Atmospheric pressure#Surface pressure

{{short description|Static pressure exerted by the weight of the atmosphere}}

{{Redirect|Air pressure|the pressure of air in other systems|Pressure}}

Atmospheric pressure, also known as air pressure or barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as {{cvt|101325|Pa|hPa|lk=on}}, which is equivalent to 1,013.25 millibars,{{cite web|url=https://www.bipm.org/en/committees/ci/cipm/90-2001/resolution- |title=Statement (2001) |publisher=BIPM |date= |accessdate=2022-03-19}} 760{{nbsp}}mm Hg, 29.9212{{nbsp}}inches{{nbsp}}Hg, or 14.696{{nbsp}}psi.International Civil Aviation Organization. Manual of the ICAO Standard Atmosphere, Doc 7488-CD, Third Edition, 1993. {{ISBN|92-9194-004-6}}. The atm unit is roughly equivalent to the mean sea-level atmospheric pressure on Earth; that is, the Earth's atmospheric pressure at sea level is approximately 1 atm.

In most circumstances, atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. As elevation increases, there is less overlying atmospheric mass, so atmospheric pressure decreases with increasing elevation. Because the atmosphere is thin relative to the Earth's radius—especially the dense atmospheric layer at low altitudes—the Earth's gravitational acceleration as a function of altitude can be approximated as constant and contributes little to this fall-off. Pressure measures force per unit area, with SI units of pascals (1 pascal = 1 newton per square metre, 1{{nbsp}}N/m²). On average, a column of air with a cross-sectional area of 1 square centimetre (cm²), measured from the mean (average) sea level to the top of Earth's atmosphere, has a mass of about 1.03 kilogram and exerts a force or "weight" of about 10.1 newtons, resulting in a pressure of 10.1 N/cm² or 101{{nbsp}}kN/m² (101 kilopascals, kPa). A column of air with a cross-sectional area of 1{{nbsp}}in² would have a weight of about 14.7{{nbsp}}lbf, resulting in a pressure of 14.7{{nbsp}}lbf/in².

Mechanism

Atmospheric pressure is caused by the gravitational attraction of the planet on the atmospheric gases above the surface and is a function of the mass of the planet, the radius of the surface, and the amount and composition of the gases and their vertical distribution in the atmosphere.{{cite magazine|title=atmospheric pressure (encyclopedic entry)|url=https://www.nationalgeographic.org/encyclopedia/atmospheric-pressure/|magazine=National Geographic|access-date=28 February 2018|archive-date=28 February 2018|archive-url=https://web.archive.org/web/20180228194703/https://www.nationalgeographic.org/encyclopedia/atmospheric-pressure/|url-status=live}}{{cite web|title=Q & A: Pressure – Gravity Matters?|url=https://van.physics.illinois.edu/qa/listing.php?id=2232|website=Department of Physics|publisher=University of Illinois Urbana-Champaign|access-date=28 February 2018|archive-date=28 February 2018|archive-url=https://web.archive.org/web/20180228194620/https://van.physics.illinois.edu/qa/listing.php?id=2232|url-status=live}} It is modified by the planetary rotation and local effects such as wind velocity, density variations due to temperature and variations in composition.{{Cite book|url=https://books.google.com/books?id=14whM9fEOzsC&q=atmospheric+pressure+density+temperature+and+composition|title=Introduction to Atmospheric Chemistry|last=Jacob|first=Daniel J.|date=1999|publisher=Princeton University Press|isbn=9780691001852|language=en|access-date=2020-10-15|archive-date=2021-10-01|archive-url=https://web.archive.org/web/20211001043009/https://books.google.com/books?id=14whM9fEOzsC&q=atmospheric+pressure+density+temperature+and+composition|url-status=live}}

Mean sea-level pressure

File:Day5pressureforecast.png

File:Mslp-jja-djf.png re-analysis.]]

File:Aircraft altimeter.JPG.]]

