I appreciate that rain can fall due to the process of coalescence described below, but I was under the impression that nucleation based around atmospheric dust particles was a vital part of the process, and is responsible for most rain showers.
Coalescence occurs when water droplets fuse to create larger water droplets. Air resistance typically causes the water droplets in a cloud to remain stationary. When air turbulence occurs, water droplets collide, producing larger droplets. As these larger water droplets descend, coalescence continues, so that drops become heavy enough to overcome air resistance and fall as rain. Coalescence generally happens most often in clouds above freezing, and is also known as the warm rain process.
In clouds below freezing, when ice crystals gain enough mass they begin to fall. This generally requires more mass than coalescence when occurring between the crystal and neighboring water droplets. This process is temperature dependent, as supercooled water droplets only exist in a cloud that is below freezing. In addition, because of the great temperature difference between cloud and ground level, these ice crystals may melt as they fall and become rain.
Formation of Raindrops
About a year ago I read An Introduction to Thermal Physics: Daniel V. Schroeder and although I can't find the book at the moment to check, I am pretty sure that the author did stress the importance of atmospheric dust, it's just I can't remember his explanation.
Cloud seeding by aircraft in drought prone areas is another factor illustrating the importance of nucleation, in this case using silver iodide.
My questions are:
Why don't clouds rain more often, as the water vapor "droplets" are randomly pushed together by both differential heating in the cloud and air turbulence, which is usually greatest at the base of the cloud?
Why the need for dust as the initiator of rain (and hail/ sleet)?
There is a related question Why does water fall as streams, but it doesn't address the seeding issue.