CONDENSATION IS THE change of phase of a substance from vapor to liquid. It is the opposite of evaporation, the change of phase from liquid to vapor. The condensation of water is one of the most important physical processes of the Earth's climate system. Condensation forms cloud particles and precipitation. It is the main sink of atmospheric water. On Earth, water can be found in the solid, liquid, and gas phases. Evaporation (and sublimation) of water substance, the transport of water vapor, and condensation away from its sources, is the most important heat transport mechanism in the Earth's climate system. Condensation is an extremely important process in the earth's water cycle because it is responsible for the formation of clouds and precipitation.

Weak forces between molecules cause them to stick to each other and produce the various phases of a substance. Random thermal motions cause some molecules to overcome these intermolecular forces and escape from the liquid and form a gas phase around it. The number of molecules that leave the liquid phase increases with its temperature because of the increase in the thermal energy, and, therefore, kinetic energy, of the substance. The molecules in the gas phase jiggle and move randomly because of their thermal energy. Some of the gas molecules stick to the liquid when they strike it. The numbers of molecules that return to the liquid phase increases with their concentration.

Anne Ferran Work
Condensation is integral to the most important heat transport mechanism in the earth's climate system.

Thus, the concentration of molecules in the gas phase increases until a balance between molecules leaving and returning to the liquid surface is reached. This is called thermodynamic equilibrium. The concentration of molecules in the gas phase, in equilibrium with the liquid phase at a given temperature, is defined as the saturation value.

The molecules that leave the liquid phase are the ones moving faster than the average molecule in the liquid. That is, they have larger kinetic energy than the average molecule. This is the reason a liquid cools while it evaporates. There is a sudden large attraction when a molecule of water vapor approaches the surface of liquid water. This speeds up the incoming molecule and increases its kinetic energy. Thus, a liquid releases heat while it condenses. This is known as the latent heat of condensation. The saturation vapor pressure depends on the curvature of the surface of the liquid phase.

This has important implications for climate processes. In a curved surface, such as that of a cloud droplet, each water molecule has fewer nearby neighbors than on a flat surface. Thus, the intermolecular attractive forces holding them together are smaller; a water molecule can escape a curved surface easier than a flat surface. At equilibrium, the concentration of water molecules in the gas phase has to be larger than over a curved surface over a flat surface to compensate for the fact that a larger number of molecules leave the liquid at a given temperature. Therefore, the saturation water vapor pressure is larger over curved surfaces.

Condensation occurs when the concentration of molecules in the gas phase exceeds the equilibrium, or saturation value, at a given temperature. Relative humidity is defined as the ratio of the water vapor concentration to the saturation value, with respect to a flat surface of pure water. Thus, the relative humidity of the air in equilibrium with a cloud droplet can be much greater than 100 percent, depending on the curvature (or size) of the droplet. This frequently occurs in clouds and is called supersaturation. Therefore, condensation and cloud droplets form preferentially over impurities that reduce the curvature of the surface of the liquid phase. These impurities are called cloud condensation nuclei.

When the temperature of the liquid phase is lowered, a smaller number of molecules leave it, and the saturation water vapor concentration is reduced. This is what occurs when moist air rises in convec-tive updrafts and cools adiabatically. Thus, when the air rises sufficiently for saturation to occur, clouds form. This is the process by which convective clouds form. The cloud base is at the saturation condensation level of the rising air parcels. However, the fraction of the condensate that falls as rain, as opposed to evaporating and moistening the environment, is not easily determined. In fact, this is one of the most uncertain processes in cloud models. Cloud processes have been identified by various researchers, and more recently by the United Nations Intergovernmental Panel on Climate Change (IPCC), as one of the most uncertain processes in climate models. Condensation partially controls the content of vertical distribution of water vapor, the most important greenhouse gas in the Earth's atmosphere.

SEE ALSO: Cloud Feedback; Clouds, Cirrus; Clouds, Cumulus; Clouds, Stratus; Evaporation and Transpiration; Evaporation Feedbacks.

BIBLIOGRApHY. J.V.D. Iribarne and W.L. Godson, Atmospheric Thermodynamics (Reidel, Dordrecht, 1981); N.O. Renno, K.A. Emanuel, and P.H. Stone, "A Radiative-Con-vective Model with an Explicit Hydrologic Cycle: 1. Formulation and Sensitivity to Model Parameters," Journal of Geophysical Research (v.99, 1994); N.O. Renno, P.H. Stone, and K.A. Emanuel. A Radiative-Convective Model with an Explicit Hydrological Cycle: 2. Sensitivity to Large Changes in Solar Forcing," Journal of Geophysical Research (v.99, 1994).

Renno Nilton University of Michigan

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