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This page contains contents from http://en.wikipedia.org/wiki/Dew_point
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<br />[[User:SLeithaeuser|SLeithaeuser]] 13:13, 28 October 2012 (CET)Hoffe das ist o.k. zitiert (oben) ich nehme die links noch heraus und überarbeite measurements.<br />[[User:SLeithaeuser|SLeithaeuser]] 09:17, 29 October 2012 (CET)Soweit erst einmal fertig- die Graphen habe ich mir von Salzwiki geholt- sie haben keinen Bezug zum Text wieviel darf ich zum ursprünglichen Text hinzufügen und wie zeige ich es an?


== Dew point ==
wer was geschrieben hat ergibt sich aus der history. Mien Vorschlag bei neuen Passagen, die über ein Korriguieren hinaus gehen siehe Texteseite. Sie können jetzt dort weiterarbeiten [[User:Hschwarz|Hschwarz]] 09:30, 29 October 2012 (CET)
 
[[File:Phasendiagramm Wasser.png|right|thumb|320px|'''Figure 1''' Phase transition of water<br />(Wasser-Water; Eis-Ice; Wasserdampf-Water vapor; Druck-Pressure; Kritischer Punkt-Critical point; Tripelpunkt- Triple point)]]<br />
[[File:Feuchte Luft.png|right|thumb|310px|'''Figure 2''' Maximum water vapor concentraten as a function of temperature]]<br />
 
The '''dew point''' is the temperature below which the water vapor in a volume of humid air at a constant barometric pressure will condense into liquid water. Condensed water is called dew when it forms on a solid surface.
 
The dew point is a water-to-air saturation temperature. The dew point is associated with relative humidity. A high relative humidity indicates that the dew point is closer to the current air temperature. 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 dew point remains constant and temperature increases, relative humidity decreases.<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, Ohio, USA | date=2006-08-15 | url=http://www.shorstmeyer.com/wxfaqs/humidity/humidity.html| accessdate=2009-08-20}}</ref>
 
At a given temperature but ''independent'' of barometric pressure, the dew point is a consequence of the absolute humidity, the mass of water per unit volume of air. If both the temperature and pressure rise, however, the dew point will rise and the relative humidity will lower accordingly. Reducing the absolute humidity without changing other variables will bring the dew point back down to its initial value. In the same way, increasing the absolute humidity after a temperature drop brings the dew point back down to its initial level.  If the temperature rises in conditions of constant pressure, then the dew point will remain constant but the relative humidity will drop.
For this reason, a constant relative humidity (%) with different temperatures implies that when it's hotter, a higher fraction of the air is water vapor than when it's cooler.
 
At a given barometric pressure but ''independent'' of temperature, the dew point indicates the [[mole fraction]] of water vapor in the air, or, put differently, determines the specific humidity of the air. If the pressure rises without changing this mole fraction, the dew point will rise accordingly; Reducing the mole fraction, i.e., making the air less humid, would bring the dew point back down to its initial value. In the same way, increasing the mole fraction after a pressure drop brings the relative humidity back up to its initial level.
Considering New York (33&nbsp;ft elevation) and Denver (5,280&nbsp;ft 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}}</ref> for example, this means that if the dew point and temperature in both cities are the same, then the mass of water vapor per cubic meter of air will be the same, but the mole fraction of water vapor in the air will be greater in Denver.<br />
 
 
 
==Measurement==
Devices called dew point meters 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. [[Moisture measurement methods]] deals with this subject in detail.
 
