coalescence The process by which the size of cloud water droplets increases as larger drops (more than 19 μm) fall and collide with smaller drops. Ice crystals in frozen cloud tops may also fall and grow by coalescence with drops below, ultimately melting into raindrops. In cumuliform cloud, turbulence may encourage this process of drop growth by the upthrust of small droplets to overtake and coalesce with other droplets. Repetition of the process can eventually produce rain-sized droplets. Factors favouring coalescent growth are high moisture content, water cloud of large vertical extent, and strong upward turbulence. See also Bergeron theory and collision theory.

coalescence Process of cloud water droplet enlargement, as larger drops (more than 19μm) fall and collide with smaller drops. Ice crystals in frozen cloud tops may also fall and grow by coalescence with drops below, ultimately melting into raindrops. In cumuliform cloud, turbulence may encourage this process of drop growth by the up-thrust of small droplets to overtake and coalesce with other droplets. Repetition of the process can eventually produce rainsized droplets. Factors favouring coalescent growth are high moisture content, water cloud of large vertical extent, and strong upward turbulence. See also BERGERON THEORY; and COLLISION THEORY.