Editing Dew point

{{Short description|Temperature below which condensation occurs}}
{{About|the meteorological dew point|the petroleum term|Hydrocarbon dew point}}
{{humidity}}
The '''dew point''' is the temperature the air needs to be cooled to (at constant pressure) in order to produce a [[relative humidity]] of 100%.<ref>{{Cite web |last=US Department of Commerce |first=NOAA |title=Dew Point vs Humidity |url=https://www.weather.gov/arx/why_dewpoint_vs_humidity |access-date=2025-02-12 |website=www.weather.gov |language=EN-US}}</ref> This temperature depends on the pressure and water content of the air. When the air at a temperature above the dewpoint is cooled, its [[moisture]] capacity is reduced and airborne water vapor will [[Condensation|condense]] to form liquid water known as [[dew]].<ref>{{cite web|url = https://thecraftsmanblog.com/how-to-eliminate-window-condensation/|title = How To: Eliminate Window Condensation| date=15 November 2021 }}</ref> When this occurs through the air's contact with a colder surface, dew will form on that surface.<ref>{{cite web|url = https://forecast.weather.gov/glossary.php?word=dew+point|work = Glossary – NOAA's National Weather Service|title = Dew Point|date = 25 June 2009}}</ref> 

The dew point is affected by the air's [[humidity]]. The more moisture the air contains, the higher its dew point.<ref name="WallaceHobbs2006">{{cite book|author1=John M. Wallace|url=https://books.google.com/books?id=HZ2wNtDOU0oC&pg=PA83|title=Atmospheric Science: An Introductory Survey|author2=Peter V. Hobbs|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 via [[deposition (phase transition)|deposition]] rather than condensation.<ref>{{cite web |url = https://forecast.weather.gov/glossary.php?word=dew+point |work =Glossary – NOAA's National Weather Service |title = Frost Point |date = 25 June 2009}}</ref>
In liquids, the analog to the dew point is the [[cloud point]].

==Humidity==
If all the other factors influencing humidity remain constant, at ground level the [[relative humidity]] rises as the temperature falls; this is because the air's capacity to hold water vapor decreases, and less vapor is needed to saturate the air. The dew point temperature equals the air temperature when the air is saturated with water; in all other cases the dew point will be less than the air temperature.<ref>{{Cite book |last=Spiridonov |first=Vlado |url=https://link.springer.com/10.1007/978-3-030-52655-9 |title=Fundamentals of Meteorology |last2=Ćurić |first2=Mladjen |date=2021 |publisher=Springer International Publishing |isbn=978-3-030-52654-2 |location=Cham |language=en |doi=10.1007/978-3-030-52655-9}}</ref>{{rp|129}} 

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>{{Cite Merriam-Webster|dew point}}</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, and no dew or fog forms, the vapor is called [[supersaturated]]. This can happen if there are not enough particles in the air to act as [[condensation nuclei]].<ref name=ctrib>{{cite news |last1= Skilling |first1=Tom|title=Ask Tom why: Is it possible for relative humidity to exceed 100 percent?|url= https://www.chicagotribune.com/2011/07/20/ask-tom-why-is-it-possible-for-relative-humidity-to-exceed-100-percent/ |access-date=24 January 2018|work=Chicago Tribune|date=20 July 2011}}</ref>

The dew point depends on how much water vapor the air contains. If the air is very dry and has few water molecules, the dew point is low and surfaces must be much cooler than the air for condensation to occur. If the air is very humid and contains many water molecules, the dew point is high and condensation can occur on surfaces that are only a few degrees cooler than the air.<ref>{{Cite book |title=Moisture Control Guidance for Building Design, Construction and Maintenance |publisher=U.S. Environmental Protection Agency }}</ref>

A high relative humidity implies that the dew point is close 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 | date=2006-08-15 | url=http://www.shorstmeyer.com/wxfaqs/humidity/humidity.html| access-date=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]].

Increasing the barometric pressure raises 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|access-date=15 February 2018|archive-date=16 February 2018|archive-url=https://web.archive.org/web/20180216030042/https://www.vaisala.com/sites/default/files/documents/Dew-point-compressed-air-Application-note-B210991EN-B-LOW-v1.pdf|url-status=dead}}</ref> This means that, if the pressure increases, the mass of water vapor per volume unit of air must be reduced in order to maintain the same dew point. For example, consider New York City ({{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 |access-date=March 19, 2007 |url-status=dead |archive-url=https://web.archive.org/web/20070203182736/http://www.denvergov.org/AboutDenver/today_factsguide.asp |archive-date=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==
When the air temperature is high, the human body uses the 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 (humid), 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 at which 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, thereby increasing comfort. By contrast, comfort decreases as unevaporated perspiration increases.

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|-5|°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,<ref>{{Cite web|url=https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=interpretations&p_id=24602|title=02/24/2003 - Reiteration of Existing OSHA Policy on Indoor Air Quality: Office Temperature/Humidity and Environmental Tobacco Smoke. |website=Occupational Safety and Health Administration |access-date=2020-01-20}}</ref> equivalent to a dew point of approximately {{convert|4.0|to|16.5|C|F|0}} (by Simple Rule calculation below).

