The slopes of cumulus clouds in relation to external wind shear

Abstract

Abstract An equation for the slope of cumulus clouds is derived. It is shown to depend upon the vertical ascent rate, the external wind shear and the horizontal frictional forces between the cloud and its surroundings. These frictional forces are of two origins: one due to entrainment or mixing with the outside air, and the other due to form or body drag. Four observational cases are presented, one concerning trade cumulus, and three involving clouds in mid‐latitudes. In two of these, measurement of updraught speed, wind shear, and cloud slope permit an estimate of the magnitude of the friction forces. The minimum contribution due to entrainment is approximately equal to that inferred by Stommel for trade cumulus.