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http://en.wikipedia.org/wiki/Psychrometrics
{{about|the meteorological dew point|the petroleum term|Hydrocarbon dew point}}


{{Distinguish2|[[Psychometrics]], a discipline of psychology and education}}
The '''dew point''' is the [[temperature]] below which the [[water vapor]] in a volume of humid [[air]] at a constant [[barometric pressure]] will [[Condensation|condense]] into liquid water. Condensed water is called [[dew]] when it forms on a solid surface.
{{Redirect-distinguish|Psychrometry|Psychometry}}
'''Psychrometrics''' or '''psychrometry''' or '''Hygrometry''' are terms used to describe the field of engineering concerned with the determination of physical and thermodynamic properties of gas-vapor mixtures. The term derives from the Greek ''psuchron'' (ψυχρόν) meaning "cold"<ref>{{citation|url=http://www.perseus.tufts.edu/cgi-bin/ptext?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3D%23115938|title=''psuchron''|work=A Greek-English Lexicon|author=Henry George Liddell, Robert Scott}}</ref> and ''metron'' (μέτρον) meaning "means of measurement".<ref>{{citation|title=''metron''|work=A Greek-English Lexicon|url=http://www.perseus.tufts.edu/cgi-bin/ptext?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3D%2367261|author=Henry George Liddell, Robert Scott}}</ref>


==Common applications==
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>
Although the principles of psychrometry apply to any physical system consisting of gas-vapor mixtures, the most common system of interest is the mixture of water vapor and air, because of its application in [[HVAC|heating, ventilating, and air-conditioning]] and [[meteorology]]. In human terms, our comfort is in large part a consequence of not just the temperature of the surrounding air, but (because we cool ourselves via perspiration) the extent to which that air is saturated with water vapor.


Many substances are [[hygroscopy|hygroscopic]], meaning they attract water, usually in proportion to the relative humidity or above a [[critical relative humidity]].  Such substances include cotton, paper, cellulose, other wood products, sugar, and many chemicals and fertilizers. Industries that use these materials are concerned with relative humidity control in production and storage of such materials.
[[General aviation]] pilots use dew-point data to calculate the likelihood of [[carburetor icing]] and [[fog]], and to estimate the height of the [[cloud base]].


In industrial drying applications, such as drying paper, manufacturers usually try to achieve an optimum between low relative humidity, which increases the drying rate, and energy usage, which decreases as exhaust relative humidity increases. In many industrial applications it is important to avoid [[condensation]] that would ruin product or cause corrosion.
[[Image:dewpoint.jpg|300px|right|thumb|This graph shows the maximum percentage, by mass, of water vapor that air at sea-level across a range of temperatures can contain.]]


Molds and fungi can be controlled by keeping relative humidity low.  Wood destroying fungi generally do not grow at relative humidities below 75%. [[Silverfish]]  (''lepisma saccharina'') cannot survive in relative humidities less than 75%.
At a given temperature but ''independent'' of [[barometric pressure]], the dew point is a consequence of the [[Humidity#Absolute_humidity_.28Volume_basis.29|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.


==Psychrometric properties==
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.
===Dry-bulb temperature===
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.
{{main|Dry-bulb temperature}}
[[Mercury-in-glass thermometer|Common thermometers]] measure what is known as the dry-bulb temperature. Electronic temperature measurement, via thermocouples, thermistors, and resistance temperature devices (RTDs), for example, have been widely used too since they became available.


===Wet-bulb temperature===
==Relationship to human comfort==
{{main|Wet-bulb temperature}}
When the air temperature is high, the body's ''[[thermoregulation]]'' uses evaporation of [[perspiration]] 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 [[humidity|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 cooling system 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 even without generating additional body heat by exercising, one can become coated with sweat on humid days. It is the unevaporated sweat that tends to make one feel uncomfortable in humid weather.
The thermodynamic wet-bulb temperature is a [[Thermodynamic properties|thermodynamic property]] of a mixture of air and water vapor.  The value indicated by a simple wet-bulb thermometer often provides an adequate approximation of the thermodynamic wet-bulb temperature.


A wet-bulb [[thermometer]] is an instrument which may be used to infer the amount of moisture in the air. If a moist cloth wick is placed over a thermometer bulb, the evaporation of moisture from the wick will lower the thermometer reading (temperature). If the air surrounding a wet-bulb thermometer is dry, evaporation from the moist wick will be more rapid than if the air is moist. When the air is [[dew point|saturated]], no water will evaporate from the wick and the temperature of the wet-bulb thermometer will be the same as the reading on the dry-bulb thermometer. However, if the air is not saturated, water will evaporate from the wick causing the temperature reading to be lower.
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.


