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.

Laws of Thermodynamics

0^th Law of Thermodynamics—if body A is in thermal equilibrium with body T, and so it body B, then A and B are in thermal equilibrium.

1^st Law of Thermodynamics—a measure of heat transferred into a system will result in an increase in temperature and in the system’s ability to do work. In other words, energy is conserved property that is neither created nor destroyed but, may change form and travel from place to place. Or, in terms of an internal combustion engine in an automobile, the first law describes the underlying principle of what occurs in the cylinder.

Other forms of the First Law…

2^nd Law of Thermodynamics—only in transferring heat from a warmer body to a cooler body can heat be converted into work, in a cyclical process. A cyclic process is a series of operations by which the state of the substance (working substance) changes but the substance is finally returned to its original state in all respects. In other words, heat is always transferred from regions of high temperature to regions of low temperature. Heat can be transferred by three processes…

1.    Conduction—the movement of heat through a substance without appreciable movement of the molecules.

2.    Convection—the transfer of heat by mixing of a fluid.

3.    Radiation—the transfer of energy by radiation that can occur through empty space.

Forms of the Second Law, considering the Carnot Cycle…

3^rd Law of Thermodynamics—there is no finite series of steps that can get you to absolute zero. In other words, since absolute zero cannot be reached an engine cannot be perfectly efficient.

Conclusion—heat can be converted into work, in a cyclic process, but can only be perfectly efficient at absolute zero, which is unattainable.