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Humidity is a measure of the quantity of water vapor in the air. There are different methods for determining this quantity and those methods are reflected in a variety of humidity indexes and readings.

The humidity reading in general use by most meteorologists is relative humidity. The relative humidity of air describes the saturation of air with water vapor. Given in terms of percent humidity (e.g., 50% relative humidity), the measurement allows a comparison of the amount of water vapor in the air with the maximum amount water vapor that—at a given temperature—represents saturation. Saturation exists when the phase state changes of evaporation and condensation are in equilibrium.

Approximately 1% of Earth's total water content is suspended in the atmosphere as water vapor, precipitation, or clouds. Humidity is a measure only of the vapor content.

Because water vapor exerts a pressure, the presence of water vapor in the air contributes vapor pressure to the overall atmospheric pressure. Actual vapor pressures are measured in millibars. One atmosphere of pressure (1 atm) equals 1013.25 mbar.

In contrast to the commonly used value of relative humidity, absolute humidity is a measure of the actual mass of water vapor in a defined volume of air. Absolute humidity is usually expressed in terms of grams of water per cubic meter.

Specific humidity is a measure of the mass of water vapor in a defined volume of air relative to the total mass of gas in the defined volume.

The amount of water vapor needed to achieve saturation increases with temperature. Correspondingly, as temperature decreases, the amount of water vapor needed to reach saturation decreases. As the temperature of a parcel of air is lowered, it will eventual reach saturation without the addition or loss of water mass. At saturation (dew point), condensation or precipitation forms. This is the fundamental mechanism for cloud formation as air moving aloft is cooled. The level of cloud formation is an indication of the humidity of the ascending air because, given the standard temperature lapse rate, a parcel of air with a greater relative humidity will experience condensation (e.g., cloud formation) at a lower altitude than a parcel of air with a lower relative humidity.

The differences in the amount of water vapor in a parcel of air can be dramatic. A parcel of air near saturation may contain 28g of water per cubic meter of air at 30° F (-1° C), but only 8g of water per cubic meter of air at 10°F (-12° C).

An increasingly popular measure of comfort, especially in the hotter summer months, is the heat index. The heat index is an integrated measurement of relative humidity and dry air temperature. The measurement is useful because higher humidity levels retard evaporation from the skin (perspiration) and lower the effectiveness of physiological cooling mechanisms.

Absolute humidity may be measured with a sling cyclometer. A hydrometer is used to measure water vapor content. Water vapor content can also be can be expressed as grains/cubic ft. Grains, a unit of weight, equals 1/7000 of a pound.

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