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{{main|Earth's atmosphere}}
{{main|Earth's atmosphere|Pressure}}
'''Atmospheric pressure''' at a 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 changes with the seasons.


'''Atmospheric pressure''' at any given point in Earth's atmosphere is defined as the force per unit area exerted upon a surface at that point by the weight of air above that surface. Atmospheric pressure at [[sea level]] will vary with geographic location as well as with weather conditions.
== Standard values of atmospheric pressure at sea level ==


Similarly, as [[elevation]] increases there is less overlying atmospheric mass, so that pressure decreases with increasing elevation.
{{main|Reference conditions of gas temperature and pressure}}
 
The [[Atmosphere (unit)|standard atmosphere]] (symbol: atm) is a [[Units of pressure|unit of pressure]] and is defined as being equal to 101 325 Pa or 101.325 [[Pascal (unit)|kPa]]. <ref>International Civil Aviation Organization, Manual of the ICAO Standard Atmosphere, Doc 7488-CD, Third Edition, 1993, ISBN 92-9194-004-6. </ref>  The following units are equivalent, but only to the number of decimal places displayed: 760 [[mmHg]] ([[torr]]), 29.92 [[inHg]], 14.696 [[Pounds per square inch|PSI]], 1013.25 [[Bar (unit)|millibars]].  One standard atmosphere is standard pressure used for pneumatic fluid power (ISO R554), and in the aerospace (ISO 2533) and petroleum (ISO 5024) industries.  
== Standard atmospheric pressure ==
 
 
The [[Atmosphere (unit)|standard atmosphere]] (symbol: atm) is a [[Units of pressure|unit of pressure]] and is defined as being equal to 101 325 Pa or 101.325 [[Pascal (unit)|kPa]]. <ref>International Civil Aviation Organization, Manual of the ICAO Standard Atmosphere, Doc 7488-CD, Third Edition, 1993, ISBN 92-9194-004-6. </ref><ref>OPCIT http://en.wikipedia.org/wiki/ICAO_Standard_Atmosphere </ref>  The following units are equivalent, but only to the number of decimal places displayed: 760 [[mmHg]] ([[torr]]), 29.92 [[inHg]], 14.696 [[Pounds per square inch|PSI]], 1013.25 [[Bar (unit)|millibars]].  One standard atmosphere is standard pressure used for pneumatic fluid power (ISO R554), and in the aerospace (ISO 2533) and petroleum (ISO 5024) industries.  


