|
|
| Line 1: |
Line 1: |
| {{Redirect|Air pressure|the pressure of air in other systems|pressure}}
| | I'm Felix and I live with my husband and our three children in La Mesa, in the CA south part. My hobbies are Figure skating, Collecting cards and Computer programming.<br><br>Feel free to surf to my web-site - [http://www.coupongift.org/stores/wpsocial november 2014 coupon] |
| {{Continuum mechanics}}
| |
| | |
| '''Atmospheric pressure''' is the force per unit area exerted on a surface by the weight of air above that surface in the [[atmosphere of Earth]] (or that of another planet). 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. On a given plane, low-pressure areas have less atmospheric mass above their location, whereas high-pressure areas have more atmospheric mass above their location. Likewise, as [[elevation]] increases, there is less overlying atmospheric mass, so that atmospheric pressure decreases with increasing elevation. On average, a column of air one square centimeter in cross-section, measured from [[sea level]] to the top of the atmosphere, has a [[mass]] of about 1.03 kg and [[weight]] of about 10.1 N (2.28 [[Pound (force)|lb<sub>f</sub>]]) (A column one square inch in cross-section would have a weight of about 14.7 lbs, or about 65.4 N). | |
| | |
| == Standard atmospheric pressure ==
| |
| The [[Atmosphere (unit)|standard atmosphere]] (symbol: atm) is a [[Units of pressure|unit of pressure]] equal to 101325 [[Pascal (unit)|Pa]]<ref name="icao">International Civil Aviation Organization, Manual of the [[ICAO Standard Atmosphere]], Doc 7488-CD, Third Edition, 1993, ISBN 92-9194-004-6.</ref> or 1013.25 [[Bar (unit)|millibars]] or [[Pascal (unit)|hectopascals]]. It is equivalent to 760 [[mmHg]] ([[torr]]), 29.92 [[inHg]], 14.696 [[Pounds per square inch|psi]].
| |
| | |
| ===SI-system===
| |
| | |
| The Pascal unit is derived from ''Newton per square-meter''. However also Newton is derived, from ''kilogram meter per square-second''. Hence, by the use of '''pure''' [[SI units]] only, the value of standard athmosperic pressure equals 101325 kg/(s<sup>2</sup> m), "''kilogram per square-second metre''". By using the [[prefix]] for 10, ''deka'' or ''da'' instead of kilogram, we'll get 1013.25 dag/(s<sup>2</sup> m) , which equals the same value in millibar or hektopascal.
| |
| 1013.25 "''dekagram per square-second meter''". When physicians use math, then it's usually written as 101325 m <sup>-1</sup>kg s<sup>-2</sup>.
| |
| But by replacing kilogram (kg) with dekagram (dag), we see that the millibar and haktopascal units equal the same value in dekagram per square-second metre:
| |
| | |
| <math> 1013.25 dag/s</math><sup>2</sup><math>m</math> <ref>table 3 at http://physics.nist.gov/cuu/Units/units.html </ref>
| |
| Please note that the basic SI-unit for [[mass]] is the ''kilogram'', kg. However 1 kg = 10 hg = 100 dag = 1000 g = 1000000 mg etc, and for each SI-unit only one prefix can be used. In the case of the kilogram the prefix is already in use, so to speak. Hence for instance prefix ''deci'' or ''centi'' cannot be used on the kilogram like ''1 kilogram = 100 centikilogram'' insted ''gram'' is "treated" as the basic unit, and ''1 hektogram = 100 gram or 0.1 kilogram'' and ''1 dekagram = 10 gram or 0.01 kilogram''.
| |
| | |
| == 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).[[ECMWF re-analysis|ERA-15]] reanalysis.]]
| |
| [[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 mean sea level pressure (MSLP) is the atmospheric pressure at [[sea level]] or (when measured at a given elevation on land) the station pressure reduced to sea level assuming that the temperature falls at a [[lapse rate]] of 6.5 K per km in the fictive layer of air between the station and sea level.
| |
| | |
| This is the atmospheric pressure normally given in weather reports on radio, television, and newspapers or on the [[Internet]]. When [[barometer]]s 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 atmospheric pressure is the same for everyone. The pressures that are considered ''high pressure'' or ''low pressure'' do not depend on geographical location. This makes [[isobar (meteorology)|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.
