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Humidity is the amount of water vapor in the air. Absolute humidity is the quantity of water vapor in a given volume of air, expressed by weight. Specific humidity is a ratio of weight quantities of water vapor to dry air, such as 1:200, for example. Relative humidity is how much water vapor can possibly be held by the air, in its current condition of temperature and pressure.

Humidity indicates the likelihood of precipitation, dew, or fog. High humidity makes people feel hotter outside in the summer because it reduces the effectiveness of sweating to cool the body by reducing the evaporation of perspiration from the skin. This effect is calculated in a heat index table.


Types of humidity

Absolute humidity (Volume basis)

Absolute humidity on a volume basis is the quantity of water in a particular volume of air. The most common units are grams per cubic meter, although any mass unit and any volume unit could be used. Pounds per cubic foot is common in the U.S., and occasionally even other units mixing the Imperial and metric systems are used.

If all the water in one cubic meter of air were condensed into a container, the mass of the water in the container could be measured with a scale to determine absolute humidity. The amount of water vapor in that cube of air is the absolute humidity of that cubic meter of air. More technically, absolute humidity on a volume basis is the mass of dissolved water vapor, mw, per cubic meter of total moist air, Vnet:

Absolute humidity ranges from 0 grams per cubic meter in dry air to 30 grams per cubic meter (0.03 ounce per cubic foot) when the vapor is saturated at 30 °C.[1] (See also Absolute Humidity table)

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