Past exam of the physics of atmospheres and oceans course of the University of Oxford / 2019 / 4 / e

A broad initial cloud-droplet size distribution can be seeded by giant or unusually soluble cloud condensation nuclei, producing a minority of larger droplets before gravitational collection becomes efficient. Variable supersaturation histories, entrainment and mixing can broaden the distribution relative to a perfectly homogeneous monodisperse model. Turbulent relative motion and preferential concentration can increase encounters, and rare successful collisions can create unusually rapid stochastic collectors. These mechanisms can work together; a single deterministic condensational law applied equally to identical droplets cannot create the initial differential fall speeds by itself. For warm rain, the subsequent amplification is liquid collision-coalescence, distinct from ice-phase precipitation mechanisms.