| |
| ;QNH: The barometric altimeter setting that 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 that will cause an altimeter to read zero when at the reference datum of a particular airfield (in general, 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 29.92 inches (inHg) or 760 millimetres of mercury ([[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]], [[Canada]], and [[Colombia]] where it is reported in inches (to two decimal places) of [[Mercury (element)|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,<ref>[http://www.cbc.ca/weather/conditions.jsp?station=YUL ]{{dead link|date=October 2012}}</ref> while [[Environment Canada]]'s standard unit of pressure is the same.<ref>{{citation|url=http://weatheroffice.ec.gc.ca/trends_table/pages/yul_metric_e.html |title=Montréal-Trudeau Int'l Airport - Past 24 Hour Conditions |publisher=Weatheroffice.ec.gc.ca |date=2012-07-30 |accessdate=2012-10-17}}</ref><ref>{{citation|url=http://weatheroffice.ec.gc.ca/forecast/trends_graph_e.html?yvr&unit=m |title=Weather |publisher=Weatheroffice.ec.gc.ca |date=2012-07-30 |accessdate=2012-10-17}}</ref>)
| |
| | |
| 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 1050.0 mbar (105.00 kPa, 30.01 inHg), with record highs close to 1085.0 mbar (108.50 kPa, 32.04 inHg). The lowest measurable ''sea-level pressure'' is found at the centers of [[tropical cyclone]]s and [[tornado]]es, with a record low of 870 mbar (87 kPa) (see [[Atmospheric_pressure#Atmospheric_pressure_records|Atmospheric pressure records]]).
| |
| | |
| == Altitude atmospheric pressure variation ==
| |
| [[File:Storm over Snæfellsjökull.jpg|thumb|left|A very local storm above Snæfellsjökull, showing clouds formed on the mountain by [[Orographic lifting|Orographic Lift]]]]
| |
| [[File:Atmospheric Pressure vs. Altitude.png|thumb|300 px|right|Variation in atmospheric pressure with altitude, computed for 15 °C and 0% relative humidity.]]
| |
| [[File:Plastic bottle at 14000 feet, 9000 feet and 1000 feet, sealed at 14000 feet.png|thumb|This plastic bottle was sealed at approximately {{convert|14000|ft|m}} altitude, and was crushed by the increase in atmospheric pressure —at {{convert|9000|ft|m}} and {{convert|1000|ft|m}}— as it was brought down towards sea level.]]
| |
| | |
| 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. As altitude increases, atmospheric pressure decreases. One can calculate the atmospheric pressure at a given altitude.<ref>[http://psas.pdx.edu/RocketScience/PressureAltitude_Derived.pdf A quick derivation relating altitude to air pressure] by Portland State Aerospace Society, 2004, accessed 05032011</ref> Temperature and humidity also affect the atmospheric pressure, and it is necessary to know these to compute an accurate figure. The graph at right was developed for a temperature of 15 °C and a relative humidity of 0%.
| |
| | |
| At low altitudes above the sea level, the pressure decreases by about 1.2 kPa for every 100 meters. For higher altitudes within the [[troposphere]], the following equation (the [[Barometric formula]]) relates atmospheric pressure ''p'' to altitude ''h''
| |
| :<math>p = p_0 \cdot \left(1 - \frac{L \cdot h}{T_0} \right)^\frac{g \cdot M}{R \cdot L} \approx p_0 \cdot \left(1 - \frac{g \cdot h}{c_p \cdot T_0} \right)^{\frac{c_p \cdot M}{R}} \approx p_0 \cdot \exp \left(- \frac{g \cdot M \cdot h}{R \cdot T_0} \right),</math>
| |
| where the constant parameters are as described below:
| |
| | |
| {| class="wikitable"
| |
| |-
| |
| !|Parameter |||Description|||Value
| |
| |-
| |
| | ''p''<sub>0</sub> || style="text-align:left;"| sea level standard atmospheric pressure|| style="text-align:right;"| 101325 Pa
| |
| |-
| |
| | ''L'' || style="text-align:left;"| temperature lapse rate, = g/c<sub>p</sub> for dry air || style="text-align:right;"| 0.0065 K/m
| |
| |-
| |
| | ''c''<sub>p</sub> || style="text-align:left;"| constant pressure specific heat || style="text-align:right;"| ~ 1007 J/(kg•K)
| |
| |-
| |
| | ''T''<sub>0</sub>|| style="text-align:left;"| sea level standard temperature || style="text-align:right;"| 288.15 K
| |
| |-
| |
| | ''g'' || style="text-align:left;"| Earth-surface gravitational acceleration|| style="text-align:right;"| 9.80665 m/s<sup>2</sup>
