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{{about|the meteorological dew point|the petroleum term|Hydrocarbon dew point}}
{{humidity}}

The '''dew point''' is the [[temperature]] to which [[air]] must be cooled to become saturated with [[water vapor]]. When further cooled, the airborne water vapor will [[Water vapor#Condensation|condense]] to form liquid water ([[dew]]). When air cools to its dew point through contact with a surface that is colder than the air, water will condense on the surface.<ref>[http://w1.weather.gov/glossary/index.php?word=dew+point Glossary – NOAA's National Weather Service<!-- Bot generated title -->]</ref><ref name="WallaceHobbs2006">{{cite book|author1=John M. Wallace|author2=Peter V. Hobbs|title=Atmospheric Science: An Introductory Survey|url=https://books.google.com/books?id=HZ2wNtDOU0oC&pg=PA83|date=24 March 2006|publisher=Academic Press|isbn=978-0-08-049953-6|pages=83–}}</ref> When the temperature is below the freezing point of water, the dew point is called the '''frost point''', as [[frost]] is formed rather than dew.<ref>[http://w1.weather.gov/glossary/index.php?word=frost+point Glossary – NOAA's National Weather Service<!-- Bot generated title -->]</ref> The measurement of the dew point is related to [[humidity]]. A higher dew point means there is more moisture in the air.<ref name="WallaceHobbs2006" />

==Humidity==
If all the other factors influencing humidity remain constant, at ground level the [[relative humidity]] rises as the temperature falls; this is because less vapor is needed to saturate the air. In normal conditions, the dew point temperature will not be greater than the air temperature, since relative humidity cannot exceed 100%.<ref>{{cite web|title=Observed Dew Point Temperature|url=http://ww2010.atmos.uiuc.edu/(Gh)/guides/maps/sfcobs/dwp.rxml|website=Department of Atmospheric Sciences (DAS) at the University of Illinois at Urbana-Champaign|accessdate=15 February 2018}}</ref>

In technical terms, the dew point is the temperature at which the water vapor in a sample of [[air]] at constant [[barometric pressure]] condenses into liquid water at the same [[reaction rate|rate]] at which it evaporates.<ref>[http://www.merriam-webster.com/dictionary/dew%20point Dew Point | Definition of dew point by Merriam-Webster<!-- Bot generated title -->]</ref> At temperatures below the dew point, the rate of condensation will be greater than that of evaporation, forming more liquid water. The condensed water is called dew when it forms on a solid surface, or frost if it freezes.  In the air, the condensed water is called either [[fog]] or a [[cloud]], depending on its altitude when it forms.  If the temperature is below the dew point, the vapor is called [[supersaturated]]. This can happen if there are not enough particles in the air to act as [[condensation nuclei]].<ref>{{cite news|last1=Skilling|first1=Tom|title=Ask Tom why: Is it possible for relative humidity to exceed 100 percent?|url=http://articles.chicagotribune.com/2011-07-20/news/ct-wea-0720-asktom-20110720_1_relative-humidity-condensation-nuclei-supersaturated-air|accessdate=24 January 2018|work=Chicago Tribune|date=20 July 2011}}</ref>

A high [[relative humidity]] implies that the dew point is closer to the current air temperature. A relative humidity of 100% indicates the dew point is equal to the current temperature and that the air is maximally saturated with water. When the moisture content remains constant and temperature increases, relative humidity decreases, but the dew point remains constant.<ref name=Horstmeyer>{{cite web| last=Horstmeyer | first=Steve | title=Relative Humidity....Relative to What? The Dew Point Temperature...a better approach | publisher=Steve Horstmeyer, Meteorologist, WKRC TV, Cincinnati, OH | date=2006-08-15 | url=http://www.shorstmeyer.com/wxfaqs/humidity/humidity.html| accessdate=2009-08-20}}</ref>

[[General aviation]] pilots use dew point data to calculate the likelihood of [[carburetor icing]] and [[fog]], and to estimate the height of a [[cumuliform]] [[cloud base]].

