtropics, but has negligible or even negative effects over desert and semi-desert areas.
The base of cumulus clouds is usually flat, because they form when moist air rising from the surface reaches its lifting condensation level. The height of the lifting condensation level depends only on the properties of the updraft air, therefore it is constant in updrafts mixed by turbulent eddies. The height of the cloud bases usually ranges from a few hundred meters over the oceans, to more than 16,404 ft. (5,000 m.)
over dry desert and semi-desert areas. The shape and size of cumulus clouds depends on the intensity of the updrafts causing them to form. Individual updrafts form the various cumulus towers that compose single cumulus clouds. The rounded tops of these towers are the boundaries of convective plumes reaching their level of neutral buoyancy. Cumulus clouds can develop into giant cumulonimbus in environments convectively unstable over large depths.
Convective circulations are heat engines because they convert heat into bulk fluid motion such as con-vective updrafts, downdrafts, and the complete con-vective circulation. The efficiency of this conversion of heat into kinetic energy depends on the depth of the convective layer. Therefore, deeper convection produces stronger updrafts and more well defined cumulus towers than shallow convection. Cumulus clouds can cause showery rain. Over deserts and semi-arid regions, the rain evaporates before reaching the surface. This is known as virga.
Global climate warming will likely produce increases in the amount of cumulus clouds. This happens because surface warming, coupled with stratospheric cooling, increases convective instability. The height of the bases of cumulus clouds might increase over land because global warming is expected to increase the surface temperature and reduce humidity. Illumination and wind has strong effects on cumulus clouds, in particular, on their appearance and organization. Changes in illumination and background cause changes in color and the apparent surface relief of cumulus clouds. Wind shears can shred the top of cumulus clouds, forming the species known as cumulus fractus. Wind can also orient clouds into rolls or cloud streets and produce waves and lenticular clouds to form above them.
SEE ALSO: Clouds, Cirrus; Clouds, Stratus.
BIBLIOGRApHY. K.A. Emanuel, Atmospheric Convection (Oxford University Press, 1994); N.O. Renno and A.P. Ingersoll, "Natural Convection as a Heat Engine: A Theory for CAPE," Journal of the Atmospheric Sciences (v.53, 1996); N.O. Renno and E.R. Williams, "Quasi-Lagrangian Measurements in Convective Boundary Layer Plumes," Monthly Weather Review (v.123, 1995).
Nilton O. Renno University of Michigan
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