Moist Air and Clouds

Abstract All the water vapor in clear air condenses to about 1 cm of liquid at midlatitudes. And yet, precipitation from a single storm can greatly exceed this—evidence that storms are giant engines for processing water. The lifting-condensation level is the height to which a parcel can rise where its temperature equals its dew point. This is often a good estimate of the base of cumulus clouds with their roots in the planetary boundary layer. The contribution of variable water vapor to the density of moist air can be accounted for by a virtual temperature, calculated but not measured with a thermometer. It is always greater than air temperature by a few degrees Celsius. If a rising, initially dry parcel becomes saturated, the lapse rate decreases from dry adiabatic to moist adiabatic. This warming process determines equivalent potential temperature, conserved in moist and dry processes and a critical parameter in the analysis of convective systems.