# Humidity

Humidity is the concentration of water vapor present in the . 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 and pressure of the of interest. The same amount of vapor results in higher humidity in cool than warm .

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

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

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

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

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

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

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

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

Humidity plays an important role for surface life.

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

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

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

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

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

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

Last Updated on 3 years by pinc