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Meteorology and Atmospheric Science (draft 2)
When studying volcanic clouds or trying to learn more about them, some concepts of atmospheric science and meteorology should be reviewed. Water vapor is a major component of volcanic clouds, therefore some basic and more advanced background information will be provided on this page. Other aspects of meteorology and atmospheric science such as cloud condensation nuclei and precipitation will also be discussed.

# Water and Water Vapor

Water exists in 3 states: liquid (water), solid (snow, ice), and gaseous (water vapor). It can frequently change its physical state by the processes of: freezing-melting, condensation-evaporation, and deposition-sublimation. Each change of state involves a transfer of heat between latent and sensible forms. For example, when snow or ice melts, it takes 80 calories to melt 1 gram of snow or ice. The heat that is drawn from the environment or atmosphere is held as sensible heat and stored as latent heat. Therefore, the environment becomes cooler when sensible heat is expended and the latent heat is accumulated. When water begins to freeze, the latent heat will be reduced by 80 cal/g and the heat released as sensible heat with cause the environment to warm up. A similar relation can be discribed in evaporation and condensation. For evaporation, latent heat is accumulated and the sensible heat is expended by 600 calories per gram. The reverse occurs for condensation.
The ratio of the mass of water vapor in a sample to the total mass of the moist air is called specific humidity. The ratio of the mass of water vapor to the mass of only the dry air in a sample is called the mixing ratio. Since both the specific humidity and the mixing ratio are ratios, they are also dimensionless numbers. Although they are both often expressed in units of grams of water vapor per kilogram of air (moist or dry).

#### Sources:

The majority of this information comes from the following site(s):

Hydrometeors and Meteorological Clouds

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http://www.DOMAIN/PATHNAME.HTM -- Revised: 20-December-2000