Humidity is the concentration of water vapor present in the air. vapor, the gaseous of , is generally invisible to the eye.

Humidity indicates the likelihood for precipitationdew, or fog to be present.

Humidity depends on the temperature and pressure of the of interest. The same amount of vapor results in higher humidity in cool air than warm air.

A related parameter is the dew point. The amount of vapor needed to achieve saturation increases as the temperature increases.

As the temperature of a parcel of air decreases it will eventually reach the saturation point without adding or losing mass.

The amount of vapor contained within a parcel of air can vary significantly.

For example, a parcel of air near saturation may contain 28 g (0.99 oz) of per cubic metre of air at 30 °C (86 °F), but only 8 g (0.28 oz) of per cubic metre of air at 8 °C (46 °F).

Three primary measurements of humidity are widely employed: absolute, relative, and specific.

Absolute humidity is expressed as either mass of vapor per volume of moist air (in grams per cubic metre) or as mass of vapor per mass of dry air (usually in grams per kilogram).

Relative humidity, often expressed as a percentage, indicates a present of absolute humidity relative to a maximum humidity given the same temperature.

Specific humidity is the ratio of vapor mass to total moist air parcel mass.

Humidity plays an important role for surface life.

For life dependent on perspiration (sweating) to regulate internal body temperature, high humidity impairs heat exchange efficiency by reducing the rate of moisture evaporation from skin surfaces.

This effect can be calculated using a heat index table, also known as a humidex.

The notion of air “holding” vapor or being “saturated” by it is often mentioned in connection with the concept of relative humidity.

This, however, is misleading—the amount of vapor that enters (or can enter) a given space at a given temperature is almost independent of the amount of air (nitrogen, , etc.) that is present.

Indeed, a vacuum has approximately the same equilibrium capacity to hold vapor as the same volume filled with air; both are given by the equilibrium vapor pressure of at the given temperature.

There is a very small difference described under “Enhancement factor” below, which can be neglected in many calculations unless high accuracy is required.