Droplet Growth Process

In categorization of aerosols and their sources at the beginning of this unit, we defined three categories of aerosols. These categories grouped aerosols ranged less than 0.2 micrometers to more than 2 micrometers in diameter. From the table below, a typical cloud condensation nucleus (CCN) is about 0.2 micrometers in diameter, while a typical raindrop is about 2000 micrometers or 2 mm in diameter. The increase in size from a CCN to a raindrop represents a growth of 10,000 times the initial size of the CCN.

Some terminology is warranted at this point. We use the term precipitation to refer to all liquid or solid ice particles that are large enough to fall to the ground. A hydrometeor refers to all liquid and solid ice particles, regardless of size and regardless of whether they remain in the cloud or fall to the earth as precipiation. A third term, virga (not to be confused with a well-known "potential"-enhancing drug!) refers to hydrometeors that do fall, but that evaporate before hitting the ground.

There are two growth processes that any droplet that grows beyond about 20 micrometers in diameter will experience: the diffusion process and the collision-coalescence process. We will briefly discuss each of these next.