The accuracy of a simple wet-bulb thermometer depends on how fast air passes over the bulb and how well the thermometer is shielded from the radiant temperature of its surroundings. Speeds up to 5,000&nbsp;ft/min (~60&nbsp;mph) are best but it may be dangerous to move a thermometer at that speed. Errors up to 15% can occur if the air movement is too slow or if there is too much radiant heat present (from sunlight, for example).
A [[wet bulb thermometer]] also uses [[evaporative cooling]], so it provides a good analog for use in evaluating comfort level.


A wet bulb temperature taken with air moving at about 1–2&nbsp;m/s is referred to as a '''screen temperature''', whereas a temperature taken with air moving about 3.5&nbsp;m/s or more is referred to as '''sling temperature'''.
Discomfort also exists when the dew point is low (below around {{convert|-30|°C|°F|abbr=on}}). The drier air can cause skin to crack and become irritated more easily.  It will also dry out the respiratory paths. [[Occupational Safety and Health Administration|OSHA]] recommends indoor air be maintained at 68 to 76°F (20 to 24.5°C) with a 20-60% relative humidity (a dew point of 24 to 60°F).<ref>http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=INTERPRETATIONS&p_id=24602</ref>


A [[psychrometer]] is a device that includes both a dry-bulb and a wet-bulb thermometer. A '''sling psychrometer''' requires manual operation to create the airflow over the bulbs, but a '''powered psychrometer''' includes a fan for this function.  Knowing both the dry-bulb temperature (DBT) and wet-bulb temperature (WBT), one can determine the relative humidity (RH) from the psychrometric chart appropriate to the air pressure.
Lower dew points, less than {{convert|10|°C|°F|abbr=on}}, 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 (see graph below), allowing for relative effective cooling.


===Relative humidity===
Those accustomed to [[continental climate]]s often begin to feel uncomfortable when the dew point reaches between {{convert|15|and|20|C|F}}. Most inhabitants of these areas will consider dew points above {{convert|21|°C|°F|abbr=on}} oppressive.
{{main|Relative humidity}}
The ratio of the vapor pressure of moisture in the sample to the saturation pressure at the dry bulb temperature of the sample.


===Dew point temperature===
{| class="wikitable"
{{main|Dew point temperature}}
|-
The saturation temperature of the moisture present in the sample of air, it can also be defined as the temperature at which the vapour changes into liquid (condensation). Usually the level at which water vapor changes into liquid marks the base of the cloud in the atmosphere hence called condensation level.So the temperature value that allow this process (Condensation) to take place is called the 'dew point temperature'.A simplified definition is the temperature at which the water wapour turns into "dew"(Chamunoda Zambuko 2012).
! Dew point °C
! Dew point °F
! Human perception</b><ref name=Horstmeyer/>
! Rel. humidity at {{convert|32|°C|°F|abbr=on}}
|-
| > Higher than 26 °C
| > Higher than 80 °F
| Severely high. Even deadly for [[asthma]] related illnesses
| 65% and higher
|-
| 24–26 °C
| 75–80 °F
| Extremely uncomfortable, fairly oppressive
| 62%
|-
| 21–24 °C
| 70–74 °F
| Very humid, quite uncomfortable
| 52–60%
|-
| 18–21 °C
| 65–69 °F
| Somewhat uncomfortable for most people at upper edge
| 44–52%
|-
| 16–18 °C
| 60–64 °F
| OK for most, but all perceive the humidity at upper edge
| 37–46%
|-
| 13–16 °C
| 55–59 °F
| Comfortable
| 38–41%
|-
| 10–12 °C
| 50–54 °F
| Very comfortable
| 31–37%
|-
| < 10 °C
| < 49 °F
| A bit dry for some
| 30%
|}


===Humidity===
==Measurement==
{{main|Humidity}}
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.


