A related parameter is the dew point. The amount of water 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 water mass.
The amount of water vapor contained within a parcel of air can vary significantly.
Three primary measurements of humidity are widely employed: absolute, relative, and specific.
Relative humidity, often expressed as a percentage, indicates a present state of absolute humidity relative to a maximum humidity given the same temperature.
Specific humidity is the ratio of water vapor mass to total moist air parcel mass.
Humidity plays an important role for surface life.
For animal 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” water vapor or being “saturated” by it is often mentioned in connection with the concept of relative humidity.
This, however, is misleading—the amount of water vapor that enters (or can enter) a given space at a given temperature is almost independent of the amount of air (nitrogen, oxygen, etc.) that is present.
Indeed, a vacuum has approximately the same equilibrium capacity to hold water vapor as the same volume filled with air; both are given by the equilibrium vapor pressure of water 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.