Instability driven by settling and evaporation in a shear flow: A model for asperitas clouds

Abstract

Designated a new cloud feature in 2017, asperitas clouds are wave-like formations on the underside of layer clouds. We propose a mechanism for asperitas cloud formation: an instability driven by the settling and evaporation of water droplets. For suitable droplet size and liquid water content, a layer of dense air forms, which in moderate ambient shear gives rise to cloud structures of the asperitas type.