BERGERONFINDEISON PROCESS

  a theory of raindrop formation. At temperatures between -5蚓 and -25蚓 both water droplets and ice crystals exist in the same space. Vapour is sublimated onto the ice crystals and the deficit in vapour is compensated by the evaporation of the water droplets leading to further sublimation and thus growth of the ice crystals. These may coalesce into snowflakes which then overcome gravity and fall. When air temperatures at the surface are greater than zero the flakes melt into water drops before they land.