Everything about Relative Humidity totally explained
Relative humidity is a term used to describe the amount of
water vapor that exists in a gaseous mixture of air and water.
Definition
The relative humidity of an air-water mixture is defined as the ratio of the
partial pressure of water vapor in the mixture to the saturated
vapor pressure of water at a given temperature. Relative humidity is expressed as a percentage and is calculated in the following manner:
is the
saturation vapor pressure of water at the temperature of the mixture.
A common misconception
Often the concept of air holding water vapor is used in the description of relative humidity. Relative humidity is wholly understood in terms of the physical properties of water alone and therefore is unrelated to this concept.
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This misconception is probably a result of the use of the word
saturation which is often mis-used in definitions of relative humidity. In the present context
saturation refers to the state of water vapor
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solubility of one material in another.
The thermophysical properties of water-air mixtures encountered at atmospheric conditions can be reasonably approximated by assuming it behaves as a mixture of
ideal gases.
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Related concepts
The term relative humidity is reserved for systems of water vapor in air. The term
relative saturation is used to describe the analogous property for systems consisting of a condensable phase other than water in a non-condensable phase other than air.
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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 is dependence demonstrated by considering the air-water system shown below. The system is closed (for example 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.
Other important facts
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.
The statement that relative humidity can never be above 100%, while a fairly good guide, isn't 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
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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's 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's called a
humidistat, or sometimes
hygrostat. (These are
analogous to a
thermometer and
thermostat for temperature, respectively.)
Further Information
Get more info on 'Relative Humidity'.
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