in global heat transport. When surface water evaporates, the heat required to change liquid water into vapor is absorbed from the surface and carried along with the vapor into the air. When water vapor condenses into a cloud and falls as rain, it releases that heat, known as latent heat, into the air. The processes that control the conversion of water vapor into cloud and precipitation particles are called cloud microphysical processes. The interaction of these processesdetermines the properties of clouds that, in turn, determine the effect of clouds on the radiative energy exchanges, whether the cloud will produce precipitation, how much and what type of precipitation it will produce, and how long the cloud will last. At temperatures above freezing (0?C), the weak vertical air motions (slow cooling) associated turbulence near the surface or with large-scale circulations lead only to the formation of rather small cloud droplets (about 5-10 um in radius, 1 micron = 1 millionth meter) covering very large areas. For typical concentrations of small aerosols on which the droplets form (from about 50-200 cm ?3 over oceans to about 500-2000 cm ?3 over land), the total amount of vapor converted to droplets is small, equivalent to about a layer of water about 0.01-0.03 mm deep. Such clouds, ranging from scattered fair-weather cumulus toextensive sheets of stratocumulus, produce no precipitation and last only as long as the upward motions continue (usually about 10-20 min for cumulus but days for stratocumulus) because such small droplets fall very slowly