Atmospheric pressure 47484 225987010 2008-07-16T09:51:29Z 194.201.81.253 /* External links */ I deleted the reference {{Commons|Atmospheric pressure}}, as when I clicked on it WikiCommons threw up a page stating that there were no resources under that name. '''Atmospheric pressure''' is the [[pressure]] at any given point in the [[Earth's atmosphere]]. In most circumstances atmospheric pressure is closely approximated by the [[Fluid pressure|hydrostatic pressure]] caused by the [[weight]] of [[Earth's atmosphere|air]] above the measurement point. Low pressure areas have less atmospheric mass above their location, whereas high pressure areas have more atmospheric mass above their location. Similarly, as elevation increases there is less overlying atmospheric mass, so that pressure decreases with increasing elevation. A column of air 1 square inch in cross section, measured from [[sea level]] to the top of the atmosphere, would weigh approximately 14.7 [[lbf]]. A {{convert|1|m2|sqft|0|abbr=on}} column of air would weigh about 100 [[kilonewton]]s (equivalent to a mass of 10.2 [[tonne]]s at the surface). == Standard atmospheric pressure == The ''[[Atmosphere (unit)|standard atmosphere]]'' (symbol: atm) is a [[Units of pressure|unit of pressure]] and is defined as being precisely equal to 101.325 [[Pascal (unit)|kPa]]. The following non-standard units are equivalent: 760 [[mmHg]] ([[torr]]), 29.92 [[inHg]] or 14.696 [[Pounds per square inch|PSI]]. 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 physical 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. However, this standard atmosphere is defined slightly differently: temperature = 20 °C (68 °F), air density = 1.225 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 = 0 °C (32 °F) instead. For natural gas, the petroleum industry uses a standard temperature of 15.6 °C (60.08 °F), pressure 101.56 kPa (14.73 psi). == Mean sea level pressure == [[Image:Mslp-jja-djf.png|thumb|right|15 year average MSLP for JJA (top) and DJF (bottom)<br>JJA: June July August<br>DJF: December January February]] Mean sea level pressure (MSLP or '''QFF''') 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. 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. See [[altimeter]]. *'''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 [[pascal (unit)|kPa]] (1013.25 mbar) 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 1032.0 mbar. The lowest measurable ''sea-level pressure'' is found at the centers of hurricanes (typhoons, baguios) == Altitude atmospheric pressure variation == [[Image:Air pressure crushing a plastic bottle p1180559.jpg|thumb|150px|This plastic bottle was closed at approximately 2,000 m altitude, then brought back to sea level. It was crushed by air pressure.]] Pressure varies smoothly from the earth's surface to the top of the [[mesosphere]]. Although the pressure changes with the weather, NASA has averaged the conditions for all parts of the earth year-round. The following is a list of air pressures (as a fraction of one atmosphere) with the corresponding average altitudes. The table gives a rough idea of air pressure at various altitudes. {| class="wikitable" |- ! rowspan="2"|fraction of 1 atm || colspan="2"|average altitude |- ! (m) !! (ft) |----- | 1 || align="right" | 0|| align="right" | 0 |----- | 1/2 || align="right"| 5,486 ||align="right" | 18,000 |----- | 1/3 || align="right" | 8,376|| align="right" | 27,480 |----- | 1/10 || align="right" | 16,132|| align="right" | 52,926 |----- | 1/100 || align="right" | 30,901|| align="right" | 101,381 |----- | 1/1000 || align="right" | 48,467|| align="right" | 159,013 |----- | 1/10000 || align="right" | 69,464|| align="right" | 227,899 |----- | 1/100000 || align="right" | 96,282|| align="right" | 283,076 |} == Calculating variation with altitude == {{seealso|Barometric formula}} There are two different equations for computing the average pressure at various height regimes below 86 km (or 278,400 ft). Equation 1 is used when the value of standard temperature lapse rate is not equal to zero and equation 2 is used when standard temperature lapse rate equals zero. Equation 1: :<math>{P}=P_b \cdot \left[\frac{T_b}{T_b + L_b\cdot(h-h_b)}\right]^{\textstyle \frac{g_0 \cdot M}{R^* \cdot L_b}}</math> Equation 2: :<math>{P}=P_b \cdot \exp \left[\frac{-g_0 \cdot M \cdot (h-h_b)}{R^* \cdot T_b}\right]</math> where :<math>P</math> = Static pressure (pascals) :<math>T</math> = Standard temperature ([[kelvin]]s) :<math>L</math> = Standard temperature lapse rate (kelvins per m) :<math>h</math> = Height above sea level (meters) :<math>R^*</math> = [[Universal gas constant]]: 8.31432×10³ N&middot;m / (kmol&middot;K) :<math>g_0</math> = [[Standard gravity]] (9.80665 m/s²) :<math>M</math> = Molar mass of Earth's air (28.9644 g/mol) Or converted to English units:<ref name=conversion>Mechtly, E. A., 1973: ''The International System of Units, Physical Constants and Conversion Factors''. NASA SP-7012, Second Revision, National Aeronautics and Space Administration, Washington, D.C.