====Specific Humidity====
==Calculating the dew point==
Specific humidity is the proportion of the mass of water vapor per unit mass of the moist air sample (dry air plus the water vapor); it is closely related to humidity ratio and always lower in value.
{{See also|Psychrometric chart}}


====Absolute humidity====
[[Image:Dewpoint-RH.svg|thum |center|Graph of the dependence of the dew point upon air temperature for several levels of relative humidity.]]
The mass of water vapor per unit volume of air containing the water vapor. This quantity is also known as the water vapor density.<ref>{{cite web|title=AMS Weather Glossary|url=http://www.ametsoc.org/amsedu/wes/glossary.html#W|publisher=American Meteorological Society|accessdate=18 September 2011}}</ref>


===Specific enthalpy===
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'':
Analogous to the specific enthalpy of a pure substance. In psychrometrics, the term quantifies the total energy of both the dry air and water vapor per pound of dry air.
:::<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};\\
===Specific volume===
T_{dp}&= \frac{c\gamma(T,R\!H)}{b-\gamma(T,R\!H)};\end{align}
Analogous to the specific volume of a pure substance. In psychrometrics, the term quantifies the total volume of both the dry air and water vapor per pound of dry air.
 
===Psychrometric ratio===
The '''psychrometric ratio''' is the ratio of the heat transfer coefficient to the product of mass transfer coefficient and humid heat at a wetted surface.  It may be evaluated with the following equation:<ref>http://www.che.iitb.ac.in/courses/uglab/manuals/coollabmanual.pdf, accessed 20080408</ref><ref>http://www.probec.org/fileuploads/fl120336971099294500CHAP12_Dryers.pdf, accessed 20080408</ref>
:<math>
r = \frac {h_c} {k_y c_s}\,
</math>
</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:


:where:
:::<small>Note: unless declared otherwise, all temperatures are expressed and worked in degrees Celsius</small>
::*'''<math>r</math>''' = Psychrometric ratio, dimensionless
::::<math>
::*'''<math>h_c</math>''' = convective heat transfer coefficient, W m<sup>-2</sup> K<sup>-1</sup>
\begin{align}
::*'''<math>k_y</math>''' = convective mass transfer coefficient, kg m<sup>-2</sup> s<sup>-1</sup>
P_s(T)& = \frac{100}{R\!H}P_\text{a}(T) = a\exp\!\!\left(\frac{bT}{c+T}\right);\\[8pt]
::*'''<math>c_s</math>''' = humid heat, J kg<sup>-1</sup> K<sup>-1</sup>
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]
The psychrometric ratio is an important property in the area of psychrometrics, as it relates the absolute humidity and saturation humidity to the difference between the dry bulb temperature and the [[adiabatic saturation temperature]].
T_\text{dp} & = \frac{c\ln(P_\text{a}(T)/a)}{b-\ln(P_\text{a}(T)/a)};\end{align}</math>
 
Mixtures of air and water vapor are the most common systems encountered in psychrometry.  The psychrometric ratio of air-water vapor mixtures is approximately unity, which implies that the difference between the adiabatic saturation temperature and wet bulb temperature of air-water vapor mixtures is small.  This property of air-water vapor systems simplifies drying and cooling calculations often performed using psychrometic relationships.
 
===Humid heat===
'''Humid heat''' is the constant-pressure specific heat of moist air, per unit mass of dry air.<ref>http://www.engin.umich.edu/class/che360/coursepack/ch13-cooltower.doc</ref>
 
===Pressure===
Many psychrometric properties are dependent on the atmospheric pressure at the location of the sample.
 
==Psychrometric charts==
[[Image:PsychrometricChart.SeaLevel.SI.svg|thumb|A psychrometric chart for sea-level elevation]]
 
===Terminology===
A psychrometric chart is a graph of the thermodynamic parameters of moist air at a constant pressure, often equated to an elevation relative to sea level. The [[ASHRAE]]-style psychrometric chart, shown here, was pioneered by [[Willis Carrier]] in 1904.<ref>Gatley, D.P. 2004. “Psychrometric chart celebrates 100th anniversary.” ASHRAE Journal 46(11):16 – 20</ref> It depicts these parameters and is thus a graphical [[equation of state]].  The parameters are:
 