[[Image:dewpoint.jpg|upright=1.5|thumb|This graph shows the maximum percentage, by mass, of water vapor that air at [[sea-level pressure]] across a range of temperatures can contain. For a lower ambient pressure, a curve has to be drawn above the current curve. A higher ambient pressure yields a curve under the current curve.]]

Increasing the barometric pressure increases the dew point.<ref>{{cite web|title=Dew Point in Compressed Air – Frequently Asked Questions|url=https://www.vaisala.com/sites/default/files/documents/Dew-point-compressed-air-Application-note-B210991EN-B-LOW-v1.pdf|website=Vaisala|accessdate=15 February 2018}}</ref> This means that, if the pressure increases, the mass of water vapor in the air must be reduced in order to maintain the same dew point. For example, consider [[New York City|New York]] ({{convert|33|ft|m|abbr=on|disp=or}} elevation) and [[Denver]] ({{convert|5280|ft|m|abbr=on|disp=or}} elevation<ref name="denfacts">{{cite web|url=http://www.denvergov.org/AboutDenver/today_factsguide.asp |title=Denver Facts Guide&nbsp;– Today |publisher=The City and County of Denver |accessdate=March 19, 2007 |url-status=dead |archiveurl=https://web.archive.org/web/20070203182736/http://www.denvergov.org/AboutDenver/today_factsguide.asp |archivedate=February 3, 2007 }}</ref>). Because Denver is at a higher elevation than New York, it will tend to have a lower barometric pressure. This means that if the dew point and temperature in both cities are the same, the amount of water vapor in the air will be greater in Denver.

==Relationship to human comfort==
{{refimprove section |date=October 2016}}
When the air temperature is high, the human body uses the evaporation of [[Perspiration|sweat]] to cool down, with the cooling effect directly related to how fast the perspiration evaporates. The rate at which perspiration can evaporate depends on how much moisture is in the air and how much moisture the air can hold. If the air is already saturated with moisture, perspiration will not evaporate. The body's [[thermoregulation]] will produce perspiration in an effort to keep the body at its normal temperature even when the rate it is producing sweat exceeds the evaporation rate, so one can become coated with sweat on humid days even without generating additional body heat (such as by exercising).

As the air surrounding one's body is warmed by body heat, it will rise and be replaced with other air. If air is moved away from one's body with a natural breeze or a fan, sweat will evaporate faster, making perspiration more effective at cooling the body. The more unevaporated perspiration, the greater the discomfort.

A [[wet bulb thermometer]] also uses [[evaporative cooling]], so it provides a good measure for use in evaluating comfort level.

Discomfort also exists when the dew point is very low (below around {{convert|-30|°C|°F|disp=or}}).{{citation needed|date=November 2016}} The drier air can cause skin to crack and become irritated more easily. It will also dry out the airways. The US [[Occupational Safety and Health Administration]] recommends indoor air be maintained at {{convert|20|-|24.5|C|F|0}} with a 20–60% relative humidity, equivalent to a dew point of {{convert|-4.5|to|15.5|C|F|0}}.{{citation needed|date=November 2016}}

Lower dew points, less than {{convert|10|°C|°F}}, correlate with lower ambient temperatures and the body requires less cooling. A lower dew point can go along with a high temperature only at extremely low relative humidity, allowing for relatively effective cooling.

People inhabiting [[tropical climate|tropical]] and [[humid subtropical climate|subtropical]] climates acclimatize somewhat to higher dew points. Thus, a resident of [[Singapore]] or [[Miami]], for example, might have a higher threshold for discomfort than a resident of a temperate climate like [[London]] or [[Chicago]]. People accustomed to temperate climates often begin to feel uncomfortable when the dew point gets above {{convert|15|C|F}}, while others might find dew points up to {{convert|18|C|F}} comfortable. Most inhabitants of temperate areas will consider dew points above {{convert|21|°C|°F}} oppressive and tropical-like, while inhabitants of hot and humid areas may not find this uncomfortable. Thermal comfort depends not just on physical environmental factors, but also on psychological factors.<ref>{{cite journal|last1=Lin|first1=Tzu-Ping|title=Thermal perception, adaptation and attendance in a public square in hot and humid regions|journal=Building and Environment|date=10 February 2009|volume=44|issue=10|pages=2017–2026|url=https://www.academia.edu/download/46325093/Thermal_perception_adaptation_and_attend20160607-13171-4gct3e.pdf|accessdate=23 January 2018|doi=10.1016/j.buildenv.2009.02.004}}</ref>

