Humidity is the amount of water present in the atmosphere in the form of vapor. As a gas, water vapor contributes to the local atmospheric pressure in accordance with Dalton’s law of partial pressures: In any mixture of gases, the humidity partial pressure of any one component is equal to the total pressure of the mixture multiplied by the fraction of the gas present in the mixture. Humidity For example, molecular oxygen constitutes 20 percent of the atmosphere, so the partial atmospheric pressure of oxygen is 20 percent of Earth’s total atmospheric pressure. The total humidity pressure is about 1.03 kilograms per square centimeter, so the partial atmospheric pressure of oxygen is about .20 kilograms per square centimeter.
Humidity water normally exists in liquid and solid as well as vaporous form. Humidity Its vapor humidity pressure is the pressure at which pure water vapor coexists in equilibrium with either the liquid or the solid state. At equilibrium, the liquid would not evaporate, Humidity the solid would not sublimate, and the vapor would not condense. By contrast, if the local partial pressure of water vapor is greater than its vapor pressure, the vapor condenses; if the local partial pressure is less than the vapor pressure, then the liquid evaporates and the solid sublimates. Humidity vapor pressure is not a constant but rather is a function of temperature.
If the local partial pressure of water is exactly equal to its vapor pressure, the air is said to be saturated. This state is defined as 100 percent humidity, and the corresponding temperature is water’s dew point. If the vapor pressure of water is equal to the total local atmospheric pressure, the water will evaporate without limit, and the corresponding temperature is water’s boiling point. Relative humidity is the ratio, expressed as a percentage, of the local partial pressure of water vapor to the vapor pressure associated humidity with the local temperature.
Humidity can exceed 100 percent, a condition known as supersaturation. Humidity in supersaturation, water vapor’s partial pressure exceeds the theoretical vapor pressure at that temperature. Condensation cannot take place, however, unless condensation nuclei are present. Humidity water droplets exceeding a certain critical size act as such nuclei, absorbing water vapor and growing; water droplets below the critical size evaporate. If no droplets larger than the critical size exist and no other condensation nuclei are present, then the supersaturated vapor is stable. Fine, dry particles, such as dust or pollutants, also act as condensation nuclei in supersaturated humidity air.