| |
| |-
| |
| | ''M'' || style="text-align:left;"| molar mass of dry air|| style="text-align:right;"| 0.0289644 kg/mol
| |
| |-
| |
| | ''R'' || style="text-align:left;"| universal gas constant|| style="text-align:right;"| 8.31447 J/(mol•K)
| |
| |}
| |
| | |
| == Local atmospheric pressure variation ==
| |
| [[File:Wilma1315z-051019-1kg12.jpg|thumb|[[Hurricane Wilma]] on 19 October 2005–{{convert|882|hPa|abbr=on}} 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 or semidiurnal (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 records ==
| |
| The highest adjusted-to-sea level barometric pressure ever recorded on Earth (above 750 meters) was {{convert|1085.7|hPa|inHg}} measured in [[Tosontsengel, Zavkhan|Tosontsengel, Mongolia]] on 19 December 2001.<ref name="wmo.asu.edu">{{citation|url=http://wmo.asu.edu/highest-sea-lvl-air-pressure-above-700m |title=World: Highest Sea Level Air Pressure Above 750m |publisher=Wmo.asu.edu |date=2001-12-19 |accessdate=2013-04-15}}</ref> The highest adjusted-to-sea level barometric pressure ever recorded (below 750 meters) was at Agata, Evenhiyskiy, Russia [66°53’N, 93°28’E, elevation: 261 m (856.3 ft)] on 31 December 1968 of {{convert|1083.3|hPa|inHg}}.<ref>{{citation|url=http://wmo.asu.edu/world-highest-sea-level-air-pressure-below-700m |title=World: Highest Sea Level Air Pressure Below 750m |publisher=Wmo.asu.edu |date=1968-12-31 |accessdate=2013-04-15}}</ref> The discrimination is due to the problematic assumptions (assuming a standard lapse rate) associated with reduction of sea level from high elevations.<ref name="wmo.asu.edu"/> The lowest non-tornadic atmospheric pressure ever measured was 870 hPa (25.69 inHg), set on 12 October 1979, during [[Typhoon Tip]] in the western Pacific Ocean. The measurement was based on an instrumental observation made from a reconnaissance aircraft.<ref name=FAQ1>{{cite web|url=http://www.aoml.noaa.gov/hrd/tcfaq/E1.html |title=Subject: E1), Which is the most intense tropical cyclone on record? |author=Chris Landsea|publisher=[[Atlantic Oceanographic and Meteorological Laboratory]]|date=2010-04-21 |accessdate=2010-11-23|authorlink=Chris Landsea| archiveurl= http://web.archive.org/web/20101206200600/http://www.aoml.noaa.gov/hrd/tcfaq/E1.html| archivedate= 6 December 2010 <!--DASHBot-->| deadurl= no}}</ref> The normal high barometric pressure at the [[Dead Sea]], as measured by a standard mercury manometer and blood gas analyzer, was found to be 799 mmHg (1065 hPa).<ref>{{cite journal|last=Kramer|first=MR|coauthors=Springer C, Berkman N, Glazer M, Bublil M, Bar-Yishay E, Godfrey S|title=Rehabilitation of hypoxemic patients with COPD at low altitude at the Dead Sea, the lowest place on earth|journal=Chest|date=March 1998|volume=113|issue=3|pages=571-575|quote=PMID: 9515826|url=http://journal.publications.chestnet.org/data/Journals/CHEST/21761/571.pdf}}</ref>
| |
| | |
| == Atmospheric pressure based on depth of water ==
| |
| Atmospheric pressure is often measured with a mercury [[barometer]], and a height of approximately 760mm (29.9213in) of mercury is often used to illustrate (and measure) atmospheric pressure. However, since mercury is not a familiar substance to most people, water may provide a more intuitive way to visualize the pressure of one atmosphere.
| |
| | |
| One atmosphere (101 kPa or 14.7 psi) is the pressure caused by the weight of a column of fresh water of approximately 10.3m (33.79265ft). Thus, a diver 10.3 m underwater experiences a pressure of about 2 atmospheres (1 atm of air plus 1 atm of water). This is the maximum height to which a column of water can be drawn up by [[suction]] at atmospheric pressure.
| |
| | |
| Low pressures such as [[natural gas]] lines are sometimes specified in inches of water, typically written as ''w.c.'' (water column) or ''W.G.'' (inches water gauge). A typical gas-using residential appliance is rated for a maximum of 14 w.c., which is approximately 35 [[hPa]].
| |
| | |
| In general, non-professional [[barometer]]s are [[Barometer#Aneroid barometers|aneroid barometers]] or [[strain gauge]] based. See ''[[pressure measurement]]'' for a description of barometers.
