Relative humidity

The ratio of the vapor pressure to the saturation vapor pressure with respect to a plane surface of pure water. Consequently, relative humidity describes the amount of water vapor in the air relative to how much water vapor is in the air when it is saturated at a given pressure and temperature.
Relative humidity can also be calculated to a high degree of accuracy by dividing the mixing ratio by the saturation mixing ratio. These two definitions yield almost identical numerical values. Relative humidity is usually expressed in percent and can be computed from psychrometric data. Unless specified otherwise, relative humidity is reported with respect to water rather than ice because most hygrometers are sensitive to relative humidity with respect to water even at subfreezing temperatures, and because the air can easily become supersaturated with respect to ice, which would require three digits in coded messages for relative humidity with respect to ice.

Retallack, B. J., 1974: Physical Meteorology. World Meteorological Organization, 211 pp. Petty, G., 2008: A First Course in Atmospheric Thermodynamics. Sundog Publishing, 337 pp. Wallace, J. M., and P. V. Hobbs, 2006: Atmospheric Science: An Introductory Survey. 2nd ed. Elsevier Science, 483 pp.

Term edited 22 March 2024.

1. The study of the physics, chemistry, and dynamics of the earth's atmosphere, including the related effects at the air–earth boundary over both land and the oceans. Fundamental topics include the composition, structure, and motion of the atmosphere. The goals ascribed to meteorology are the complete understanding and accurate prediction of atmospheric phenomena.

The direction of a line over the earth with reference to north (true or magnetic).

A collection of ideas and axioms, leading to differential equations specifying rates of change, that describes our experience with processes that involve fluxes of heat and changes in energy content.