Lower dew points, less than {{convert|10|°C|°F}}, correlate with lower ambient temperatures and cause the body to require 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|doi=10.1016/j.buildenv.2009.02.004|bibcode=2009BuEnv..44.2017L }}</ref>

==Altitude and clouds==
A rising air mass in the atmosphere will [[adiabatic cooling|cool]] as the pressure reduces: its temperature will move closer to the dew point temperature. The relative humidity of this air increases as the pressure drops with the same amount of water vapor. This combination causes the dew point temperature to fall 4.5 degrees F per 1000 ft or 0.2C per 100m. At the elevation where the dew point is reached condensation begins, creating clouds, or fog as it is called a low altitudes. [[Cumulus cloud]]s tend to have flat bottoms marking the altitude where the temperature crosses the dew point. The condensation of water vapor into water droplets releases 600 calories of energy per gram of water, heating the surrounding air.<ref name=Strahler-1963>Strahler, A. N. (1963). [https://archive.org/details/earthsciences0002arth/page/292/mode/2up?q=%22dew+point%22 The Earth Sciences]. Japan: Harper & Row.</ref>{{rp|292}}

==Dew point weather records==
* '''Highest dew point temperature:''' A dew point of {{convert|35|°C|F}} — while the temperature was {{convert|42|°C|F}} — was observed at [[Dhahran]], Saudi Arabia, at 3:00&nbsp;p.m. on 8 July 2003.<ref>{{cite news|url=https://www.washingtonpost.com/news/capital-weather-gang/wp/2017/06/29/iran-city-soars-to-record-of-129-degrees-near-hottest-ever-reliably-measured-on-earth/|title=Iranian city soars to record 129 degrees: Near hottest on Earth in modern measurements|newspaper=Washington Post|access-date=3 July 2017|archive-url=https://web.archive.org/web/20170702063507/https://www.washingtonpost.com/news/capital-weather-gang/wp/2017/06/29/iran-city-soars-to-record-of-129-degrees-near-hottest-ever-reliably-measured-on-earth/|archive-date=2 July 2017|url-status=live}}</ref><ref>{{Cite web |date=4 August 2015 |title=Iran city hits suffocating heat index of 165 degrees, near world record |url=https://kleanindustries.com/resources/market-analysis-research/iran-city-hits-suffocating-heat-index-of-165-degrees-near-world-record/ |access-date=2023-12-28 |website=Klean Industries |language=en}}</ref>

==Measurement==
Devices called [[hygrometer]]s 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 dew point is revealed by observing the loss of clarity in the reflection cast by the mirror. 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.

{| class="wikitable" style="text-align:center;"
|-
! colspan="2"|Dew point
! Relative humidity at {{convert|32|°C|°F|abbr=on}}
|-
| Over 27&nbsp;°C
| Over 80&nbsp;°F
| 73% and higher
|-
| 24–26&nbsp;°C
| 75–79&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
|}

==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 empirical approximation used to calculate the dew point, ''T''<sub>d</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{d}&= \frac{c\gamma(T,\mathrm{RH})}{b-\gamma(T,\mathrm{RH})};\end{align}
</math>
where ''b'' = 17.625 and ''c'' = 243.04&nbsp;°C.<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=1 February 2005 |volume=86 |issue=2 |pages=225–234 |doi=10.1175/BAMS-86-2-225 |bibcode=2005BAMS...86..225L |access-date=15 March 2024 |url=https://journals.ametsoc.org/view/journals/bams/86/2/bams-86-2-225.xml?tab_body=pdf}}</ref> The values of ''b'' and ''c'' were selected by minimizing the maximum deviation over the range −40&nbsp;°C to +50&nbsp;°C.

The more complete formulation and origin of this approximation involves the interrelated [[Saturated fluid|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{d} & = \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_{d} & = \frac{c\ln\frac{P_\mathrm{a}(T)}{a}}{b-\ln\frac{P_\mathrm{a}(T)}{a}} = \frac{c\ln\left(\frac\mathrm{RH}{100}\frac{P_\mathrm{s,m}(T)}{a}\right)}{b-\ln\left(\frac\mathrm{RH}{100}\frac{P_\mathrm{s,m}(T)}{a}\right)} = \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>[https://www.weather.gov/media/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|access-date=2012-07-04|archive-url=https://web.archive.org/web/20120915053830/http://www.rsmas.miami.edu/users/pzuidema/Bolton.pdf|archive-date=2012-09-15|url-status=dead}}</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''.<ref>{{cite web |url=http://www.paroscientific.com/dewpoint.htm |title=MET4 and MET4A Calculation of Dew Point |access-date=7 October 2014 |url-status=dead |archive-url=https://archive.today/20120526034637/http://www.paroscientific.com/dewpoint.htm |archive-date=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|access-date=2016-01-15|archive-date=2016-03-04|archive-url=https://web.archive.org/web/20160304093405/http://www.public.iastate.edu/~bkh/teaching/505/arden_buck_sat.pdf|url-status=dead}}</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{d} &\approx T-\frac{100-\mathrm{RH}}{5}; \\[5pt]
\mathrm{RH} &\approx 100-5(T-T_\mathrm{d});
\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, can be found in an article published in the ''[[Bulletin of the American Meteorological Society]]''.<ref>{{cite journal|author1-link=Mark G. Lawrence|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|doi-access=free}}</ref> 

For temperatures in degrees Fahrenheit, these approximations work out to
<math display="block">\begin{align}
T_\mathrm{d,^\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{d,^\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 breaking the stronger bonding between water molecules on the surface of ice compared to the surface of ([[supercooled]]) liquid water requires a higher temperature.<ref>{{cite web |url=http://www.theweatherprediction.com/habyhints/347/ |title=Frost point and dew point |last=Haby |first=Jeff |access-date=September 30, 2011}}</ref>

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

==References==
{{Reflist|}}

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

{{Meteorological variables}}
{{Authority control}}

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

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