Chapter 6- Atmospheric Moisture

Chapter 6- Atmospheric Moisture - The process whereby...

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The process whereby molecules break free of liquid water is known as evaporation . The opposite process is condensation , wherein water vapor molecules become a liquid. 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 .
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Consider a hypothetical jar containing pure water with a flat surface and an overlying volume that initially contains no water vapor (a). As evaporation begins, water vapor starts to accumulate above the surface of the liquid. With increasing water vapor content, the condensation rate likewise increases (b). Eventually, the amount of water vapor above the surface is enough for the rates of condensation and evaporation to become equal. The resulting equilibrium state is called saturation (c).
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Humidity refers to the amount of water vapor in the air. The part of the total atmospheric pressure due to water vapor is referred to as the vapor pressure . The vapor pressure of a volume of air depends on both the temperature and the density of water vapor molecules. The saturation vapor pressure is an expression of the maximum water vapor that can exist. The saturation vapor pressure depends only on temperature.
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Absolute humidity is the density of water vapor, expressed as the number of grams of water vapor contained in a cubic meter of air. Specific humidity expresses the mass of water vapor existing in a given mass of air. Saturation specific humidity is the maximum specific humidity that can exist and is directly analogous to the saturation vapor pressure. The mixing ratio is a measure of the mass of water vapor relative to the mass of the other gases of the atmosphere. The maximum possible mixing ratio is called the saturation mixing ratio .
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Relative humidity , RH, relates the amount of water vapor in the air to the maximum possible at the current temperature. RH = (specific humidity/saturation specific humidity) X 100%
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Chapter 6- Atmospheric Moisture - The process whereby...

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