Humidity is the amount of water vapor present in the atmosphere. Relative humidity, expressed as a percentage, is the most commonly reported measure, indicating how close the air is to saturation.
Humidity refers to the concentration of water vapor in the atmosphere and is expressed using several measures. Absolute humidity is the mass of water vapor per unit volume of air (g/m3). Specific humidity is the mass of water vapor per unit mass of moist air (g/kg). Relative humidity (RH) is the ratio of the actual water vapor pressure to the saturation vapor pressure at the same temperature, expressed as a percentage, and is the most widely reported measure. Relative humidity varies inversely with temperature for a given moisture content: as temperature rises, RH decreases even though the actual moisture content remains unchanged. Humidity is measured using hygrometers, psychrometers (wet and dry bulb thermometers), or electronic capacitive and resistive sensors. In hydrology, humidity data are essential inputs for calculating evaporation and evapotranspiration using methods such as the Penman-Monteith equation. High humidity reduces the evaporative demand on plants and water surfaces, while low humidity increases water loss and fire risk. Humidity also affects human comfort, building moisture management, and the formation of precipitation, dew, and fog.