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| − | {{Redirect|Air pressure|the pressure of air in other systems|pressure}}
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| − | {{Continuum mechanics}}
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| − | '''Atmospheric pressure''', sometimes also called '''barometric pressure''', is the pressure exerted by the weight of air 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. 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 {{convert|spell=in|1|cm2|abbr=~}} in cross-section, measured from [[sea level]] to the top of the atmosphere, has a [[mass]] of about {{convert|1.03|kg}} and weight of about {{convert|10.1|N|lk=on}}. That force (across one square centimeter) is a pressure of 10.1 N/cm<sup>2</sup> or 101,000 N/m<sup>2</sup>. A column {{convert|1|sqin}} in cross-section would have a weight of about {{cvt|14.7|lb}} or about 65.4 N.
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| − | == Standard atmospheric ==
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| − | The [[Atmosphere (unit)|standard atmosphere]] (symbol: atm) is a [[Units of pressure|unit of pressure]] defined as {{convert|101,325|Pa|bar|abbr=on|comma=gaps|lk=in}}, equivalent to 760 [[Millimeter of mercury|mmHg]] ([[torr]]), 29.92 [[inHg]] and 14.696 [[Pounds per square inch|psi]].<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>
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| − | == Mean sea level pressure ==
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| − | [[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]] re-analysis.]]
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| − | [[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}}.]]
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| − | The mean sea level pressure (MSLP) is the atmospheric pressure at [[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 adjusted to sea level, not the actual local atmospheric pressure.
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| − | The ''[[altimeter setting]]'' in aviation, is an atmospheric pressure adjustment.
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| − | Average ''sea-level pressure'' is {{convert|1013.25|mbar|kPa inHg mmHg|abbr=on|comma=off}}. In aviation weather reports ([[METAR]]), [[QNH]] is transmitted around the world in millibars or hectopascals (1 hectopascal = 1 millibar), 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 adjusted to sea level by a different method, in the remarks section, not in the 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>{{citation|url=http://www.cbc.ca/montreal/weather/s0000635.html |title=Montreal Current Weather |publisher=CBC Montreal, Canada |accessdate=2014-03-30}}</ref>
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| − | In the US weather code remarks, three digits are all that are transmitted; decimal points and the one or two most significant digits are omitted: {{convert|1013.2|mbar|kPa|abbr=on|comma=off}} is transmitted as 132; {{convert|1000.0|mbar|kPa|abbr=on|comma=off}} is transmitted as 000; 998.7 mbar 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 {{convert|1050|mbar|kPa inHg|abbr=on|comma=off}}, with record highs close to {{convert|1085|mbar|kPa inHg|abbr=on|comma=off}}. The lowest measurable ''sea-level pressure'' is found at the centers of [[tropical cyclone]]s and [[tornado]]es, with a record low of {{convert|870|mbar|kPa inHg|abbr=on|comma=off}} (see [[Atmospheric pressure#Records|Atmospheric pressure records]]).
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| − | == Altitude variation ==
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| − | [[File:Storm over Snæfellsjökull.jpg|thumb|left|A very local storm above Snæfellsjökull, showing clouds formed on the mountain by [[orographic lift]]]]
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| − | [[File:Atmospheric Pressure vs. Altitude.png|thumb|300 px|right|Variation in atmospheric pressure with altitude, computed for 15 °C and 0% relative humidity.]]
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| − | [[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.]]
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| − | 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%.
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| − | 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''
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| − | :<math>p = p_0 \cdot \left(1 - \frac{L \cdot h}{T_0} \right)^\frac{g \cdot M}{R_0 \cdot L} \approx p_0 \cdot \left(1 - \frac{g \cdot h}{c_p \cdot T_0} \right)^{\frac{c_p \cdot M}{R_0}},</math>
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| − | :<math>p \approx p_0 \cdot \exp \left(- \frac{g \cdot M \cdot h}{R_0 \cdot T_0} \right)</math>
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| − | where the constant parameters are as described below:
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| − | {| class="wikitable"
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| − | !|Parameter |||Description|||Value
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| − | | ''p''<sub>0</sub> || style="text-align:left;"| sea level standard atmospheric pressure|| style="text-align:right;"| 101325 Pa
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| − | | ''L'' || style="text-align:left;"| temperature lapse rate, = g/c<sub>p</sub> for dry air || style="text-align:right;"| 0.0065 K/m
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| − | | ''c''<sub>p</sub> || style="text-align:left;"| constant pressure specific heat || style="text-align:right;"| ~ 1007 J/(kg•K)
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| − | | ''T''<sub>0</sub>|| style="text-align:left;"| sea level standard temperature || style="text-align:right;"| 288.15 K
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| − | |-
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| − | | ''g'' || style="text-align:left;"| Earth-surface gravitational acceleration|| style="text-align:right;"| 9.80665 m/s<sup>2</sup>
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| − | | ''M'' || style="text-align:left;"| molar mass of dry air|| style="text-align:right;"| 0.0289644 kg/mol
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| − | | ''R''<sub>0</sub> || style="text-align:left;"| [[universal gas constant]]|| style="text-align:right;"| 8.31447 J/(mol•K)
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| − | |}
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| − | == Local variation ==
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| − | [[File:Wilma1315z-051019-1kg12.jpg|thumb|[[Hurricane Wilma]] on 19 October 2005; {{convert|882|hPa|abbr=on}} in the storm's eye]]
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| − | 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.