*[[Dry-bulb temperature]] (''DBT'') is that of an air sample, as determined by an ordinary thermometer. It is typically plotted as the [[x-axis|abscissa (horizontal axis)]] of the graph. The SI units for temperature are [[kelvin]]s or [[degrees Celsius]]; other units are [[degrees Fahrenheit]] and [[degrees Rankine]].
*[[Wet-bulb temperature]] (''WBT'') is that of an air sample after it has passed through a constant-pressure, ideal, adiabatic saturation process, that is, after the air has passed over a large surface of liquid water in an insulated channel.  In practice this is the reading of a thermometer whose sensing bulb is covered with a wet sock evaporating into a rapid stream of the sample air (see [[Hygrometer]]). When the air sample is saturated with water, the WBT will read the same as the DBT. The slope of the line of constant WBT reflects the heat of vaporization of the water required to saturate the air of a given relative humidity.
*[[Dew point]] temperature (''DPT'') is the temperature at which a moist air sample at the same pressure would reach water vapor “saturation.”  At this point further removal of heat would result in water vapor condensing into liquid water fog or, if below [[freezing point]], solid hoarfrost.  The dew point temperature is measured easily and provides useful information, but is normally not considered an independent property of the air sample as it duplicates information available via other humidity properties and the saturation curve.
*[[Relative humidity]] (''RH'') is the ratio of the mole fraction of water vapor to the mole fraction of saturated moist air at the same temperature and pressure. RH is dimensionless, and is usually expressed as a percentage. Lines of constant RH reflect the physics of air and water: they are determined via experimental measurement. The concept that air "holds" moisture, or that moisture “dissolves” in dry air and saturates the solution at some proportion, is erroneous (albeit widespread); see [[relative humidity]] for further details.
*[[Mixing ratio|Humidity ratio]] is the proportion of mass of water vapor per unit mass of dry air at the given conditions (DBT, WBT, DPT, RH, etc.). Also known as moisture content or mixing ratio. It is typically plotted as the [[y-axis|ordinate (vertical axis)]] of the graph. For a given DBT there will be a particular humidity ratio for which the air sample is at 100% relative humidity: the relationship reflects the physics of water and air and must be determined by measurement. The dimensionless humidity ratio is typically expressed as grams of water per kilogram of dry air, or grains of water per pound of air (7000 grains equal 1 pound).
*[[Enthalpy|Specific enthalpy]], symbolized by ''h'', is the sum of the internal (heat) energy of the moist air in question, including the heat of the air and water vapor within. Also called heat content per unit mass. In the approximation of ideal gases, lines of constant enthalpy are parallel to lines of constant WBT. Enthalpy is given in (SI) joules per kilogram of air, or BTU per pound of dry air.
*[[Specific volume]] is the volume of the mixture (dry air plus the water vapor) containing one unit of mass of “dry air”.  The SI units are cubic meters per kilogram of dry air; other units are cubic feet per pound of dry air.  The inverse of specific volume is usually confused as the density of the mixture (see "Applying the Psychrometric Relationships" CIBSE, August 2009).  However, to obtain the actual mixture density one must multiply the inverse of the specific volume by unity plus the humidity ratio value at the point of interest (see ASHRAE Fundamentals 1989 6.6, equation 9).
 
The psychrometric chart allows all the parameters of some moist air to be determined from any three independent parameters, one of which must be the pressure.  Changes in ''state'', such as when two air streams mix, can be modeled easily and somewhat graphically using the correct psychrometric chart for the location's air pressure or elevation relative to sea level.  For locations at not more than 2000&nbsp;ft (600 m) of altitude it is common practice to use the sea-level psychrometric chart.
 
In the ''ω''-''t'' chart, the dry bulb temperature (''t'') appears as the abscissa (horizontal axis) and the humidity ratio (''ω'') appear as the ordinate (vertical axis). A chart is valid for a given air pressure (or elevation above sea level). From any two independent ones of the six parameters dry bulb temperature, wet bulb temperature, relative humidity, humidity ratio, specific enthalpy, and specific volume, all the others can be determined.  There are <math>\left({6 \atop 2}\right) = 15</math> possible [[combination]]s of independent and derived parameters.
===Locating parameters on chart===
'''* Dry bulb temperature:''' These lines are drawn straight, not always parallel to each other, and slightly inclined from the vertical position.  This is the ''t''–axis, the abscissa (horizontal) axis.  Each line represents a constant temperature.
 
'''* Dew point temperature:''' From the state point follow the horizontal line of constant humidity ratio to the intercept of 100% RH, also known as the ''saturation curve''.  The dew point temperature is equal to the fully saturated dry bulb or wet bulb temperatures.
 
'''* Wet bulb temperature:''' These lines are oblique lines that differ slightly from the enthalpy lines.  They are identically straight but are not exactly parallel to each other. These intersect the saturation curve at DBT point.
 