{| class="wikitable" style="text-align:center;"
|-
! colspan="2"|Dew point
! Relative humidity at {{convert|32|°C|°F|abbr=on}}
|-
| Over 26&nbsp;°C
| Over 80&nbsp;°F
| 73% and higher
|-
| 24–26&nbsp;°C
| 75–80&nbsp;°F
| 62–72%
|-
| 21–24&nbsp;°C
| 70–74&nbsp;°F
| 52–61%
|-
| 18–21&nbsp;°C
| 65–69&nbsp;°F
| 44–51%
|-
| 16–18&nbsp;°C
| 60–64&nbsp;°F
| 37–43%
|-
| 13–16&nbsp;°C
| 55–59&nbsp;°F
| 31–36%
|-
| 10–12&nbsp;°C
| 50–54&nbsp;°F
| 26–30%
|-
| Under 10&nbsp;°C
| Under 50&nbsp;°F
| 25% and lower
|}

==Measurement==
Devices called hygrometers are used to measure dew point over a wide range of temperatures. These devices consist of a polished metal mirror which is cooled as air is passed over it.  The temperature at which dew forms is, by definition, the dew point. Manual devices of this sort can be used to calibrate other types of humidity sensors, and automatic sensors may be used in a control loop with a humidifier or dehumidifier to control the dew point of the air in a building or in a smaller space for a manufacturing process.

==Calculating the dew point==
[[Image:Dewpoint-RH.svg|thumb|upright=1.5|Graph of the dependence of the dew point upon air temperature for several levels of relative humidity.]]
{{See also|Psychrometric chart}}

A well-known approximation used to calculate the dew point, ''T''<sub>dp</sub>, given just the actual ("dry bulb") air temperature, ''T'' (in degrees Celsius) and [[relative humidity]] (in percent), RH, is the Magnus formula:
<math display="block">\begin{align}
\gamma(T,\mathrm{RH})&=\ln\left(\frac\mathrm{RH}{100}\right)+\frac{bT}{c+T};\\[8pt]
T_\mathrm{dp}&= \frac{c\gamma(T,\mathrm{RH})}{b-\gamma(T,\mathrm{RH})};\end{align}
</math>
The more complete formulation and origin of this approximation involves the interrelated [[Saturation (chemistry)|saturated]] water [[vapor pressure]] (in units of [[bar (unit)|millibar]]s, also called [[Pascal (unit)|hectopascals]]) at ''T'', ''P''<sub>s</sub>(''T''), and the actual vapor pressure (also in units of millibars), ''P''<sub>a</sub>(''T''), which can be either found with ''RH'' or approximated with the barometric pressure (in millibars), ''BP''<sub>mbar</sub>, and "[[wet-bulb temperature|wet-bulb]]" temperature, ''T''<sub>w</sub> is (unless declared otherwise, all temperatures are expressed in [[degrees Celsius]]):
<math display="block">
\begin{align}
P_\mathrm{s}(T)& = \frac{100}\mathrm{RH}P_\mathrm{a}(T) = ae^{\frac{bT}{c+T}};\\[8pt]
P_\mathrm{a}(T) & = \frac\mathrm{RH}{100}P_\mathrm{s}(T)=ae^{\gamma(T,\mathrm{RH})}\\
&\approx P_\mathrm{s}(T_\mathrm{w}) - BP_\mathrm{mbar} 0.00066 \left(1 + 0.00115T_\mathrm{w} \right)\left(T-T_\mathrm{w}\right);\\[8pt]
T_\mathrm{dp} & = \frac{c\ln\frac{P_\mathrm{a}(T)}{a}}{b-\ln\frac{P_\mathrm{a}(T)}{a}};
\end{align}</math>