| |
| | |
| == Boiling point of water ==
| |
| [[File:Kochendes wasser02.jpg|thumb|[[Boiling water]]]]
| |
| Clean fresh water [[boiling|boils]] at about {{convert|100|C}} at earth's standard atmospheric pressure. The boiling point is the temperature at which the [[vapor pressure]] is equal to the atmospheric pressure around the water.<ref>{{citation |url=http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/vappre.html |title=Vapour Pressure |publisher=Hyperphysics.phy-astr.gsu.edu |date= |accessdate=2012-10-17}}</ref> Because of this, the boiling point of water is lower at lower pressure and higher at higher pressure. This is why cooking at elevations more than {{convert|1100|m|abbr=on}} above [[sea level]] requires adjustments to recipes.<ref>{{citation |url=http://www.crisco.com/Cooking_Central/Cooking_Tips/Prep_High_Alt.aspx |title=High Altitude Cooking |publisher=Crisco.com |date=2010-09-30 |accessdate=2012-10-17}}</ref> A rough approximation of elevation can be obtained by measuring the temperature at which water boils; in the mid-19th century, this method was used by explorers.<ref>{{Cite journal |first=M. N. |last=Berberan-Santos |first2=E. N. |last2=Bodunov |first3=L. |last3=Pogliani |title=On the barometric formula |journal=American Journal of Physics |volume=65 |issue=5 |pages=404–412 |year=1997 |doi=10.1119/1.18555 |bibcode = 1997AmJPh..65..404B |postscript=<!-- Bot inserted parameter. Either remove it; or change its value to "." for the cite to end in a ".", as necessary. -->{{inconsistent citations}} }}</ref>
| |
| | |
| == Atmospheric pressure measurement and maps ==
| |
| An important application of the knowledge that atmospheric pressure varies directly with altitude and the availability of reliable pressure measurement devices was in determining the height of hills and mountains. While in 1774 Maskelyne was confirming Newton's theory of gravitation at and on Schiehallion in Scotland (using plumb bob deviation to show the effect of "gravity") and accurately measure elevation, William Roy using barometric pressure was able to confirm his height determinations, the agreement being to within one meter (3.28084 feet). This was then a useful tool for survey work and map making and long has continued to be useful. It was part of the "application of science" which gave practical people the insight that applied science could easily and relatively cheaply be "useful".<ref>Hewitt, Rachel, ''Map of a Nation - a Biography of the Ordnance Survey'' ISBN 1847080987</ref>
| |
| | |
| == See also ==
| |
| {{portal|Underwater diving}}
| |
| * [[High-pressure area]]
| |
| * [[Low-pressure area]]
| |
| * [[Atmosphere (unit)]]
| |
| * [[Barometric formula]]
| |
| * [[Barotrauma]] physical damage to body tissues caused by a difference in pressure between an air space inside or beside the body and the surrounding gas or liquid.
| |
| * [[International Standard Atmosphere]] – a tabulation of typical variation of principal thermodynamic variables of the atmosphere (pressure, density, temperature etc.) with altitude, at mid latitudes.
| |
| * [[NRLMSISE-00]]
| |
| * [[Plenum chamber]]
| |
| * [[Subtropical high belts]]
| |
| * [[Cabin pressurization]]
| |
| | |
| == References ==
| |
| {{Reflist|30em}}
| |
| | |
| == External links ==
| |
| * [http://www.magnet.fsu.edu/education/community/slideshows/belljar/index.html How Atmospheric Pressure Affects Objects] (Audio slideshow from the National High Magnetic Field Laboratory)
| |
| * [[NASA]] [http://modelweb.gsfc.nasa.gov/atmos/us_standard.html page on the 1976 Standard Atmosphere]
| |
| * [http://www.pdas.com/atmos.html 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]
| |
| * [http://www.csgnetwork.com/pressurealtcalc.html Calculator giving standard air pressure at a specified altitude, or altitude at which a pressure would be standard]
| |
| * [http://www.physics.org/facts/air.asp Some of the effects of air pressure]
| |
| * [http://www.newbyte.co.il/calc.html Atmospheric calculator and Geometric to Pressure altitude converter]
| |
| | |
| === Experiments ===
| |
| * [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]]
| |
| * [http://www.teachertube.com/viewVideo.php?video_id=62613 Test showing a can being crushed after boiling water inside it, then moving it into a tub of ice cold water.]
| |
| | |
| {{Meteorological variables}}
| |
| {{Diving medicine, physiology and physics}}
| |
| | |
| [[Category:Atmosphere]]
| |
| [[Category:Atmospheric thermodynamics]]
| |
| [[Category:Pressure]]
| |
| [[Category:Underwater diving physics]]
| |