Humidity Control
Air is a mixture of gases and water vapor. Dry air (air without water vapor) is composed mainly of nitrogen (78% by volume), and oxygen (21%), the remaining 1% being made up of carbon dioxide and minute quantities of other gases. With regard to these dry air components, the composition of air is practically the same everywhere.
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The amount of water vapor in the air however, varies greatly between particular locations and prevailing weather conditions. Since all air in the natural state contains a certain amount of water vapor, there is no such thing as "dry air". Nevertheless, the concept of dry air is useful in that it simplifies psychrometric calculations. The term "dry air" is normally only used when referring to air without water vapor, whereas the terms "air" and "moist air" refer to the natural mixture of dry air and water vapor. Dalton's Law states that in any mechanical mixture of gases and vapors that are not combined chemically, each gas or vapor in the mixture exerts an individual partial pressure that is equal to the pressure that the gas would exert if it occupied the space alone, and the total pressure of the gaseous mixture is equal to the sum of the partial pressures exerted by the individual gases or vapors. Air, being a mechanical mixture of gases and water vapor, obeys Dalton's law. Total barometric pressure may therefore be considered the sum of the pressures exerted by the dry gases and the partial pressure exerted by the water vapor.