'''* Relative humidity:''' These hyperbolic lines are shown in intervals of 10%.  The saturation curve is at 100% RH, while dry air is at 0% RH.
 
'''* Humidity ratio:''' These are the horizontal lines on the chart.  Humidity ratio is usually expressed as mass of moisture per mass of dry air (pounds or kilograms of moisture per pound or kilogram of dry air, respectively). The range is from 0 for dry air up to 0.03 (lbmw/lbma) on the right hand ''ω''-axis, the ordinate or vertical axis of the chart.
 
'''* Specific enthalpy:''' These are oblique lines drawn diagonally downward from left to right across the chart that are parallel to each other.  These are not parallel to wet bulb temperature lines.


'''* Specific volume:''' These are a family of equally spaced straight lines that are nearly parallel.
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>)''


The region above the saturation curve is a two-phase region that represents a mixture of saturated moist air and liquid water, in thermal equilibrium.
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>


The protractor on the upper left of the chart has two scales.  The inner scale represents sensible-total heat ratio (SHF).  The outer scale gives the ratio of enthalpy difference to humidity difference.  This is used to establish the slope of a condition line between two processes.  The horizontal component of the condition line is the change in sensible heat while the vertical component is the change in latent heat.<ref>Kutz, Myer (Ed). (2006)  The Mechanical Engineers’ Handbook. New Jersey: John Wiley & Sons.</ref><ref>American Society of Heating, Refrigerating and Air-Conditioning Engineers (1997). ASHRAE Fundamentals Handbook</ref><ref>Biasca, Karyn. [http://www.uwsp.edu/papersci/biasca/currentpages/ "Psychrometric Chart Tutorial"], accessed November 20, 2010.</ref>
===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>


===How to read the chart: fundamental examples===
This can be expressed as a simple rule of thumb:
Psychrometric charts are available in SI (metric) and  IP (U.S./English) units.  They are also available in low and high temperature ranges and for different pressures. 
<blockquote>
*Determining relative humidity: The percent relative humidity can be located at the intersection of the vertical dry bulb and diagonally down sloping wet bulb temperature lines. Metric (SI): Using a dry bulb of 25° C and a wet bulb of 20° C, read the relative humidity at approximately 63.5%.  English/U.S (IP): Using a dry bulb of 77° F and a wet bulb of 68° F, read the relative humidity at approximately 63.5%.  In this example the humidity ratio is 0.0126 kg water per kg dry air.
''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.''
*Determining the effect of temperature change on relative humidity:  For air of a fixed water composition or ''moisture ratio'', find the starting relative humidity from the intersection of the wet and dry bulb temperature lines.  Using the conditions from the previous example, the relative humidity at a different dry bulb temperatures can be found along the horizontal humidity ratio line of 0.0126, either in kg water per kg dry air or pounds water per pound dry air.
</blockquote>
:A common variation of this problem is determining the final humidity of air leaving an air conditioner evaporator coil then heated to a higher temperature.  Assume that the temperature leaving the coil is 10°C (50°F) and is heated to room temperature (not mixed with room air), which is found by following the horizontal humidity ratio from the dew point or saturation line to the room dry bulb temperature line and reading the relative humidity.  In typical practice the conditioned air is mixed with room air that is being infiltrated with outside air.


*Determining the amount of water to be removed in lowering relative humidity: This is the difference in humidity ratio between the initial and final conditions times the weight of dry air.
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>


===Mollier diagram===
For temperatures in degrees Fahrenheit, these approximations work out to
{{main|Enthalpy-entropy chart}}
:<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>


[[File:Mollier.pdf|thumb|Mollier Diagram (Chart), IP Units]]
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.


The Mollier ''h''-''x'' diagram], developed by [[Richard Mollier]] in 1923,<ref>Mollier, R. 1923. “Ein neues diagram für dampfluftgemische.”  ZVDI 67(9)</ref> is an alternative psychrometric chart, preferred by many users in Scandinavia, Eastern Europe, and Russia.<ref>Todorovic, B., ASHRAE Transactions DA-07-024 (113-1), 2007</ref>
== Frost point ==


