On a Criterion Governing the Mode of Cloud Formation in Planetary Atmospheres

Abstract

Condensation in vertically moving planetary atmospheric gases containing small quantities of any condensable vapor will occur in updrafts only if the latent heat of the viper is sufficiently large, otherwise the condensation will require adiabatic compression and will occur only in downdrafts, as first noted by Brunt. Several methods are presented for viewing and deriving a quantitative criterion for the intermediate case in which the vapor behaves in neutral fashion in either ascending or descending currents. Put in its most general form, relating the molar entropy of vaporization S of the condensable vapor and the molar specific heat Cp, of the carrier gas, the null criterion requires S= Cp. The peculiar nature of meteorological processes in a planetary atmosphere in which latent heat considerations imply downdraft condensation is examined in some detail, and some implications of the null condensation criterion for studies of planetary atmospheres and their evolution are discussed.