For greater accuracy, ''P''<sub>s</sub>(''T'') (and therefore ''γ''(''T'', RH)) can be enhanced, using part of the ''Bögel modification'', also known as the [[Arden Buck equation]], which adds a fourth constant ''d'':
<math display="block">\begin{align}P_\mathrm{s,m}(T)&=ae^{\left(b-\frac{T}{d}\right)\left(\frac{T}{c+T}\right)};\\[8pt]
\gamma_\mathrm{m}(T,\mathrm{RH})&=\ln\left(\frac\mathrm{RH}{100}e^{\left(b-\frac{T}{d}\right)\left(\frac{T}{c+T}\right)}\right);\\[8pt]
T_{dp}&= \frac{c\gamma_m(T,\mathrm{RH})}{b-\gamma_m(T,\mathrm{RH})};
\end{align}</math>
where
*''a'' = 6.1121&nbsp;mbar, ''b'' = 18.678, ''c'' = 257.14&nbsp;°C, ''d'' = 234.5&nbsp;°C.

There are several different constant sets in use. The ones used in [[NOAA]]'s presentation<ref>[http://www.crh.noaa.gov/images/epz/wxcalc/rhTdFromWetBulb.pdf ''Relative Humidity and Dewpoint Temperature from Temperature and Wet-Bulb Temperature'']</ref> are taken from a 1980 paper by David Bolton in the ''Monthly Weather Review'':<ref>{{cite journal|last1=Bolton|first1=David|title=The Computation of Equivalent Potential Temperature|journal=Monthly Weather Review|date=July 1980|volume=108|issue=7|pages=1046–1053|doi=10.1175/1520-0493(1980)108<1046:TCOEPT>2.0.CO;2|url=https://www.rsmas.miami.edu/users/pzuidema/Bolton.pdf|bibcode=1980MWRv..108.1046B}}</ref>
*''a'' = 6.112&nbsp;mbar, ''b'' = 17.67, ''c'' = 243.5&nbsp;°C.
These valuations provide a maximum error of 0.1%, for {{nowrap|−30 °C ≤ ''T'' ≤ 35°C}} and {{nowrap|1% < RH < 100%}}.
Also noteworthy is the Sonntag1990,<ref>[http://irtfweb.ifa.hawaii.edu/~tcs3/tcs3/Misc/Dewpoint_Calculation_Humidity_Sensor_E.pdf SHTxx Application Note Dew-point Calculation]</ref>
*''a'' = 6.112&nbsp;mbar, ''b'' = 17.62, ''c'' = 243.12&nbsp;°C; for {{nowrap|−45 °C ≤ ''T'' ≤ 60&nbsp;°C}} (error ±0.35&nbsp;°C).
Another common set of values originates from the 1974 ''Psychrometry and Psychrometric Charts'', as presented by '''''Paroscientific''''',<ref>{{cite web |url=http://www.paroscientific.com/dewpoint.htm |title=MET4 and MET4A Calculation of Dew Point |accessdate=7 October 2014 |url-status=dead |archiveurl=https://archive.is/20120526034637/http://www.paroscientific.com/dewpoint.htm |archivedate=May 26, 2012 }}</ref>
*''a'' = 6.105&nbsp;mbar, ''b'' = 17.27, ''c'' = 237.7&nbsp;°C; for {{nowrap|0 °C ≤ ''T'' ≤ 60&nbsp;°C}} (error ±0.4&nbsp;°C).
Also, in the ''Journal of Applied Meteorology and Climatology'',<ref>{{cite journal|last1=Buck|first1=Arden L.|title=New Equations for Computing Vapor Pressure and Enhancement Factor|journal=Journal of Applied Meteorology|date=December 1981|volume=20|issue=12|pages=1527–1532|doi=10.1175/1520-0450(1981)020<1527:NEFCVP>2.0.CO;2|url=http://www.public.iastate.edu/~bkh/teaching/505/arden_buck_sat.pdf|bibcode=1981JApMe..20.1527B}}</ref> Arden Buck presents several different valuation sets, with different maximum errors for different temperature ranges.  Two particular sets provide a range of −40&nbsp;°C to +50&nbsp;°C between the two, with even lower maximum error within the indicated range than all the sets above:
*''a'' = 6.1121&nbsp;mbar, ''b'' = 17.368, ''c'' = 238.88&nbsp;°C; for {{nowrap|0 °C ≤ ''T'' ≤ 50&nbsp;°C}} (error ≤ 0.05%).
*''a'' = 6.1121&nbsp;mbar, ''b'' = 17.966, ''c'' = 247.15&nbsp;°C; for {{nowrap|−40 °C ≤ ''T'' ≤ 0&nbsp;°C}} (error ≤ 0.06%).

