Humidity

Humidity refers to the concentration of water vapour present in the air. Water vapour, the gaseous form of water, is generally invisible to the human eye, yet it plays an essential role in atmospheric processes, weather formation, and biological comfort. Humidity determines the likelihood of precipitation, fog, dew, and cloud formation, and varies with both temperature and atmospheric pressure. Warm air can contain a greater quantity of water vapour than cool air, making humidity temperature-dependent. A closely related concept is the dew point, the temperature at which air becomes saturated with water vapour and condensation begins.
The atmosphere can contain widely varying amounts of water vapour: air near saturation may hold as little as 8 g/m³ at low temperatures or as much as 28 g/m³ at higher temperatures. Three main measures of humidity—absolute, relative, and specific—are used in meteorology, engineering, and environmental science to characterise moisture content in air.
Humidity has major implications for ecosystems and human life. For perspiring organisms, particularly humans, high humidity significantly reduces the efficiency of evaporative cooling, increasing the body’s heat stress. Indices such as the heat index and the humidex quantify this perceived temperature rise under humid conditions. The common idea that air “holds” water vapour is misleading, as water vapour capacity is governed by its saturation vapour pressure at a given temperature and is essentially independent of the presence of air; even a vacuum has a saturation limit determined solely by temperature.

Absolute humidity

Absolute humidity measures the total mass of water vapour contained in a given volume of air. It is expressed either as grams of water vapour per cubic metre of air or as a mass ratio in chemical engineering contexts. Absolute humidity does not account for temperature changes, and since air volume varies with temperature and pressure according to Boyle’s law, the absolute humidity of a fixed mass of air changes as conditions vary even if moisture content remains constant.
Formally, absolute humidity is defined as the mass of water vapour divided by the volume of moist air:
AH = m(H₂O) / V
As this definition causes ambiguity in processes where temperature changes significantly, fields such as chemical engineering often prefer the humidity ratio (or mass mixing ratio), defined as the mass of water vapour per mass of dry air. British Standard BS 1339 recommends avoiding the term “absolute humidity” when possible due to its variable usage. In atmospheric contexts, absolute humidity ranges from nearly zero in extremely dry environments, such as the Atacama Desert, to values near saturation at warm temperatures.

Relative humidity

Relative humidity is the most widely reported measure of atmospheric moisture and expresses the amount of water vapour in air relative to the maximum it could contain at the same temperature and pressure. It is defined as the ratio of the partial pressure of water vapour to the saturation vapour pressure at a given temperature, expressed as a percentage:
RH = (Pw / Ps) × 100
Relative humidity varies strongly with temperature. Cooling an air parcel increases relative humidity, while warming it decreases relative humidity. If cooling continues to the point where RH reaches 100%, the air achieves saturation and condensation or deposition will occur when suitable nuclei are present. Relative humidity only accounts for the vapour phase; suspended liquid droplets in fogs or clouds do not directly influence its numerical value.
Under certain conditions, relative humidity can exceed 100%, producing supersaturated air. This situation is unstable and will resolve once condensation nuclei trigger droplet or ice crystal formation. Relative humidity is a key meteorological variable for predicting precipitation, dew formation, and fog. It also influences human comfort: high relative humidity reduces the evaporation of sweat, making conditions feel hotter. For example, at high temperatures, a relative humidity of 75% may cause conditions to feel substantially warmer than recorded. In materials science, relative humidity is used to estimate moisture-related dimensional changes in wood and other hygroscopic materials.

Specific humidity

Specific humidity, also known as moisture content, is the ratio of the mass of water vapour to the total mass of an air parcel (water vapour plus dry air). It is nearly equivalent to the mixing ratio, which expresses the mass of water vapour per unit mass of dry air. Specific humidity is a more stable measure than absolute humidity because it does not change with temperature or pressure, making it valuable for engineering applications such as heating, ventilation, and air conditioning (HVAC) design.
Specific humidity is typically represented by symbols such as q and is used to determine heat and mass balances in atmospheric and thermodynamic systems. Unlike relative humidity, specific humidity directly quantifies moisture content and remains constant during adiabatic temperature variations.

Related concepts and distinctions

Humidity terminology varies across scientific fields. Relative humidity applies strictly to water vapour in air, while the analogous ratio for other condensable vapours in other gases is referred to as relative saturation. Moisture content, humidity ratio, and mixing ratio are used when mass-based descriptions are required.
The misconception that air “holds” water vapour arises from the temperature dependence of saturation vapour pressure. In reality, the equilibrium vapour pressure of water is determined solely by temperature, not by the amount of air present. Air merely provides an inert background gas.

Measurement techniques

Humidity is measured using specialised instruments. Hygrometers and psychrometers determine moisture content using principles such as evaporation and thermal differences. A psychrometer employs both a dry-bulb and a wet-bulb thermometer; the temperature difference between them, due to evaporative cooling on the wet bulb, is used to infer humidity through psychrometric charts.
Humidistats are devices that trigger control systems such as humidifiers or dehumidifiers to maintain desired humidity levels. Saturation vapour pressure of water at different temperatures can be estimated using empirical formulas such as the Antoine equation, enabling calculation of relative humidity and dew point.
Humidity measurement and control are critical in meteorology, HVAC engineering, agriculture, manufacturing, and environmental monitoring. Accurate humidity assessment supports weather forecasting, building design, crop management, and the preservation of sensitive materials.