Humidity and Water Vapor

Water vapor is an invisible but potent ingredient of the air, and the many ways of measuring it, from dew point to relative humidity, capture how close the atmosphere is to condensing its moisture into cloud, fog, or rain.

Definition

Humidity is the amount of water vapor present in air, expressed through measures such as relative humidity, specific humidity, mixing ratio, and dew-point temperature, all of which relate the actual vapor content to the saturation value.

Scope

This topic covers the thermodynamics of water vapor in air, including saturation vapor pressure and its temperature dependence through the Clausius-Clapeyron relation, the various humidity measures such as relative and specific humidity, mixing ratio, and dew point, and their measurement.

Core questions

• What governs how much water vapor air can hold before saturating?
• How do the different humidity measures relate to one another?
• Why does relative humidity change as temperature changes even at constant moisture?
• How is atmospheric humidity measured in practice?

Key theories

Clausius-Clapeyron relation

Saturation vapor pressure rises nearly exponentially with temperature, so warmer air can hold much more water vapor, a relation that underlies humidity, cloud formation, and the intensification of the water cycle in a warming climate.

Humidity measures and dew point

Specific humidity and mixing ratio quantify the actual vapor content, relative humidity compares it to saturation, and dew point is the temperature to which air must cool to saturate, together describing the moisture state of the air.

Mechanisms

The maximum water vapor air can hold is set by the saturation vapor pressure, which increases steeply with temperature following the Clausius-Clapeyron relation. The actual vapor present is measured by mixing ratio or specific humidity, while relative humidity is the ratio of actual to saturation vapor pressure. Because saturation depends on temperature, cooling moist air raises its relative humidity until, at the dew point, it saturates and condensation begins, forming dew, fog, or cloud.

Clinical relevance

Humidity measures are basic inputs to forecasting fog, frost, cloud bases, and precipitation, to heat-stress indices that combine temperature and moisture, and to understanding how a warming atmosphere holds more water vapor and can produce heavier rainfall.

History

John Dalton's early-nineteenth-century studies of vapor pressure, together with the thermodynamic relation derived by Clapeyron and Clausius, established the quantitative description of water vapor in air; the hygrometer and later electronic and remote-sensing instruments made routine measurement of atmospheric humidity possible.

Key figures

• Rudolf Clausius
• Benoit Paul Emile Clapeyron
• John Dalton

Seminal works

• wallace2006
• bohren1998

Frequently asked questions

Why does relative humidity rise at night even when it does not rain?

Relative humidity compares the moisture in the air to the maximum it can hold, which falls as the air cools; so on a clear night, as temperatures drop, relative humidity rises even though the actual amount of water vapor has not changed.

What is the dew point?

The dew point is the temperature to which air must be cooled, at constant pressure and moisture, for it to become saturated; a higher dew point means more water vapor in the air and a greater chance of dew, fog, or precipitation.