Editing Humidity (section)

== Measurement ==
[[File:Umidaderelativa.jpg|thumb|upright|A [[hygrothermograph]] for humidity and temperature recording]]
[[File:Wetdryhygrometer.JPG|thumb|Hygrometer for domestic use, wet/dry psychrometer type]]
[[File:Nippara Limestone Cave Thermo Hygrometer.jpg|thumb|Thermo hygrometer displaying temperature and relative humidity]]
A device used to measure humidity of air is called a psychrometer or [[hygrometer]]. A [[humidistat]] is a humidity-triggered switch, often used to control a [[humidifier]] or a [[dehumidifier]].

The humidity of an air and water vapor mixture is determined through the use of psychrometric charts if both the [[Dry-bulb temperature|dry bulb temperature]] (''T'') and the [[Wet-bulb temperature|wet bulb temperature]] (''T''<sub>w</sub>) of the mixture are known. These quantities are readily estimated by using a sling [[psychrometer]].

There are several empirical formulas that can be used to estimate the equilibrium vapor pressure of water vapor as a function of temperature. The [[Antoine equation]] is among the least complex of these, having only three parameters (''A'', ''B'', and ''C''). Other formulas, such as the [[Goff–Gratch equation]] and the [[Clausius–Clapeyron relation#Meteorology and climatology|Magnus–Tetens approximation]], are more complicated but yield better accuracy.{{Citation needed|date=January 2010}}

The [[Arden Buck equation]] is commonly encountered in the literature regarding this topic:{{sfn|Buck|1981|pp=1527–1532}}
<math display="block"> e^*_w = \left(1.0007 + 3.46 \times 10^{-6} P\right) \times 6.1121\, e^{17.502 T/(240.97 + T)},</math>
where <math> T </math> is the dry-bulb temperature expressed in degrees Celsius (°C), <math> P </math> is the absolute pressure expressed in millibars, and <math> e^*_w </math> is the equilibrium vapor pressure expressed in millibars. Buck has reported that the maximal relative error is less than 0.20% between {{convert|−20|and|+50|C}} when this particular form of the generalized formula is used to estimate the equilibrium vapor pressure of water.

There are various devices used to measure and regulate humidity. Calibration standards for the most accurate measurement include the gravimetric hygrometer, [[chilled mirror hygrometer]], and electrolytic hygrometer. The gravimetric method, while the most accurate, is very cumbersome. For fast and very accurate measurement the chilled mirror method is effective.<ref>Pieter R. Wiederhold. 1997. Water Vapor Measurement, Methods and Instrumentation. Marcel Dekker, New York, NY {{ISBN|9780824793197}}</ref>  For process on-line measurements, the most commonly used sensors nowadays are based on [[capacitance]] measurements to measure relative humidity,<ref>"BS1339" Part 3</ref> frequently with internal conversions to display absolute humidity as well. These are cheap, simple, generally accurate and relatively robust. All humidity sensors face problems in measuring dust-laden gas, such as exhaust streams from clothes dryers.

Humidity is also measured on a global scale using remotely placed satellites. These satellites are able to detect the [[concentration]] of water in the [[troposphere]] at altitudes between {{cvt|4|and|12|km}}. Satellites that can measure water vapor have sensors that are sensitive to [[infrared|infrared radiation]]. Water vapor specifically absorbs and re-radiates radiation in this spectral band. Satellite water vapor imagery plays an important role in monitoring climate conditions (like the formation of thunderstorms) and in the development of [[weather forecasting|weather forecasts]].