The Bergeron Process

The Bergeron process relies primarily on the fact that the saturation vapor pressure with respect to ice is less than the saturation vapor pressure with respect to water. Pure water droplets do not freeze at 0°C due to surface tension and the structure of water, thus in order to get a pure water droplet to freeze, it will require a temperature of -40°C.

Liquid water that is cooler than 0°C is considered supercooled. In the atmosphere, similar to cloud condensation nuclei (CCN), there exist freezing nuclei. Most of these freezing nuclei "activate" at about -10°C, which allows the cloud droplets to freeze around them. Due to the relative sparseness of the freezing nuclei, ice crystals and supercooled water droplets can coexist at the same time when the temperature is between -10°C and -40°C. This is where Bergeron's primary fact becomes important, when air reaches saturation, some of the resulting droplets will come in contact with the freezing nuclei.

From the perspective of the supercooled water droplets, the air is considered at equilibrium (saturation). Whereas, for the ice crystals, the air is considered supersaturated; when water vapor deposits onto ice crystals (deposition), thus decreasing the amount of water vapor in the air. But, for the supercooled water droplets, the air is now considered subsaturated, resulting in evaporation of droplets until the air is, once again, at saturation. This cycle, the Bergeron process (in cool clouds), continues to result in the growth of the ice crystals by deposition (or sublimation) at the expense of water droplets.

To summarize, when a cloud extends or is entirely above the 0°C isotherm, it is considered a cold cloud. In such clouds, ice crystals grow at the expense of supercooled water droplets. If vapor pressure is such that water droplets have an equilibrium that is between evaporation and condensation, then there will be excess of deposition over sublimation for ice crystals. Thus, ice crystals grow by deposition of water vapor, but that removes the water vapor from the air, which causes the water droplets to become smaller. Eventually, allowing the ice crystals to become large enough to fall from the cloud. This process causes ice crystals to take on platelike or prismlike shapes. Changes in these shapes correlate to air temperature and supersaturations. So, the shapes of ice crystals may be altered while they experience environmental changes falling through the cloud.