Relative humidity

Relative humidity is the ratio of water vapor present in air to the amount needed for saturation at the same temperature, usually expressed as a percentage.

Water vapor pressure: The pressure exerted by water vapor within a gas mixture. Relative humidity can be calculated by comparing actual vapor pressure with saturation vapor pressure.
Heat index: An apparent-temperature measure combining air temperature and humidity to estimate heat stress in shaded conditions. High humidity slows sweat evaporation, making hot air feel hotter.
Humidity: The amount of water vapor present in air or another gas. Relative humidity is one way to describe atmospheric humidity.
Absolute humidity: The mass of water vapor per unit volume of air. It reports vapor mass per volume rather than a fraction of the temperature-dependent saturation amount.
Hygrometer: An instrument that measures humidity in air or another gas. Different hygrometers infer relative humidity through distinct physical properties and require calibration.
Saturation vapor pressure: The equilibrium vapor pressure of water above a flat surface of liquid water at a specified temperature. It supplies the temperature-dependent maximum used in the relative-humidity ratio.
Wet-bulb temperature: The lowest temperature attainable by evaporative cooling of a wetted surface exposed to moving air. It connects humidity to evaporative cooling and human heat limits.
Water vapor: Water in its gaseous state, dispersed in air or another gas. It is the atmospheric constituent whose abundance relative humidity compares with saturation.
Mixing ratio: The mass of water vapor divided by the mass of dry air in a sample. It tracks moisture content relative to dry air, not relative to saturation.
Humidity sensor: A device that detects water vapor in air and converts the response into a humidity reading. Sensor accuracy can drift with contamination, aging, and changing temperature.

Linked from 44 pages