Section 4.4 Formation of precipitation
Most condensed droplets and ice crystals in a cloud are too small to fall through the atmosphere (and would rapidly evaporate): condensed droplets are typically on the order of 10-20 microns.
Rain droplets form primarily through collision-coalescence where droplets collide and merge until heavy enough to fall through the atmosphere (100s microns)
Ice crystals can grow by accretion around nuclei or aggregation of ice crystals.
The Bergeron-Wegener Process can occur if a cloud contains both liquid water and ice. Because the vapor pressure is greater over water than ice (that is, it is easier for water molecules to evaporate from the liquid), the ice can grow at the expense of the liquid. This can lead to rapid growth of ice crystals in the cloud.
There are different types of precipitation:
rain: the precipitations reaches the surface while staying in the liquid phase (it may begin as ice higher in the atmosphere)
snow: the precipitation falls as ice/snow crystals all the way from the cloud to the surface
freezing rain: the precipitation falls as a liquid for part of its descent, but low surface temperatures cause the rain to freeze on contact. This is often associated with an inversion.
sleet: the precipitation falls as a liquid for part of its descent, but freezes into ice pellets before reaching the surface. This is often associated with an inversion.