</ref> where :<math>P</math> = Static pressure (inches of mercury) :<math>T</math> = Standard temperature ([[kelvin]]s) :<math>L</math> = Standard temperature lapse rate (kelvins per ft) :<math>h</math> = Height above sea level (feet) :<math>R^*</math> = [[Universal gas constant]] (using feet and kelvins and gram [[mole (unit)|moles]]: 8.9494596×10<sup>4</sup> kg&middot;sq&nbsp;ft&middot;s<sup>-2</sup>&middot;K<sup>-1</sup>&middot;kmol<sup>-1</sup>) :<math>g_0</math> = [[Standard gravity]] (32.17405 ft/s²) :<math>M</math> = Molar mass of Earth's air (28.9644 g/mol) The value of subscript ''b'' ranges from 0 to 6 in accordance with each of seven successive layers of the atmosphere shown in the table below. In these equations, ''g<sub>0</sub>'', ''M'' and ''R''<sup>*</sup> are each single-valued constants, while ''P,'' ''L,'' ''T,'' and ''h'' are multivalued constants in accordance with the table below. (Note that according to the convention in this equation, ''L<sub>0</sub>'', the tropospheric lapse rate, is negative.) It should be noted that the values used for ''M,'' ''g<sub>0</sub>'', and <math>R^*</math> are in accordance with the [[U.S. Standard Atmosphere]], 1976, and that the value for <math>R^*</math> in particular does not agree with standard values for this constant.<ref name=USSA1976>[http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770009539_1977009539.pdf U.S. Standard Atmosphere], 1976, U.S. Government Printing Office, Washington, D.C., 1976. (Linked file is very large.) </ref> The reference value for ''P<sub>b</sub>'' for ''b'' = 0 is the defined sea level value, ''P<sub>0</sub>'' = 101325 [[pascal (unit)|pascals]] or 29.92126 [[inHg]]. Values of ''P<sub>b</sub>'' of ''b'' = 1 through ''b'' = 6 are obtained from the application of the appropriate member of the pair equations 1 and 2 for the case when <math>h = h_{b+1}</math>.:<ref name=USSA1976/> {| class="wikitable" |- ! rowspan="2"|Subscript ''b'' ! colspan="2"|Height Above Sea Level ! colspan="2"|Static Pressure ! rowspan="2"|Standard Temperature<br> (K) ! colspan="2"|Temperature Lapse Rate |- ! (m) !! (ft)!! (pascals) !! (inHg) !! (K/m) !! (K/ft) |- | align="center" |0 | align="center" |0 | align="center" |0 | align="right" |101325 | align="center" |29.92126 | align="center" |288.15 | align="center" |-0.0065 | align="center" |-0.0019812 |- | align="center" |1 | align="center" |11,000 | align="center" |36,089 | align="right" |22632 | align="center" |6.683245 | align="center" |216.65 | align="center" |0.0 | align="center" |0.0 |- | align="center" |2 | align="center" |20,000 | align="center" |65,617 | align="right" |5474 | align="center" |1.616734 | align="center" |216.65 | align="center" |0.001 | align="center" |0.0003048 |- | align="center" |3 | align="center" |32,000 | align="center" |104,987 | align="right" |868 | align="center" |0.2563258 | align="center" |228.65 | align="center" |0.0028 | align="center" |0.00085344 |- | align="center" |4 | align="center" |47,000 | align="center" |154,199 | align="right" |110 | align="center" |0.0327506 | align="center" |270.65 | align="center" |0.0 | align="center" |0.0 |- | align="center" |5 | align="center" |51,000 | align="center" |167,323 | align="right" |66 | align="center" |0.01976704 | align="center" |270.65 | align="center" |-0.0028 | align="center" |-0.00085344 |- | align="center" |6 | align="center" |71,000 | align="center" |232,940 | align="right" |4 | align="center" |0.00116833 | align="center" |214.65 | align="center" |-0.002 | align="center" |-0.0006097 |} == Local atmospheric pressure variation == [[Image:Wilma1315z-051019-1kg12.jpg||thumb|[[Hurricane Wilma]] on [[19 October]] [[2005]] &ndash; 88.2 kPa in eye]] Atmospheric pressure varies widely on [[Earth]], and these changes are important in studying [[weather]] and [[climate]]. See [[pressure system]] for the effects of air pressure variations on weather. Atmospheric pressure shows a diurnal (twice-daily) cycle caused by global atmospheric tides. This effect is strongest in tropical zones, with amplitude of a few