Cloud Droplet Growth

Natural Condensation -- Condensation commences at RH=100% when air is cooled. (Would take 24 hours for a droplet to grow to the size of a small raindrop)

Condensation on Hygroscopic Nuclei -- Condensation commences with the presence of Cloud Condensation Nuclei at RH=80%, but droplets only grow rapidly at RH=100%. (Still takes over 10 hours for droplets to grow to the size of a small raindrop.)

Collision and Coalescence -- Droplets grow by accretion when they strike other small droplets. Chiefly a process in the Clouds of Vertical Development family, but also with altocumulus and cirrocumulus. Droplet growth is rapid the greater the volume of the cloud and the more turbulent the flow in that cloud. Best developed in cumulonimbus clouds.

Ice Crystal (Three Phase) Process -- When RH<100% and water droplets occur simultaneously with ice crystals (between 0 and -20C) water droplets evaporate, but ice crystals can continue to grow. Ice crystals resist evaporation (actually sublimation) even at RH as low as 80%. Ice crystals can continue to grow as random collisions between water vapor molecules leaving the evaporating water droplets collide with the ice crystals. Eventually the water droplets disappear, but the ice crystals grow to such a size that they fall out through the freezing level and arrive at the ground as small to moderate sized rain drops.