View source for Standard atmosphere (unit)

{{short description|Unit of pressure defined as 101325 Pa}}
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{{Infobox unit
| name         = Atmosphere
| image        = 
| caption      = 
| standard     = 
| quantity     = [[Pressure]]
| symbol       = atm
| symbol2      = 
| extralabel   = 
| extradata    = 
| units1       = SI units
| inunits1     = {{convert|1|atm|sigfig=6|disp=out}}
| units2       = US customary units
| inunits2     = {{convert|1|atm|psi|sigfig=7|disp=out}}<br/>
   {{convert|1|atm|inHg|sigfig=7|abbr=on|disp=out}}
| units3       = other metric units
| inunits3     = {{convert|1|atm|bar|sigfig=7|disp=out}}<br/> {{convert|1|atm|mmHg|disp=out|abbr=on}}

}}
[[File:Wohnzimmerbarometer.jpg | thumb | 220x124px | right | Aneroid barometer for household use from {{Circa|1925}}]]
The '''standard atmosphere''' (symbol: '''atm''') is a [[measurement unit|unit]] of [[pressure]] defined as {{val|101325}} [[pascal (unit)|Pa]]. It is sometimes used as a ''reference pressure'' or ''standard pressure''. It is approximately equal to [[Earth]]'s average [[atmospheric pressure]] at [[sea level]].<ref>{{Cite web |title=Water Pressures at Ocean Depths |url=https://www.pmel.noaa.gov/eoi/nemo1998/education/pressure.html |access-date=11 October 2022 |website=NOAA Pacific Marine Environmental Laboratory}}</ref>

==History==
The standard atmosphere was originally defined as the pressure exerted by a 760 mm column of [[Mercury (element)|mercury]] at {{Convert|0|C|F|abbr=|sp=us}} and standard gravity (''g''<sub>n</sub> = {{val|9.80665|u=m/s2}}).<ref name=Resnick>{{cite book |last1=Resnick |first1=Robert |last2=Halliday |first2=David |date=1960 |title=Physics for Students of Science and Engineering Part 1 |location=New York |publisher=Wiley |page=364 }}</ref>  It was used as a reference condition for physical and chemical properties, and the definition of the [[Celsius|centigrade]] temperature scale set 100 °C as the boiling point of water at this pressure. In 1954, the 10th [[General Conference on Weights and Measures]] (CGPM) adopted ''standard atmosphere'' for general use and affirmed its definition of being precisely equal to {{val|1013250}} [[dyne]]s per [[square centimetre]] ({{val|101325|ul=Pa}})<!-- {{convert|1,013,250|dyn/cm2|Pa|lk=on}} -->.<ref name=BIPM>{{Cite web|url=https://www.bipm.org/en/committees/cg/cgpm/10-1954/resolution-4|title=BIPM - Resolution 4 of the 10th CGPM|website=www.bipm.org}}</ref> This defined pressure in a way that is independent of the properties of any particular substance. In addition, the CGPM noted that there had been some misapprehension that the previous definition (from the 9th CGPM) "led some physicists to believe that this definition of the standard atmosphere was valid only for accurate work in [[thermometry]]."<ref name=BIPM/>

In [[chemistry]] and in various industries, the reference pressure referred to in ''[[standard temperature and pressure]]'' was commonly {{convert|101.325|kPa|atm|0|abbr=on|order=flip}} prior to 1982, but standards have since diverged; in 1982, the [[International Union of Pure and Applied Chemistry]] recommended that for the purposes of specifying the physical properties of substances, ''standard pressure'' should be precisely {{convert|100|kPa|bar|0|abbr=on|lk=on}}.<ref>IUPAC.org, Gold Book, ''[http://goldbook.iupac.org/S05921.html Standard Pressure]''</ref>

==Pressure units and equivalencies ==

{{Pressure Units}}

A pressure of 1&nbsp;atm can also be stated as:

:≈ {{val|1.033}} [[Kilogram-force|kgf]]/cm<sup>2</sup>
:≈ {{val|10.33}} [[Centimetre of water|m H<sub>2</sub>O]]<ref name="H2O">As a unit of measurement, the conventional metre of water (mH<sub>2</sub>O) is defined as an ideal column of water with density of 1000&nbsp;kg/m<sup>3</sup> under standard gravity g<sub>n</sub> of 9.80665&nbsp;m/s<sup>2</sup> i.e. 1&nbsp;m × 1000&nbsp;kg/m<sup>3</sup> × 9.80665&nbsp;m/s<sup>2</sup> = 9806.65&nbsp;Pa (though in practice the density of pure water is always less). 1&nbsp;cmH<sub>2</sub>O = 0.01&nbsp;mH<sub>2</sub>O and 1&nbsp;inH<sub>2</sub>O  = 0.0254&nbsp;mH<sub>2</sub>O. {{Cite book |title=BS 350:Part 1:1974 Conversion factors and tables, Part 1. Basis of tables. Conversion factors |publisher=British Standards Institution |year=1974 |page=49}}</ref>
:≈ {{val|760}} [[millimetre of mercury|mmHg]]<ref name="Hg">As a unit of measurement, the conventional millimetre of mercury (mmHg) is defined as an ideal column of mercury with density of {{val|13595.1|u=kg/m<sup>3</sup>}} under standard gravity g<sub>n</sub> of 9.80665&nbsp;m/s<sup>2</sup> i.e. 0.001&nbsp;m × {{val|13595.1|u=kg/m<sup>3</sup>}} × 9.80665&nbsp;m/s<sup>2</sup> ≈ 133.322&nbsp;Pa. 1&nbsp;inHg = 25.4&nbsp;mmHg. {{Cite book |title=BS 350:Part 1:1974 Conversion factors and tables, Part 1. Basis of tables. Conversion factors |publisher=British Standards Institution |year=1974 |page=49}}</ref>
:
:≈ {{val|29.92}} [[inch of mercury|inHg]]<ref name="Hg" />
:≈ {{val|406.782}} [[inch of water|in H<sub>2</sub>O]]<ref name="H2O" />
:≈ {{val|2116.22}} [[pounds per square foot|pounds-force per square foot]] (lbf/ft<sup>2</sup>)

The notation '''ata''' has been used to indicate an [[absolute pressure]] measured in either standard atmospheres (atm)<ref>{{cite web |website=Scuba Diving & Other Fun Activities |url=http://scubadoobydoo.blogspot.com.br/2008/03/difference-between-atm-ata.html |title=The Difference Between An ATM & An ATA |date=March 2, 2008}}</ref>{{better source needed|reason=blog entry posted by "Anonymous"|date=October 2024}} or [[technical atmosphere]]s (at).<ref>{{Cite book |title=BS 350:Part 1:1974 Conversion factors and tables, Part 1. Basis of tables. Conversion factors |publisher=British Standards Institution |year=1974 |page=50}}</ref>

==See also==

*[[International Standard Atmosphere]]

== References ==

{{reflist}}

[[Category:Units of pressure]]
[[Category:Atmospheric pressure]]