==Calculating the dew point==
 
 
A well-known approximation used to calculate the dew point, ''T<sub>dp</sub>'', given just the actual ("dry bulb") air temperature, ''T'' and [[relative humidity]] (in percent), ''RH'', is the ''Magnus formula'':
:::<math>\begin{align}
\gamma(T,R\!H)&=\ln\left(\frac{R\!H}{100}\exp\!\!\left(\frac{bT}{c+T}\right)\right)=\ln\left(\frac{R\!H}{100}\right)+\frac{bT}{c+T};\\
T_{dp}&= \frac{c\gamma(T,R\!H)}{b-\gamma(T,R\!H)};\end{align}
</math>
The more fuller formulation and origin of this approximation involves the interrelated [[Saturation (chemistry)|saturated]] water vapor pressure (in units of [[bar (unit)|millibar]], which is also [[Pascal (unit)|hPa]]) at ''T'', ''P<sub>s</sub>''(''T''), and the actual water vapor pressure (also in units of millibar), ''P<sub>a</sub>''(''T''), which can be either found with ''RH'' or approximated with the barometric pressure (in millibar units), ''BP<sub>mb</sub>'', and "[[wet-bulb temperature|wet-bulb]]" temperature, ''T<sub>w</sub>'' is:
 
:::<small>Note: unless declared otherwise, all temperatures are expressed and worked in degrees Celsius</small>
::::<math>
\begin{align}
P_s(T)& = \frac{100}{R\!H}P_\text{a}(T) = a\exp\!\!\left(\frac{bT}{c+T}\right);\\[8pt]
P_\text{a}(T) & = \frac{R\!H}{100}P_s(T)=a\exp(\gamma(T,R\!H)),\\
&\approx P_s(T_\text{w}) - B\!P_\text{mb} 0.00066 \left[1 + (0.00115T_\text{w} \right)]\left(T-T_\text{w}\right);\\[5pt]
T_\text{dp} & = \frac{c\ln(P_\text{a}(T)/a)}{b-\ln(P_\text{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, ''d'' constant:
:::<math>\begin{align}P_{s:m}(T)&=a\exp\!\!\bigg(\left(b-\frac{T}{d}\right)\left(\frac{T}{c+T}\right)\bigg);\\[8pt]
\gamma_m(T,R\!H)&=\ln\Bigg(\frac{R\!H}{100}\exp\!\!
\bigg(\left(b-\frac{T}{d}\right)\left(\frac{T}{c+T}\right)\bigg)
\Bigg);\\
T_{dp}&= \frac{c\gamma_m(T,R\!H)}{b-\gamma_m(T,R\!H)};\end{align}</math>
:::''(where <math>\scriptstyle{a=6.1121;\quad\;b= 18.678;\quad\;c= 257.14^\circ C;\quad\;d=234.5^\circ C.}</math>)''
 
There are several different  constant sets in use, the ones used in [[NOAA]]'s presentation <ref>http://www.srh.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>[https://www.rsmas.miami.edu/users/pzuidema/Bolton.pdf "''The computation of equivalent potential temperature''", Monthly Weather Review], vol.108, pg.1047, Eq.10</ref>:
:<math>\begin{align}a&=6.112;\quad\;b&= 17.67;\quad\;c&= 243.5^\circ C;\end{align}</math>
These valuations provide a minimum accuracy of 0.1%, for
:::::-30°C ≤ ''T'' ≤ +35°C;
::::::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>
:<math>\scriptstyle{a=6.112;\quad\;b= 17.62;\quad\;c= 243.12^\circ C:\quad -45^\circ C\le T\le +60^\circ C\quad (<-0.35^\circ C)}</math>
Another common set of values originates from the 1974 ''Psychrometry and Psychrometric Charts'', as presented by '''''Paroscientific''''',<ref>[http://www.paroscientific.com/dewpoint.htm MET4 AND MET4A CALCULATION OF DEW POINT]</ref>
:<math>\scriptstyle{a=6.105;\quad\;b= 17.27;\quad\;c= 237.7^\circ C:\quad 0^\circ C\le T\le +60^\circ C\quad (\pm0.4^\circ C)}</math>
Also, in the ''Journal of Applied Meteorology and Climatology'',<ref>Buck, A. L. (1981), [http://www.public.iastate.edu/~bkh/teaching/505/arden_buck_sat.pdf "New equations for computing vapor pressure and enhancement factor"], J. Appl. Meteorol. 20: 1527–1532</ref> Arden Buck presents several different valuation sets, with different minimum accuracies for different temperature ranges.  Two particular sets provide a range of -40°C → +50°C between the two, with even greater minimum accuracy than all of the other, above sets (maximum error at given |C°| extreme):
:<math>\scriptstyle{a=6.1121;\quad\;b= 17.368;\quad\;c= 238.88^\circ C:\quad\quad\! 0^\circ C\le T\le +50^\circ C\;\;(\le0.05%)}</math>
:<math>\scriptstyle{a=6.1121;\quad\;b= 17.966;\quad\;c= 247.15^\circ C:\quad -40^\circ C\le T\le 0^\circ C\quad\! \;\;(\le0.06%)}</math>
 
