Relative humidity is a measurement of the amount of water vapor that exists in a gaseous mixture of air and water.
where the partial pressure of water vapor in the mixture is estimated by :
and the saturated vapor pressure of water at the temperature of the mixture is estimated by :
In practice both T and Td are readily estimated by using a sling psychrometer and the relative humidity of the atmosphere can be calculated
The misconception that air holds water is likely the result of the use of the word saturation which is often misused in descriptions of relative humidity. In the present context the word saturation refers to the state of water vapor, not the solubility of one material in another.
The thermophysical properties of water-air mixtures encountered at atmospheric conditions are reasonably approximated by assuming they behave as a mixture of ideal gases. For many practical purposes the assumption that both components (air and water) behave independently of each other is reasonable. Therefore the physical properties of an air-water mixture can be estimated by considering the physical properties of each component separately.
For example, if the air temperature is 24 °C (75 °F) and the relative humidity is zero percent then the air temperature feels like 21 °C (69 °F). At the same air temperature if the relative humidity is 100 percent then we feel like it is 27 °C (80 °F). In other words, at a temperature of 24 °C air is saturated with water vapor and the human body cools itself at the same rate as it would if it were 27 °C and with no moisture in the air. The humidex is a measurement that reflects the combined effect of temperature and humidity on cooling of the atmosphere.
When the temperature is high and the relative humidity is low evaporation of water is rapid; soil dries, wet clothes hanging outdoors dry quickly, and perspiration readily evaporates from the skin. Wooden furniture can shrink causing the paint that covers these surfaces to fracture.
When the temperature is high and the relative humidity is high evaporation of water is slow. When relative humidity approaches 100 percent condensation can occur on surfaces leading to problems with mold, corrosion, decay, and other moisture-related deterioration.
Certain production and technical processes and treatments in factories, laboratories, hospitals and other facilities require specific relative humidity levels to be maintained using humidifiers, dehumidifiers and associated control systems.
In sealed vehicles and pressure vessels such as pressurised airliners, submersibles and spacecraft these considerations may be critical to safety, and complex environmental control systems including equipment to maintain pressure are needed. For example, airliner fuselages are susceptible to corrosion from humidity, and avionics are susceptible to condensation, and as the failure of either is potentially catastrophic, airliners operate with low internal relative humidity, often under 10%, especially on long flights. The low humidity is a consequence of drawing in the very dry air, often 5% relative humidity or below, which is found at airliner cruising altitudes. This causes discomfort such as sore eyes, dry skin, and drying out of mucosa, but humidifiers are not employed to raise it to comfortable mid-range levels because essentially dry air in an airliner is safe air.
The relative humidity of an air-water system is dependent not only on the temperature but also on the absolute pressure of the system of interest. This dependence is demonstrated by considering the air-water system shown below. The system is closed (i.e. no matter enters or leaves the system).
If the system at State A is isobariacally heated (heating with no change in system pressure) then the relative humidity of the system decreases because the saturated vapor pressure of water increases with increasing temperature. This is shown in State B.
If the system at State A is isothermally compressed (compressed with no change in system temperature) then the relative humidity of the system increases because the partial pressure of water in the system increases with increasing system pressure. This is shown in State C.
Therefore a change in relative humidity can be explained by a change in system temperature, a change in the absolute pressure of the system, or change in both of these system properties.
A gas in this context is referred to as saturated when the vapor pressure of water in the air is at the equilibrium vapor pressure for water vapor at the temperature of the gas and water vapor mixture; liquid water (and ice, at the appropriate temperature) will fail to lose mass through evaporation when exposed to saturated air. It may also correspond to the possibility of dew or fog forming, within a space that lacks temperature differences among its portions, for instance in response to decreasing temperature. Fog consists of very minute droplets of liquid, primarily held aloft by isostatic motion (in other words, the droplets fall through the air at terminal velocity, but as they are very small, this terminal velocity is very small too, so it doesn't look to us like they are falling and they seem to be being held aloft).
The statement that relative humidity (RH%) can never be above 100%, while a fairly good guide, is not absolutely accurate, without a more sophisticated definition of humidity than the one given here. An arguable exception is the Wilson cloud chamber which uses, in nuclear physics experiments, an extremely brief state of "supersaturation" to accomplish its function.
For a given dewpoint and its corresponding absolute humidity, the relative humidity will change inversely, albeit nonlinearly, with the temperature. This is because the partial pressure of water increases with temperature – the operative principle behind everything from hair dryers to dehumidifiers.
Due to the increasing potential for a higher water vapor partial pressure at higher air temperatures, the water content of air at sea level can get as high as 3% by mass at 30 °C (86 °F) compared to no more than about 0.5% by mass at 0 °C (32 °F). This explains the low levels (in the absence of measures to add moisture) of humidity in heated structures during winter, indicated by dry skin, itchy eyes, and persistence of static electric charges. Even with saturation (100% relative humidity) outdoors, heating of infiltrated outside air that comes indoors raises its moisture capacity, which lowers relative humidity and increases evaporation rates from moist surfaces indoors (including human bodies.)
Similarly, during summer in humid climates a great deal of liquid water condenses from air cooled in air conditioners. Warmer air is cooled below its dewpoint and the excess water vapor condenses. This phenomenon is the same as that which causes water droplets to form on the outside of a cup containing an ice-cold drink.
A useful rule of thumb is that the maximum absolute humidity doubles for every 20 °F (11.1 °C) increase in temperature. Thus, the relative humidity will drop by a factor of 2 for each 20 °F (11.1 °C) increase in temperature, assuming conservation of absolute moisture. For example, in the range of normal temperatures, air at 70 °F (21.1 °C) and 50% relative humidity will become saturated if cooled to 50°F (10 °C), its dewpoint and 40 °F (4.4 °C) air at 80% relative humidity warmed to 70 °F (21.1 °C) will have a relative humidity of only 29% and feel dry. By comparison, a relative humidity between 40% and 60% is considered healthy and comfortable in comfort controlled environments (ASHRAE Standard 55 - see thermal comfort).
Water vapor is a lighter gas than air at the same temperature, so humid air will tend to rise by natural convection. This is a mechanism behind thunderstorms and other weather phenomena. Relative humidity is often mentioned in weather forecasts and reports, as it is an indicator of the likelihood of precipitation, dew, or fog. In hot summer weather, it also increases the apparent temperature to humans (and other animals) by hindering the evaporation of perspiration from the skin as the relative humidity rises. This effect is calculated as the heat index or humidex.
A device used to measure humidity is called a hygrometer, one used to regulate it is called a humidistat, or sometimes hygrostat. (These are analogous to a thermometer and thermostat for temperature, respectively.)