Formation Processes MET

There are two processes involved for forming precipitation. The first of these is extremely easy to understand; the second, very difficult.

Collision and coalescence process is exactly what it sounds like: small cloud (water) droplets hit each other, then merge. Eventually, the water droplets get too big and heavy and fall out of the cloud. This is similar to what happens when rain falls on a window - the droplets hit each other, get bigger and eventually run down the window.

The Bergeron process (more fully the Bergeron-Findeisen-Wegener process) is much more complicated. There are numerous facts you may get examined on. Firstly, it can only take place in clouds where there are both ice crystals and water droplets - meaning the temperature must be below freezing/0 degrees or the cloud must be above the freezing level. These ice crystals and water droplets will naturally be undergoing a state of flux back to water vapour, raising the vapour pressure. However because the saturated vapour pressure with respect to ice is lower than that with respect to water it will be saturated with respect to ice first. Hence, the water vapour now wants to sublimate to ice, resulting in rapid growth of ice crystals.