Condensation of Water Vapor

The condensation of water vapor in the atmosphere is the transition of water vapor in the air to the liquid state (droplets). In a more general sense, the term “condensation of water vapor” is applied to the transition of water vapor to both the liquid and the solid states. Sublimation is the meteorological term for the transition of water vapor to the solid state (crystals, snowflakes), while in physical science it describes the reverse process.

Water is always present in the atmosphere, where it can exist in the gaseous, liquid, and solid states simultaneously. Even though the lower atmospheric layers contain several hundred kg of water vapor per km³ of air (several thousand kg during the summer), the condensation of water vapor in the atmosphere is only possible when the vapor pressure e (or partial pressure) exceeds the saturation pressure E. The saturation pressure is primarily a function of temperature, decreasing with decreasing temperature; it is also dependent on the presence of dissolved impurities in the water as well as on the curvature of the droplet surface. For example, the smaller the water droplets, the greater the E. Usually e < E in the atmosphere, although sufficient cooling of air masses under specific conditions will cause e to exceed E. This occurs when (1) there is a drop in the temperature of air owing to adiabatic expansion during its rise, with E decreasing at the same time (most clouds are formed in this manner), (2) the air is cooled as a result of contact with a colder ground surface (fogs are often formed in this manner), and (3) water evaporates from a warmer ground surface, during which the water vapor pressure e increases to values exceeding E (evaporation mists are formed).

It is known that great supersaturation is required for the condensation of water vapor in absolutely pure air. However, the atmosphere always contains dust particles, sea salt droplets, products of partial combustion, and the like which act as condensation nuclei. These nuclei facilitate the condensation of water vapor during extremely negligible supersaturation (fractions of a percent). The direct condensation of water vapor on cloud crystals can play an important role at negative temperatures in clouds. E is considerably lower for crystals than for supercooled droplets at the same temperature; therefore, crystal growth and droplet evaporation occur in a mixed cloud composed of droplets and crystals. The condensation of water vapor on the earth’s surface and on objects on the earth results in the formation of dew, frost, rime, and the like.

The condensation of water vapor, ensuring the formation of clouds and precipitation, serves as an important link in the hydrological cycle of the earth. The heat removed from the earth’s surface during evaporation and liberated during the condensation of water vapor plays a major role in the heat transfer between the earth and the atmosphere.