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{{main|Earth's atmosphere|Pressure}}
'''Atmospheric pressure''' at any given point in Earth's atmosphere is the downward [[force]] per unit [[area]]  exerted upon a horizontal surface at that point by the weight of air above that surface.


Atmospheric [[pressure]] at [[sea level]] will vary with geographic location, the temperature and humidity of the air and with the weather conditions. In fact, a change in the sea level atmospheric pressure usually indicates an upcoming change in the weather. Since air [[temperature]] and [[humidity]] as well as the weather change with the annual seasons (i.e., winter, spring, summer and fall), the sea level atmospheric pressure also changes with the seasons.
== Standard values of atmospheric pressure at sea level ==
{{main|Reference conditions of gas temperature and pressure|Atmosphere (unit)}}
In 1954, the 10th Conférence Générale des Poids et Mesures (CGPM) adopted a ''standard atmosphere'' for general use and defined it as being precisely 1,013,250 [[dyne]]s per [[centimeter]] (101,325 [[Pascal (unit)|Pa]]).<ref name=BIPM>[http://www.bipm.org/jsp/en/ViewCGPMResolution.jsp?CGPM=10&RES=4 BIPM Definition of the standard atmosphere]</ref> This value was intended to represent the average atmospheric pressure at the average  sea level at the latitude of [[Paris|Paris, France]], and as a practical matter, truly reflects the average sea level pressure for many of the industrialized nations (those with [[latitude]]s similar to Paris).
The ''International standard atmosphere'' (ISA) as used by [[International Civil Aviation Organization]] (ICAO) is also  defined as being 101,325 Pa  at sea level.<ref>International Civil Aviation Organization, Manual of the ICAO Standard Atmosphere, Doc 7488-CD, Third Edition, 1993, ISBN 92-9194-004-6</ref> The [[U.S. Standard Atmosphere]] also defines the sea level atmospheric pressure as being 101,325 Pa.<ref name=StdAtm>[http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770009539_1977009539.pdf U.S. Standard Atmosphere, 1976] Scroll to pdf page 28 of 241 pdf pages.</ref>
In [[chemistry]], the original definition of "Standard Temperature and Pressure" by the [[International Union of Pure and Applied Chemistry]] (IUPAC) was a reference temperature of 0 [[Celsius|°C]] (273.15 [[kelvin|K]]) and pressure of 101,325 Pa. However, in 1982, the IUPAC recommended that for the purposes of specifying the physical properties of substances, the "standard pressure" should be defined as 100,000 Pa (1 [[Bar (unit)|bar]]).<ref name=IUPAC>[[IUPAC]] Gold Book, [http://goldbook.iupac.org/S05921.html Standard Pressure]</ref>
However, the sea level atmospheric pressure of 101,325 Pa (as defined by the CGPM, the ICAO and also the IUPAC prior to 1982) continues to be very commonly used.
Some equivalent values of the sea level atmospheric pressure (in other units of measurement) are 760 mm-[[Mercury|Hg]] (or 760 torr), 29.92 [[U.S. customary units#Units of length|in]]-Hg, 1.01325 bar, 14.696 [[U.S. customary units#Units of pressure|psi]] and 1013.25 millibars.
As a related matter of interest, the '''''[[atmosphere (unit)|atmosphere]]''''' (symbol: atm), defined as being 101,325 Pa, is a measurement unit of pressure. The difference between an atm and a bar is about 1%, which is not significant for many applications, and is within the error range of many common pressure gauges.
== Variation of atmospheric pressure with altitude ==
{| border="0" width="435" align="right" cellpadding="0" cellspacing="0" style="wrap=no"
|
{| class = "wikitable" align="right"
|+ Base Values for the U.S. Standard Atmosphere<ref name=StdAtm/>
!Region<br/>Number!!Altitude Range<br/>(m)!!<math>P_b</math><br/>(Pa)!!<math>T_b</math><br/>(K)!!<math>L_b</math><br/>(K/m)!!<math>h_b</math><br/>(m)
|- align="center"
|1||0.00 to 10,999||101,325||288.15||-0.0065||0.00
|- align="center"
|2||11,000 to 19,999||22,632||216.65||0.00||11,000
|- align="center"
|3||20,000 to 31,999||5,474||216.65||0.001||20,000
|- align="center"
|4||32,000 to 46,999||868||228.65||0.0028||32,000
|- align="center"
|5||47,000 to 50,999||110||270.65||0.00||47,000
|- align="center"
|6||51,000 to 70,999||66||270.65||-0.0028||51,000
|- align="center"
|7||71,000 to 85,000||4||214.65||-0.002||71,000
|}
|}
As the altitude  of a point in the atmosphere is increased, the weight of the overlying atmosphere decreases and hence the atmospheric pressure decreases. More simply put, the atmospheric pressure at the top of a tall mountain is lower than the atmospheric pressure at sea level.
The U.S. Standard Atmosphere<ref name=StdAtm/> provides two [[barometric formula|equations]] for calculating the atmospheric pressure at any given altitude up to 86 kilometres (53 miles). Equation 1 is used when the [[Atmospheric lapse rate|lapse rate]]<ref>Very simply put, the lapse rate is the rate at which the atmospheric temperature changes with altitude. It is often expressed in K/m, K/km or [[Fahrenheit and Rankine temperatures|°F]]/1000 [[U.S. customary units|ft]].</ref> is not equal to zero and equation 2 is used when the lapse rate equals zero:<ref name=StdAtm/> The two equations are valid for seven different altitude regions of the Earth's atmosphere by using the designated base values (from the adjacent table) for <math>P_b</math>, <math>T_b</math>, <math>L_b</math> and <math>h_b</math> for each of the seven regions:<ref name=StdAtm/><ref>Equation 2 can be obtained from the [[Boltzmann distribution]].</ref>
:Equation 1:
:<math>P = P_b\cdot\left[\frac{T_b}{T_b + L_b\cdot(h - h_b)}\right]^{\textstyle\frac{g\cdot M}{R\cdot L_b}}</math>
:Equation 2:
::<math>P = P_b \cdot \exp \left[\frac{-\,g \cdot M \cdot (h-h_b)}{R \cdot T_b}\right]</math>
{|border="0" cellpadding="2"
|-
|align=right|where:
|-
|align=right|<math>P</math>
|align=left|= Atmospheric pressure at any given altitude <font style="vertical-align:+5%;"><math>\scriptstyle h</math></font>, [[Pascal (unit)|Pa]]
|-
|align=right|<math>P_b</math>
|align=left|= Base atmospheric pressure, Pa
|-
|align=right|<math>T_b</math>
|align=left|= Base atmospheric temperature, [[Kelvin|K]]
|-
|align=right|<math>L_b</math>
|align=left|= Base [[lapse rate]], K/[[Metre (unit)|m]]
|-
|align=right|<math>h_b</math>
|align=left|= Base height (i.e., altitude), m
|-
|align=right|<math>h</math>
|align=left|= Any given altitude, m
|-
|align=right|<math>g</math>
|align=left|= [[Gravitational acceleration]] = 9.8067 m/s<sup>2</sup>
|-
|align=right|<math>M</math>
|align=left|= [[Molecular weight]] of air = 0.028964 kg/[[Mole|mol]]
|-
|align=right|<math>R</math>
|align=left|= [[Universal gas constant]] = 8.3144 [[Joule|J]]/(K <math>\cdot</math> mol)
|}
For example, the atmospheric pressure at an altitude of 10,000 metres is obtained as 26,437 Pa by using Equation 1 and the appropriate base values for the altitude region number 1.
==References==
{{reflist}}

Revision as of 17:01, 20 October 2009

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