Relative humidity is a measure of how much water vapour is present in air compared with the maximum amount of water vapour that air could contain at the same temperature before becoming saturated. It is normally expressed as a percentage. Air with a relative humidity of 50 percent contains approximately half the water vapour required for saturation at that particular temperature, whereas air at 100 percent relative humidity is saturated.
Relative humidity therefore does not directly describe the absolute quantity of water vapour in the atmosphere because the amount of water vapour required for saturation changes strongly with temperature. Warm air can hold considerably more water vapour before reaching saturation than cold air. Relative humidity is consequently a temperature dependent measure of atmospheric moisture and is widely used in meteorology, climate science, agriculture, building science, aviation and everyday weather reporting.
The technical concept of relative humidity depends on saturation vapour pressure. Saturation vapour pressure is the equilibrium vapour pressure of water over a flat surface of liquid water or, at sufficiently low temperatures, over ice. Relative humidity can be expressed approximately as the actual water vapour pressure divided by the saturation vapour pressure at the same temperature, multiplied by 100. The actual vapour pressure depends on the amount of water vapour present, while saturation vapour pressure increases rapidly with temperature. This means that adding no water vapour at all can still cause relative humidity to change substantially if temperature changes. Cooling air raises relative humidity because the saturation value falls, while warming air lowers relative humidity because the saturation value rises.
Relative humidity is closely connected with condensation, clouds, fog, dew and precipitation. When air cools until its water vapour reaches saturation, the relative humidity approaches 100 percent. If cooling continues, excess water vapour cannot remain entirely in the gas phase under equilibrium conditions and can condense into liquid droplets or deposit as ice, provided suitable surfaces or particles are available. The temperature at which air becomes saturated through cooling is related to the dew point. Saturation does not automatically mean that visible condensation must occur immediately because condensation can depend on condensation nuclei, surface properties, mixing and microscopic conditions. Nevertheless, air approaching 100 percent relative humidity is generally associated with an increased likelihood of condensation when additional cooling occurs.
Temperature is therefore essential when interpreting relative humidity. A parcel of air can have exactly the same quantity of water vapour in the morning and afternoon while its relative humidity changes greatly because the temperature changes. For example, if air cools overnight while its moisture content remains nearly constant, relative humidity can rise towards saturation and dew or fog may develop. During the daytime, solar heating can raise air temperature and reduce relative humidity even without removing any water vapour.
This explains why a statement such as air has 80 percent humidity is incomplete unless the temperature and preferably the measurement conditions are also known. Relative humidity is a ratio relative to temperature dependent saturation rather than a direct measurement of the mass of water vapour in a given volume of air.
Relative humidity can be measured with instruments such as hygrometers and psychrometers. Modern electronic hygrometers commonly use sensors whose electrical properties change with atmospheric moisture. Psychrometers use two thermometers, traditionally called the dry bulb and wet bulb thermometers, and determine humidity from the temperature difference produced by evaporative cooling at the wet bulb. Other methods infer humidity from dew point, chilled mirror measurements or changes in the physical properties of moisture sensitive materials. Accurate measurements require appropriate calibration, ventilation and protection from direct radiation or heat sources. Meteorological stations routinely measure humidity because it contributes to weather analysis, forecasting and assessments of atmospheric stability.
Relative humidity has important practical effects on people, organisms and materials. High humidity can reduce the rate at which sweat evaporates from human skin, making warm conditions feel more oppressive because evaporative cooling becomes less effective. Very high humidity can also promote mould growth, condensation and corrosion when surfaces become sufficiently cool. Low humidity can increase evaporation and drying and can contribute to static electrical effects and discomfort in some indoor environments. In agriculture, humidity influences plant transpiration, fungal disease and irrigation requirements. In manufacturing and storage, humidity control can be essential for food, pharmaceuticals, electronics, paper, textiles and other moisture sensitive materials.
Relative humidity should be distinguished from absolute humidity and specific humidity. Absolute humidity commonly refers to the mass of water vapour per unit volume of air, while specific humidity is the mass of water vapour per unit mass of moist air. Mixing ratio describes the mass of water vapour relative to the mass of dry air. These measures are less directly dependent on temperature than relative humidity and are often more useful for tracing moisture transport in meteorology.
Relative humidity is nevertheless especially useful for assessing proximity to saturation, condensation potential and human comfort. A relative humidity of 100 percent does not mean that the air is made entirely of water, nor does it mean that the atmosphere contains an unusually large absolute amount of water vapour.
A common misconception is that relative humidity tells us how much water vapour the air contains in an absolute sense. It instead tells us how close the air is to saturation at its current temperature. Another misconception is that relative humidity can exceed 100 percent under ordinary equilibrium conditions. Supersaturation can occur, particularly in clouds or carefully controlled environments, but it is usually temporary because condensation or ice formation reduces the excess vapour.
Relative humidity is also not the same as the probability of rain. A location can have very high humidity without precipitation if lifting and cloud microphysical processes are insufficient. Conversely, precipitation can occur when near surface relative humidity is less than 100 percent because conditions higher in the atmosphere may be saturated and falling precipitation can evaporate before reaching the ground.
