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.

Focus on the Environment and Societal Impacts: Forecasting the Winter of 2013-2014

Understanding Weather and Climate (7th Edition) (MasteringMeteorology Series) by Edward Aguado, James E. Burt 

Evaporation and Condensation

Figure 5–2b shows what happens when we remove the covering on the liquid water surface. Without the covering, some of the molecules at the surface can escape into the overlying volume as water vapor. The process whereby molecules break free of the liquid volume is known as evaporation. The opposite process is condensation, wherein water vapor molecules randomly collide with the water surface and bond with adjacent molecules. At the beginning of our hypothetical experiment, no condensation could occur because no water vapor was present. As evaporation begins, however, water vapor starts to accumulate above the surface of the liquid.

At the early stages of evaporation, the low water vapor content prevents much condensation from occurring, and the rate of evaporation exceeds that of condensation. This leads to an increase in the amount of water vapor present. With increasing water vapor content, however, the condensation rate likewise increases. Eventually, the amount of water vapor above the surface is enough for the rates of condensation and evaporation to become equal, as shown in Figure 5–2c. A constant amount of water vapor now exists in the volume above the water surface due to offsetting gains and losses by evaporation and condensation. The resulting equilibrium state is called saturation. When this equilibrium exists in the atmosphere, the air is said to be saturated.

The state of saturation described here can occur whether or not air (or other gases, for that matter) exists in the container. In other words, the water vapor is not “held” by the air (although this erroneous statement is frequently made). Water vapor is a gas, just like the other components of the air. Thus, it does not need to be “held” by air any more than the oxygen, nitrogen, argon, and other gases of the atmosphere need to be held by water vapor! When the air is saturated, there is simple an equilibrium between evaporation and condensation; the dry air plays no role in achieving this state. It is also important to realize that the exchange of water vapor and liquid described here applies as well to the change of phase between water vapor and ice. The change of phase directly from ice to water vapor, without passing into the liquid phase, is called sublimation. The reverse process (from water vapor to ice) is called deposition (Figure 5–3).

Understanding Weather and Climate (7th Edition) (MasteringMeteorology Series) 7th Edition by Edward Aguado (Author), James E. Burt (Author)