The underlying psychrometric parameter data for the psychrometric chart and the Mollier diagram are identical. At first glance there is little resemblance between the charts, but if the a chart is rotated by ninety degrees and looked at in a mirror the resemblance becomes apparent. The Mollier diagram coordinates are enthalpy ''h'' and humidity ratio ''x''. The enthalpy coordinate is ''skewed'' and the lines of constant enthalpy are parallel and evenly spaced. The ASHRAE psychrometric charts since 1961 use similar plotting coordinates. Some psychrometric charts use ''dry-bulb temperature'' and ''humidity ratio'' coordinates.
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>


==See also==
== See also ==
*[[Air]]
* [[Bubble point]]
*[[Air conditioning]]
* [[Carburetor heat]]
*[[Dalton's law|Dalton's law of Partial Pressures]]
* [[Hydrocarbon dew point]]
*[[Dew point]]
* [[Psychrometrics]]
*[[Dry-bulb temperature]]
* [[Thermodynamic diagrams]]
*[[Evaporative cooling]]
*[[Humidity]]
*[[Relative humidity]]
*[[Wet-bulb temperature]]
*[[Operative temperature]]


==References==
==References==
{{reflist}}
{{Reflist|}}


==External links==
==External links==
*[http://www.xchanger.com/tools_psych.htm Xchanger Inc, webpage] Calculator for humidity, dew point, mass flows & heat flux for variable pressure systems with compressors, blowers, vacuum pumps and heat exchangers.
* [http://weathersavvy.com/Q-dew_point1.html What is the dew point?]
*[http://www.truetex.com/psychrometric_chart.htm Psychrometric Chart] - Detailed psychrometric chart including curves for enthalpy, air mass, and water mass.
* [http://weather.gov/glossary/index.php?word=dew+point Dew point definition] NOAA Glossary
*[http://www.ashrae.org American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.]
* [http://www.paroscientific.com/dewpoint.htm Dew point formula]
*[http://www.uigi.com/technology.html Universal Industrial Gases, Inc. webpage] -  Links to physical properties tables, to psychrometric charts that  depict inter-relationships among the various physical properties of air, to online psychrometric properties calculators for easy calculation of properties, and to individual gas information pages.
* [http://www.faqs.org/faqs/meteorology/temp-dewpoint/ Often Needed Answers about Temp, Humidity & Dew Point] from the sci.geo.meteorology
*[http://www.issi1.com/corwin/calculator/vapor.html Corwin's Calculators] Calculator for humidity, dew point.
* [http://www.humidity-calculator.com/index.php Humidity calculator]
*[https://www.educate-sustainability.eu/kb/it/content/anatomy-psychrometric-chart Anatomy of the Psychrometric chart, from educate-sustainability.eu]
* [http://www.dew-point.us Dew point calculator in construction]


[[Category:Psychrometrics| ]]
{{Meteorological variables}}
[[Category:Heating, ventilating, and air conditioning]]


[[ca:Psicrometria]]
{{DEFAULTSORT:Dew Point}}
[[de:Psychrometrie]]
[[Category:Atmospheric thermodynamics]]
[[es:Psicrometría]]
[[Category:Psychrometrics]]
[[fa:سایکرومتری]]
[[fr:Psychrométrie]]
[[gl:Psicrometría]]
[[hr:Psihrometrija]]
[[it:Psicrometria]]
[[ja:湿り空気線図]]
[[pt:Psicrometria]]
[[sr:Psihrometrija]]

Revision as of 12:01, 28 October 2012

This page contains contents from http://en.wikipedia.org/wiki/Dew_point it is therefore released under the licence: „Creative Commons Attribution/Share Alike“

Template:About

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.[1]

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

File:Dewpoint.jpg
This graph shows the maximum percentage, by mass, of water vapor that air at sea-level across a range of temperatures can contain.

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 ft elevation) and Denver (5,280 ft elevation),[2] 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.

Relationship to human comfort[edit]

When the air temperature is high, the body's thermoregulation uses evaporation of perspiration 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 cooling system 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 even without generating additional body heat by exercising, one can become coated with sweat on humid days. It is the unevaporated sweat that tends to make one feel uncomfortable in humid weather.

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 analog for use in evaluating comfort level.

Discomfort also exists when the dew point is low (below around Template:Convert). The drier air can cause skin to crack and become irritated more easily. It will also dry out the respiratory paths. OSHA recommends indoor air be maintained at 68 to 76°F (20 to 24.5°C) with a 20-60% relative humidity (a dew point of 24 to 60°F).[3]

Lower dew points, less than Template:Convert, 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 (see graph below), allowing for relative effective cooling.

