Humidity describes the amount of water vapour present in the air, and it can be expressed using several different physical measures. Because air can contain different quantities of water vapour at different temperatures, the word humidity by itself is incomplete unless the specific measure is identified. The most familiar measure is relative humidity, which expresses the amount of water vapour in air as a percentage of the maximum amount that air could contain at the same temperature before becoming saturated.
Absolute humidity describes the mass of water vapour per unit volume of air, while specific humidity describes the mass of water vapour per unit mass of moist air. Mixing ratio compares the mass of water vapour with the mass of dry air. These measures are related but not interchangeable. Humidity is fundamental to weather because water vapour participates in cloud formation, precipitation, evaporation, condensation, latent heat transfer and atmospheric energy transport.
Relative humidity is particularly useful for everyday weather descriptions, but it is frequently misunderstood. If air has a relative humidity of 50 percent, this does not mean that the air is half water. It means that the actual water vapour pressure is approximately half the saturation vapour pressure at that temperature. Saturation vapour pressure increases strongly with temperature, so warmer air can contain more water vapour before reaching saturation than colder air.
Consequently, relative humidity can rise during cooling even if the actual amount of water vapour remains unchanged. Conversely, relative humidity can fall when air warms without gaining or losing water vapour. This is why a cool morning can have very high relative humidity while containing less total water vapour than a warm afternoon with a lower relative humidity. Relative humidity therefore describes moisture relative to temperature rather than acting as a direct measurement of the absolute amount of water in the atmosphere.
The dew point is another important humidity measure. It is the temperature to which air must be cooled, at approximately constant pressure and water vapour content, for saturation to occur. A high dew point indicates that the air contains a relatively large amount of water vapour, while a low dew point indicates drier air.
Dew point is often more useful than relative humidity for assessing how humid the air actually feels because it is less directly affected by the current temperature. If air is cooled to its dew point, condensation can begin provided suitable nucleation conditions exist, potentially producing dew, fog or cloud. The spread between air temperature and dew point is called the dew point depression and provides a simple indication of how close the air is to saturation. A small difference suggests that only modest additional cooling may be required to reach saturation, whereas a large difference indicates drier conditions.
Water vapour enters the atmosphere primarily through evaporation from oceans, lakes, rivers, soils and other water surfaces and through transpiration from plants. The combined transfer of water from land surfaces and vegetation to the atmosphere is commonly described as evapotranspiration. Atmospheric circulation then transports water vapour horizontally and vertically. Water vapour can condense into cloud droplets, deposit into ice, freeze, evaporate again or eventually fall as precipitation. These phase changes involve latent heat.
Evaporation absorbs energy, while condensation and deposition release energy to the surrounding atmosphere. Because water vapour is a powerful greenhouse gas, its concentration also affects the radiative behaviour of the atmosphere. Humidity is therefore not merely a measure of how damp the weather feels; it is a fundamental component of the atmospheric energy and water cycles and an important link between surface conditions, clouds, precipitation and climate.
Temperature strongly controls the maximum amount of water vapour that air can hold in equilibrium with liquid water or ice. The common statement that warm air can hold more moisture than cold air is useful but can be misleading if interpreted as a physical container analogy. Air does not have a simple fixed capacity that becomes filled with water.
Rather, the saturation vapour pressure of water increases with temperature because the equilibrium between water molecules in the vapour and condensed phases changes with temperature. The Clausius Clapeyron relationship describes this temperature dependence and explains why saturation vapour pressure rises rapidly as temperature increases. This thermodynamic relationship is important for understanding evaporation, condensation, cloud formation and precipitation. It also means that a warmer atmosphere can support higher water vapour concentrations when moisture is available, which can influence the potential intensity of heavy precipitation and other hydrological processes.
Humidity influences human comfort because evaporation of sweat is one of the primary mechanisms by which the human body loses heat. When relative humidity is high, the surrounding air is closer to saturation, reducing the efficiency with which sweat evaporates from the skin. This can make warm conditions feel more oppressive even when the air temperature is unchanged. When humidity is very low, evaporation can proceed efficiently but skin, eyes, mucous membranes and some materials may dry more rapidly.
Indoor humidity also affects buildings and objects. Excessive moisture can promote condensation, mould growth and material degradation, while very dry conditions can affect wood, paper and some instruments. Humidity is therefore measured and controlled in homes, museums, laboratories, industrial facilities, greenhouses and storage environments. The appropriate humidity range depends on the people, organisms, materials or processes involved.
Meteorologists measure humidity using instruments such as hygrometers, psychrometers, capacitive humidity sensors, chilled mirror hygrometers and radiosondes. A psychrometer uses the temperature difference between dry and wetted sensors to infer moisture content through evaporative cooling, while modern electronic sensors detect changes in electrical properties associated with humidity. Chilled mirror instruments determine the dew point by cooling a surface until condensation forms and then measuring the corresponding temperature.
Radiosondes carry humidity sensors through the atmosphere, providing vertical profiles used in weather forecasting and research. Satellites can estimate atmospheric water vapour over large regions using infrared and microwave measurements, although retrieval accuracy depends on atmospheric conditions and instrument characteristics. Combining ground, upper air, aircraft, satellite and model data allows scientists to monitor humidity across the three dimensional atmosphere.
A common misconception is that 100 percent relative humidity means the atmosphere is completely saturated with liquid water or that rain must immediately occur. Saturation means that the air is at equilibrium with respect to water or ice under the relevant conditions, but precipitation requires additional microphysical growth processes. Air can also temporarily become supersaturated, especially relative to ice, without immediate precipitation. Another misconception is that relative humidity is a fixed property of a particular air mass.
It changes whenever temperature or water vapour content changes, even if the other variable remains constant. Humidity should therefore be described using the appropriate measure and context. Understanding the differences between relative humidity, dew point, absolute humidity, specific humidity and mixing ratio is essential for interpreting weather reports accurately and for understanding why humidity affects clouds, fog, precipitation, evaporation, human comfort, atmospheric radiation and the wider hydrological cycle.