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| − | Atmospheric pressure shows a diurnal or semidiurnal (twice-daily) cycle caused by global [[atmospheric tides]]. This effect is strongest in tropical zones, with an 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.
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| − | == Records ==
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| − | 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 750 m |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, [[Evenk Autonomous Okrug|Evenkiyskiy]], Russia [66°53’N, 93°28’E, elevation: 261 m (856.3 ft)] on 31 December 1968 of {{convert|1083.8|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 750 m |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" />
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| − | The [[Dead Sea]], the lowest place on Earth at 425 metres (1400 feet) below sea level, has a correspondingly high typical atmospheric pressure of 1065 hPa.<ref>{{cite journal|last=Kramer|first=MR|author2=Springer C|author3=Berkman N|author4=Glazer M|author5=Bublil M|author6=Bar-Yishay E|author7=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|doi=10.1378/chest.113.3.571|pmid=9515826}}</ref>
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| − | The lowest non-tornadic atmospheric pressure ever measured was 0.858 atm (25.69 inHg), 870 hPa, 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= https://web.archive.org/web/20101206200600/http://www.aoml.noaa.gov/hrd/tcfaq/E1.html| archivedate= 6 December 2010 <!--DASHBot-->| deadurl= no}}</ref>
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| − | == Measurement based on depth of water ==
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| − | One atmosphere (101 kPa or 14.7 psi) is the pressure caused by the weight of a column of fresh water of approximately 10.3 m (33.8 ft). Thus, a diver 10.3 m underwater experiences a pressure of about 2 atmospheres (1 atm of air plus 1 atm of water). Conversely, 10.3 m is the maximum height to which water can be raised using [[suction]] under standard atmospheric conditions.
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| − | Low pressures such as [[natural gas]] lines are sometimes specified in [[Inch of water|inches of water]], typically written as ''w.c.'' (water column) or ''w.g.'' (inches water gauge). A typical gas-using residential appliance in the US is rated for a maximum of 14 w.c., which is approximately 35 [[hPa]]. Similar metric units with a wide variety of names and notation based on [[Millimeters, water gauge|millimetres]], [[Centimetre of water|centimetres]] or metres are now less commonly used.
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| − | == Boiling point of water ==
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| − | [[File:Kochendes wasser02.jpg|thumb|[[Boiling water]]]]
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| − | Pure water [[boiling|boils]] at {{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 |accessdate=2012-10-17}}</ref> Because of this, the boiling point of water is lower at lower pressure and higher at higher pressure. Cooking at high elevations, therefore, 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>
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| − | == Measurement and maps ==
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| − | An important application of the knowledge that atmospheric pressure varies directly with altitude was in determining the height of hills and mountains thanks to the availability of reliable pressure measurement devices. 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 (3.28 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 1-84708-098-7</ref>
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| − | == See also ==
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| − | {{portal|Underwater diving}}
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| − | {{div col|2}}
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| − | * [[Atmosphere (unit)]]
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| − | * [[Barometric formula]]
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| − | * [[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.
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| − | * [[Cabin pressurization]]
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| − | * [[Effects of high altitude on humans]]
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| − | * [[High-pressure area]]
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| − | * [[International Standard Atmosphere]] - a tabulation of typical variation of principal thermodynamic variables of the atmosphere (pressure, density, temperature, etc.) with altitude, at middle latitudes.
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| − | * [[Low-pressure area]]
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| − | * [[NRLMSISE-00]]
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| − | * [[Plenum chamber]]
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| − | * [[Pressure]]
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| − | * [[Subtropical high belts]]
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| − | {{div col end}}
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| − | == References ==
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| − | {{Reflist|35em}}
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| − | == External links ==
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| − | * [https://nationalmaglab.org/education/magnet-academy/watch-play/demos/how-atmospheric-pressure-affects-objects How Atmospheric Pressure Affects Objects] (Audio slideshow from the National High Magnetic Field Laboratory)
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| − | * [http://modelweb.gsfc.nasa.gov/atmos/us_standard.html 1976 Standard Atmosphere] from [[NASA]]
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| − | * [http://www.pdas.com/atmos.html Source code and equations for the 1976 Standard Atmosphere]
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| − | * [http://www.atmosculator.com/The%20Standard%20Atmosphere.html? A mathematical model of the 1976 U.S. Standard Atmosphere]
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| − | * [http://www.luizmonteiro.com/StdAtm.aspx Calculator using multiple units and properties for the 1976 Standard Atmosphere]
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| − | * [http://www.csgnetwork.com/pressurealtcalc.html Calculator giving standard air pressure at a specified altitude, or altitude at which a pressure would be standard]
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| − | * [http://www.physics.org/facts/air.asp Some of the effects of air pressure]
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| − | * [http://www.newbyte.co.il/calc.html Atmospheric calculator and Geometric to Pressure altitude converter]
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| − | === Experiments ===
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| − | * [http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/patm.html#atm Movies on atmospheric pressure experiments from] [[Georgia State University]]'s [[QuickTime|HyperPhysics website – requires QuickTime]]
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| − | * [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.]
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| − | {{Meteorological variables}}
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| − | {{Diving medicine, physiology and physics}}
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| − | {{Authority control}}
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| − | [[Category:Atmosphere]]
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| − | [[Category:Atmospheric thermodynamics]]
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| − | [[Category:Pressure]]
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| − | [[Category:Underwater diving physics]]
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