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.