millibars, and almost zero in polar areas. These variations have two superimposed cycles, a circadian (24 h) cycle and semi-circadian (12 h) cycle. == Atmospheric pressure based on height of water == Atmospheric pressure is often measured with a mercury [[barometer]], and a height of approximately 760 mm (30 [[inch]]es) of mercury is often used to teach, make visible, and illustrate (and measure) atmospheric pressure. However, since mercury is not a substance that humans commonly come in contact with, [[water]] often provides a more intuitive way to conceptualize the amount of pressure in one atmosphere. One atmosphere (101.325 kPa or 14.7 lbf/sq&nbsp;in) is the amount of pressure that can lift water approximately 10.3 m (33.9 ft). Thus, a diver at a depth 10.3 meters under water in a fresh-water lake experiences a pressure of about 2 atmospheres (1 atm for the air and 1 atm for the water). This is also the maximum height to which a column of water can be drawn up by [[suction]]. Low pressures such as [[natural gas]] lines are sometimes specified in inches of water, typically written as w.c. ([[water column]]). A typical gas using residential appliance is rated for a maximum of 14 w.c. which is approximately 0.5 PSI. Non-professional [[barometer]]s are generally aneroid barometer (Figure 3) or [[strain gauge]] based. See [[Pressure measurement]] for a description of barometers. == Atmospheric pressure's relation to water's boiling point == Although water is generally considered to boil at 100° C (212° F), water actually boils when the vapor pressure is equal to the atmospheric pressure around the water. <ref>[http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/vappre.html Vapor Pressure<!-- Bot generated title -->]</ref> Because of this, the boiling point of water is decreased in lower pressure and raised at higher pressure. This is why baking cookies at altitudes beyond 3,500 feet above [[sea level]] requires special baking directions. <ref>[http://www.crisco.com/Articles_Tips/Cooking_Tips/Prep_High_Alt.aspx Crisco® - Articles & Tips - Cooking Tips - High Altitude Cooking<!-- Bot generated title -->]</ref> == See also == * [[Plenum]] * [[NRLMSISE-00]] * [[Barometric formula]] * [[International Standard Atmosphere]] - a tabulation of typical variation of principal thermodynamic variables of the atmosphere (pressure, density, temperature etc.) with altitude, at mid latitudes. == References == <references/> :* US Department of Defense Military Standard 810E :* Burt, Christopher C., (2004). ''Extreme Weather, A Guide & Record Book''. W. W. Norton & Company ISBN 0-393-32658-6 :* ''U.S. Standard Atmosphere, 1962'', U.S. Government Printing Office, Washington, D.C., 1962. == External links == * [[NASA]] [http://modelweb.gsfc.nasa.gov/atmos/us_standard.html page on the 1976 Standard Atmosphere] * [http://www.pdas.com/atmos.htm Source code and equations for the 1976 Standard Atmosphere] * [http://www.atmosculator.com/The%20Standard%20Atmosphere.html? A mathematical model of the 1976 U.S. Standard Atmosphere] * [http://www.luizmonteiro.com/StdAtm.aspx Calculator using multiple units and properties for the 1976 Standard Atmosphere] ===Experiments=== * [http://avc.comm.nsdlib.org/cgi-bin/wiki_print.pl? An exercise in air pressure] * [http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/patm.html#atm Movies on atmospheric pressure experiments from] [[Georgia State University|Georgia State University's]] [[Quicktime|HyperPhysics website - requires QuickTime]] {{Meteorological variables}} [[nan:Khì-ap]] [[Category:Atmosphere]] [[Category:Atmospheric thermodynamics]] [[Category:Pressure]] [[Category:Underwater diving]] [[af:Lugdruk]] [[ar:ضغط جوي]] [[zh-min-nan:Khì-ap]] [[be-x-old:Атмасфэрны ціск]] [[bg:Атмосферно налягане]] [[ca:Pressió atmosfèrica]] [[cs:Atmosférický tlak]] [[de:Luftdruck]] [[et:Õhurõhk]] [[el:Ατμοσφαιρική πίεση]] [[es:Presión atmosférica]] [[eu:Presio atmosferiko]] [[fr:Pression atmosphérique]] [[ko:대기압]] [[hr:Atmosferski tlak]] [[id:Tekanan atmosfer]] [[it:Pressione atmosferica]] [[he:לחץ אטמוספירי]] [[ka:ატმოსფერული წნევა]] [[lo:ຄວາມກົດອາກາດ]] [[lt:Atmosferos slėgis]] [[hu:Légnyomás]] [[nl:Luchtdruk]] [[ja:気圧]] [[no:Atmosfærisk trykk]] [[nn:Lufttrykk]] [[pl:Ciśnienie atmosferyczne]] [[pt:Pressão atmosférica]] [[ro:Presiune atmosferică]] [[ru:Атмосферное давление]] [[simple:Atmospheric pressure]] [[sk:Atmosférický tlak]] [[sl:Zračni tlak]] [[sr:Атмосферски притисак]] [[fi:Ilmanpaine]] [[sv:Lufttryck]] [[th:ความกดอากาศ]] [[uk:Атмосферний тиск]] [[ur:ہوا کا دباؤ]] [[zh:气压]]