In 1999,  the [[International Union of Pure and Applied Chemistry]] (IUPAC) recommended that for the purposes of specifying the properties of substances, “''the standard pressure''” should be defined as precisely 100&nbsp;kPa (≈750.01&nbsp;[[torr]]) or 29.53 [[inHg]] rather than the 101.325&nbsp;kPa value of “one standard atmosphere”.<ref>IUPAC.org, Publications, ''Standard Pressure'' (''[http://www.iupac.org/goldbook/S05921.pdf 20&nbsp;kB PDF]'')</ref> This value is used as the standard pressure for the compressor and the pneumatic tool industries (ISO 2787).<ref>Compressor.co.za, ''[http://www.compressor.co.za/news/may%20news.htm May 2003 Newsletter]''</ref> (See also [[Standard temperature and pressure]].) In the [[United States]], compressed air flow is often measured in "standard cubic feet" per unit of time, where the "standard" means the equivalent quantity of moisture at standard temperature and pressure. For every 1,000 feet you ascend the atmospheric pressure decreases 4%.  However, this standard atmosphere is defined slightly differently: temperature = {{convert|20|C}}, air density = 1.225&nbsp;kg/m³ (0.0765 lb/cu&nbsp;ft), altitude = sea level, and relative humidity = 20%. In the air conditioning industry, the standard is often temperature = {{convert|0|C}} instead. For natural gas, the petroleum industry uses a standard temperature of {{convert|15.6|C}}, pressure {{convert|101.56|kPa|psi|abbr=on}}. (air pressure)
In 1999,  the [[International Union of Pure and Applied Chemistry]] (IUPAC) recommended that for the purposes of specifying the properties of substances, “''the standard pressure''” should be defined as precisely 100&nbsp;kPa (≈750.01&nbsp;[[torr]]) or 29.53 [[inHg]] rather than the 101.325&nbsp;kPa value of “one standard atmosphere”.<ref>IUPAC.org, Publications, ''Standard Pressure'' (''[http://www.iupac.org/goldbook/S05921.pdf 20&nbsp;kB PDF]'')</ref> This value is used as the standard pressure for the compressor and the pneumatic tool industries (ISO 2787).<ref>Compressor.co.za, ''[http://www.compressor.co.za/news/may%20news.htm May 2003 Newsletter]''</ref> (See also [[Standard temperature and pressure]].) In the [[United States]], compressed air flow is often measured in "standard cubic feet" per unit of time, where the "standard" means the equivalent quantity of moisture at standard temperature and pressure. For every 1,000 feet you ascend the atmospheric pressure decreases 4%.  However, this standard atmosphere is defined slightly differently: temperature = {{convert|20|C}}, air density = 1.225&nbsp;kg/m³ (0.0765 lb/cu&nbsp;ft), altitude = sea level, and relative humidity = 20%. In the air conditioning industry, the standard is often temperature = {{convert|0|C}} instead. For natural gas, the petroleum industry uses a standard temperature of {{convert|15.6|C}}, pressure {{convert|101.56|kPa|psi|abbr=on}}. (air pressure)
== Mean sea level pressure ==
[[File:Mslp-jja-djf.png|thumb|right|15 year average mean sea level pressure for June, July, and August (top) and December, January, and February (bottom).]]
Mean sea level pressure (MSLP) is the pressure at sea level or (when measured at a given elevation on land) the station pressure reduced to sea level assuming an isothermal layer at the station temperature.
This is the pressure normally given in weather reports on radio, television, and newspapers or on the Internet. When barometers in the home are set to match the local weather reports, they measure pressure reduced to sea level, not the actual local atmospheric pressure. See [[altimeter#BarometerVsAbsolute|Altimeter (barometer vs. absolute)]].
The reduction to sea level means that the ''normal range of fluctuations'' in pressure is the same for everyone. The pressures which are considered ''high pressure'' or ''low pressure'' do not depend on geographical location. This makes [[isobar]]s on a weather map meaningful and useful tools.
[[File:Aircraft altimeter.JPG|right|thumb|Kollsman-type barometric aircraft [[altimeter]] as used in North America displaying an [[altitude]] of {{convert|80|ft|abbr=on}}.]]
The ''[[altimeter]] setting'' in aviation, set either [[QNH]] or QFE, is another atmospheric pressure reduced to sea level, but the method of making this reduction differs slightly.
;QNH: The barometric altimeter setting which will cause the altimeter to read airfield elevation when on the airfield. In ISA temperature conditions the altimeter will read altitude above mean sea level in the vicinity of the airfield
;QFE: The barometric altimeter setting which will cause an altimeter to read zero when at the reference datum of a particular airfield (generally a runway threshold). In ISA temperature conditions the altimeter will read height above the datum in the vicinity of the airfield.
QFE and QNH are arbitrary [[Q codes]] rather than abbreviations, but the [[mnemonic]]s "Nautical Height" (for QNH) and "Field Elevation" (for QFE) are often used by pilots to distinguish them.
Average ''sea-level pressure'' is '''101.325 kPa''' (1013.25 mbar, or hPa) or '''29.921 inches''' of mercury (inHg) or '''760 millimeters (mmHg)'''. In aviation weather reports ([[METAR]]), QNH is transmitted around the world in millibars or hectopascals (1 millibar = 1 hectopascal), except in the United States and in Canada where it is reported in inches (or hundredths of inches) of mercury. (The United States and Canada also report ''sea level pressure'' SLP, which is reduced to sea level by a different method, in the remarks section, not an internationally transmitted part of the code, in hectopascals or millibars<ref>[http://www.flightplanning.navcanada.ca/cgi-bin/Fore-obs/metar.cgi?NoSession=NS_Inconnu&format=dcd&Langue=anglais&Region=can&Stations=CYVR&Location= Sample METAR of CYVR] Nav Canada</ref>.  However, in Canada's public weather reports, sea level pressure is instead reported in kilopascals [http://www.cbc.ca/weather/conditions.jsp?station=YUL], while [[Environment Canada]]'s standard unit of pressure is the same [http://weatheroffice.ec.gc.ca/trends_table/pages/yul_metric_e.html] [http://weatheroffice.ec.gc.ca/forecast/trends_graph_e.html?yvr&unit=m].)  In the weather code, three digits are all that is needed; decimal points and the one or two most significant digits are omitted: 1013.2 mbar or 101.32 kPa is transmitted as 132; 1000.0 mbar or 100.00 kPa is transmitted as 000; 998.7 mbar or 99.87 kPa is transmitted as 987; etc. The highest ''sea-level pressure'' on Earth occurs in [[Siberia]], where the [[Siberian High]] often attains a ''sea-level pressure'' above 1087.0 mbar. The lowest measurable ''sea-level pressure'' is found at the centers of [[tropical cyclone]]s.

Revision as of 18:25, 19 October 2009

For more information, see: Earth's atmosphere and Pressure.

Atmospheric pressure at a 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 changes with the seasons.

Standard values of atmospheric pressure at sea level

For more information, see: Reference conditions of gas temperature and pressure.

The standard atmosphere (symbol: atm) is a unit of pressure and is defined as being equal to 101 325 Pa or 101.325 kPa. [1] The following units are equivalent, but only to the number of decimal places displayed: 760 mmHg (torr), 29.92 inHg, 14.696 PSI, 1013.25 millibars. One standard atmosphere is standard pressure used for pneumatic fluid power (ISO R554), and in the aerospace (ISO 2533) and petroleum (ISO 5024) industries.

In 1999, the International Union of Pure and Applied Chemistry (IUPAC) recommended that for the purposes of specifying the properties of substances, “the standard pressure” should be defined as precisely 100 kPa (≈750.01 torr) or 29.53 inHg rather than the 101.325 kPa value of “one standard atmosphere”.[2] This value is used as the standard pressure for the compressor and the pneumatic tool industries (ISO 2787).[3] (See also Standard temperature and pressure.) In the United States, compressed air flow is often measured in "standard cubic feet" per unit of time, where the "standard" means the equivalent quantity of moisture at standard temperature and pressure. For every 1,000 feet you ascend the atmospheric pressure decreases 4%. However, this standard atmosphere is defined slightly differently: temperature = 20 C (Expression error: Missing operand for round. {{{3}}}), air density = 1.225 kg/m³ (0.0765 lb/cu ft), altitude = sea level, and relative humidity = 20%. In the air conditioning industry, the standard is often temperature = 0 C (Expression error: Missing operand for round. {{{3}}}) instead. For natural gas, the petroleum industry uses a standard temperature of 15.6 C (Expression error: Missing operand for round. {{{3}}}), pressure 101.56 kPa (Expression error: Missing operand for round. psi). (air pressure)

  1. ↑ International Civil Aviation Organization, Manual of the ICAO Standard Atmosphere, Doc 7488-CD, Third Edition, 1993, ISBN 92-9194-004-6.
  2. ↑ IUPAC.org, Publications, Standard Pressure (20 kB PDF)
  3. ↑ Compressor.co.za, May 2003 Newsletter
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