===Simple approximation===
There is also a very simple approximation that allows conversion between the dew point, temperature, and relative humidity. This approach is accurate to within about ±1&nbsp;°C as long as the relative humidity is above 50%:
<math display="block">\begin{align}
T_\mathrm{dp} &\approx T-\frac{100-\mathrm{RH}}{5}; \\[5pt]
\mathrm{RH} &\approx 100-5(T-T_\mathrm{dp});
\end{align}</math>

This can be expressed as a simple rule of thumb:
<blockquote>
For every 1&nbsp;°C difference in the dew point and dry bulb temperatures, the relative humidity decreases by 5%, starting with RH&nbsp;=&nbsp;100% when the dew point equals the dry bulb temperature.
</blockquote>

The derivation of this approach, a discussion of its accuracy, comparisons to other approximations, and more information on the history and applications of the dew point are given in the Bulletin of the American Meteorological Society.<ref>{{cite journal|last1=Lawrence|first1=Mark G.|title=The Relationship between Relative Humidity and the Dewpoint Temperature in Moist Air: A Simple Conversion and Applications|journal=Bulletin of the American Meteorological Society|date=February 2005|volume=86|issue=2|pages=225–233|doi=10.1175/BAMS-86-2-225|bibcode=2005BAMS...86..225L}}</ref>

For temperatures in degrees Fahrenheit, these approximations work out to
<math display="block">\begin{align}
T_\mathrm{dp,^\circ F} &\approx T_\mathrm{{}^\circ F}-\tfrac{9}{25}\left(100-\mathrm{RH}\right);\\[5pt]
\mathrm{RH} &\approx 100-\tfrac{25}{9}\left(T_\mathrm{{}^\circ F}-T_\mathrm{dp,^\circ F}\right);
\end{align}</math>

For example, a relative humidity of 100% means dew point is the same as air temp. For 90% RH,  dew point is 3&nbsp;°F lower than air temperature. For every 10 percent lower, dew point drops 3&nbsp;°F.

== Frost point ==
The frost point is similar to the dew point in that it is the temperature to which a given parcel of humid air must be cooled, at constant [[atmospheric pressure]], for [[water vapor]] to be [[deposition (phase transition)|deposited]] on a surface as [[ice crystals]] without undergoing the liquid phase (compare with [[sublimation (phase transition)|sublimation]]). The [[frost]] point for a given parcel of air is always higher than the dew point, as the stronger bonding between water molecules on the surface of ice requires higher temperature to break.<ref>{{cite web |url=http://www.theweatherprediction.com/habyhints/347/ |title=Frost point and dew point |last=Haby |first=Jeff |accessdate=September 30, 2011}}</ref>

== See also ==
* [[Bubble point]]
* [[Carburetor heat]]
* [[Hydrocarbon dew point]]
* [[Psychrometrics]]
* [[Thermodynamic diagrams]]

==References==
{{Reflist|}}

==External links==
* [http://www.faqs.org/faqs/meteorology/temp-dewpoint/ Often Needed Answers about Temp, Humidity & Dew Point] from the sci.geo.meteorology

{{Meteorological variables}}

{{DEFAULTSORT:Dew Point}}
[[Category:Atmospheric thermodynamics]]
[[Category:Temperature]]
[[Category:Psychrometrics]]
[[Category:Threshold temperatures]]
[[Category:Gases]]
[[Category:Meteorological data and networks]]
[[Category:Humidity and hygrometry]]

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