Those accustomed to continental climates often begin to feel uncomfortable when the dew point reaches between Template:Convert. Most inhabitants of these areas will consider dew points above Template:Convert oppressive.

Dew point °C Dew point °F Human perception[1] Rel. humidity at Template:Convert
> Higher than 26 °C > Higher than 80 °F Severely high. Even deadly for asthma related illnesses 65% and higher
24–26 °C 75–80 °F Extremely uncomfortable, fairly oppressive 62%
21–24 °C 70–74 °F Very humid, quite uncomfortable 52–60%
18–21 °C 65–69 °F Somewhat uncomfortable for most people at upper edge 44–52%
16–18 °C 60–64 °F OK for most, but all perceive the humidity at upper edge 37–46%
13–16 °C 55–59 °F Comfortable 38–41%
10–12 °C 50–54 °F Very comfortable 31–37%
< 10 °C < 49 °F A bit dry for some 30%

Measurement[edit]

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.

Calculating the dew point[edit]

Template:See also

Graph of the dependence of the dew point upon air temperature for several levels of relative humidity.

A well-known approximation used to calculate the dew point, Tdp, given just the actual ("dry bulb") air temperature, T and relative humidity (in percent), RH, is the Magnus formula:

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \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} }

The more fuller formulation and origin of this approximation involves the interrelated saturated water vapor pressure (in units of millibar, which is also hPa) at T, Ps(T), and the actual water vapor pressure (also in units of millibar), Pa(T), which can be either found with RH or approximated with the barometric pressure (in millibar units), BPmb, and "wet-bulb" temperature, Tw is:

Note: unless declared otherwise, all temperatures are expressed and worked in degrees Celsius
Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \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}}

For greater accuracy, Ps(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:

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \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}}
(where Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \scriptstyle{a=6.1121;\quad\;b= 18.678;\quad\;c= 257.14^\circ C;\quad\;d=234.5^\circ C.}} )

There are several different constant sets in use, the ones used in NOAA's presentation [4] are taken from a 1980 paper by David Bolton in the Monthly Weather Review[5]:

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{align}a&=6.112;\quad\;b&= 17.67;\quad\;c&= 243.5^\circ C;\end{align}}

These valuations provide a minimum accuracy of 0.1%, for

-30°C ≤ T ≤ +35°C;
1% < RH < 100%;

Also noteworthy is the Sonntag1990,[6]

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \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)}}

Another common set of values originates from the 1974 Psychrometry and Psychrometric Charts, as presented by Paroscientific,[7]

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \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)}}

Also, in the Journal of Applied Meteorology and Climatology,[8] 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):

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \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%)}}
Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \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%)}}

Simple approximation[edit]

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%:

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle T_{dp}\approx T-\frac{100-R\!H}{5};}

and

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle R\!H\approx 100-5(T-T_{dp});\,}

This can be expressed as a simple rule of thumb:

For every 1°C difference in the dew point and dry bulb temperatures, the relative humidity decreases by 5%, starting with RH = 100% when the dew point equals the dry bulb temperature.

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.[9]

For temperatures in degrees Fahrenheit, these approximations work out to

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle T_{dp:f}\approx T_{f}-\frac{9}{25}(100-R\!H);}

and

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle R\!H\approx 100-\frac{25}{9}(T_{f}-T_{dp:f});}

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 °F.

Frost point[edit]

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 deposited on a surface as ice without going through the liquid phase. (Compare with 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.[10]

See also[edit]

References[edit]

Template:Reflist

External links[edit]

Template:Meteorological variables

  1. ↑ 1.0 1.1 Template:Cite web
  2. ↑ Template:Cite web
  3. ↑ http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=INTERPRETATIONS&p_id=24602
  4. ↑ http://www.srh.noaa.gov/images/epz/wxcalc/rhTdFromWetBulb.pdf Relative Humidity and Dewpoint Temperature from Temperature and Wet-Bulb Temperature
  5. ↑ "The computation of equivalent potential temperature", Monthly Weather Review, vol.108, pg.1047, Eq.10
  6. ↑ SHTxx Application Note Dew-point Calculation
  7. ↑ MET4 AND MET4A CALCULATION OF DEW POINT
  8. ↑ Buck, A. L. (1981), "New equations for computing vapor pressure and enhancement factor", J. Appl. Meteorol. 20: 1527–1532
  9. ↑ 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, 225–233, 2005
  10. ↑ Template:Cite web

relative humidity

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