===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°C as long as the relative humidity is above 50%:
:<math>T_{dp}\approx T-\frac{100-R\!H}{5};</math>
and
:<math>R\!H\approx 100-5(T-T_{dp});\,</math>
 
This can be expressed as a simple rule of thumb:
<blockquote>
''For every 1°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>M. G. Lawrence, "The relationship between relative humidity and the dew point temperature in moist air: A simple conversion and applications", Bull. Am. Meteorol. Soc., 86, 225&ndash;233, 2005</ref>
 
For temperatures in degrees Fahrenheit, these approximations work out to
:<math> T_{dp:f}\approx T_{f}-\frac{9}{25}(100-R\!H);</math>
and
:<math> R\!H\approx 100-\frac{25}{9}(T_{f}-T_{dp:f});</math>
 
For example, a relative humidity of 100% means dew point is the same as air temp. For 90% RH,  dew point is 3 degrees Fahrenheit lower than air temp. 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 barometric pressure, for water vapor to be [[Deposition (phase transition)|deposited]] on a surface as ice without going through 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 | accessdate=September 30, 2011 | author=Haby, Jeff}}</ref>
 
 
==References==
{{Reflist|}}
 
==External links==
* [http://weathersavvy.com/Q-dew_point1.html What is the dew point?]
* [http://weather.gov/glossary/index.php?word=dew+point Dew point definition] NOAA Glossary
* [http://www.paroscientific.com/dewpoint.htm Dew point formula]
* [http://www.faqs.org/faqs/meteorology/temp-dewpoint/ Often Needed Answers about Temp, Humidity & Dew Point] from the sci.geo.meteorology
* [http://www.humidity-calculator.com/index.php Humidity calculator]
* [http://www.dew-point.us Dew point calculator in construction]
<br />[[User:SLeithaeuser|SLeithaeuser]] 13:13, 28 October 2012 (CET)Hoffe das ist o.k. zitiert (oben) ich nehme die links noch heraus und überarbeite measurements.<br />
 
== Relationship between human comfort and critical point for object damage/deliquescent humidity for salts ==
'''would be an interesting point- I would have to research this- have you got some data more readily available?'''

Latest revision as of 08:30, 29 October 2012

text moved to the content pageHschwarz 09:30, 29 October 2012 (CET)


SLeithaeuser 13:13, 28 October 2012 (CET)Hoffe das ist o.k. zitiert (oben) ich nehme die links noch heraus und überarbeite measurements.
SLeithaeuser 09:17, 29 October 2012 (CET)Soweit erst einmal fertig- die Graphen habe ich mir von Salzwiki geholt- sie haben keinen Bezug zum Text wieviel darf ich zum ursprünglichen Text hinzufügen und wie zeige ich es an?

wer was geschrieben hat ergibt sich aus der history. Mien Vorschlag bei neuen Passagen, die über ein Korriguieren hinaus gehen siehe Texteseite. Sie können jetzt dort weiterarbeiten Hschwarz 09:30, 29 October 2012 (CET)

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