The mean sea-level pressure (MSLP) is the atmospheric pressure at mean sea level. This is the atmospheric pressure normally given in weather reports on radio, television, and newspapers or on the Internet.{{Citation needed|date=December 2024}}

The altimeter setting in aviation is an atmospheric pressure adjustment.

Average sea-level pressure is {{cvt|1013.25|hPa|inHg mmHg}}. In aviation weather reports (METAR), QNH is transmitted around the world in hectopascals or millibars (1 hectopascal = 1 millibar). In the United States, Canada, and Japan altimeter setting is reported in inches of mercury (to two decimal places). 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.[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] {{Webarchive|url=https://web.archive.org/web/20190525162442/https://flightplanning.navcanada.ca/cgi-bin/Fore-obs/metar.cgi?NoSession=NS_Inconnu&format=dcd&Langue=anglais&Region=can&Stations=CYVR&Location= |date=2019-05-25 }} Nav Canada However, in Canada's public weather reports, sea level pressure is instead reported in kilopascals.{{citation |url=http://www.cbc.ca/montreal/weather/s0000635.html |title=Montreal Current Weather |publisher=CBC Montreal, Canada |access-date=2014-03-30 |archive-date=2014-03-30 |archive-url=https://web.archive.org/web/20140330055543/http://www.cbc.ca/montreal/weather/s0000635.html |url-status=live }}

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: {{cvt|1013.2|hPa}} is transmitted as 132; {{cvt|1000|hPa|kPa}} is transmitted as 000; 998.7{{nbsp}}hPa 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 {{cvt|1050|hPa|psi inHg}}, with record highs close to {{cvt|1085|hPa|psi inHg}}. The lowest measurable sea-level pressure is found at the centres of tropical cyclones and tornadoes, with a record low of {{cvt|870|hPa|psi inHg|comma=off}}. A system transmitting the last three digits transmits the same code (800) for 1080.0 hPa as for 980.0 hPa.

Surface pressure {{anchor|Surface}}

{{About|section=yes|the atmospheric surface pressure|other uses|Surface pressure (disambiguation)}}

Surface pressure is the atmospheric pressure at a location on Earth's surface (terrain and oceans). It is directly proportional to the mass of air over that location.

For numerical reasons, atmospheric models such as general circulation models (GCMs) usually predict the nondimensional logarithm of surface pressure.

The average value of surface pressure on Earth is 985 hPa.Jacob, Daniel J. [https://books.google.com/books?id=7B8EEn_sj0cC&q=%22surface+pressure%22 Introduction to Atmospheric Chemistry] {{Webarchive|url=https://web.archive.org/web/20200725014305/https://books.google.com/books?id=7B8EEn_sj0cC&printsec=frontcover#v=onepage&q=%22surface%20pressure%22&f=false |date=2020-07-25 }}. Princeton University Press, 1999. This is in contrast to mean sea-level pressure, which involves the extrapolation of pressure to sea level for locations above or below sea level. The average pressure at mean sea level (MSL) in the International Standard Atmosphere (ISA) is 1,013.25 hPa, or 1 atmosphere (atm), or 29.92 inches of mercury.

Pressure (P), mass (m), and acceleration due to gravity (g) are related by P = F/A = (m*g)/A, where A is the surface area. Atmospheric pressure is thus proportional to the weight per unit area of the atmospheric mass above that location.

Altitude variation

{{See also|Blaise Pascal#First atmospheric pressure vs. altitude experiment}}{{further|Barometric formula|Vertical pressure variation}}

File:Storm over Snæfellsjökull.jpg (Iceland), formed above the mountain by orographic lift]]

File:Atmospheric Pressure vs. Altitude.png

File:Plastic bottle at 14000 feet, 9000 feet and 1000 feet, sealed at 14000 feet.png

Pressure on Earth varies with the altitude of the surface, so air pressure on mountains is usually lower than air pressure at 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.[http://archive.psas.pdx.edu/RocketScience/PressureAltitude_Derived.pdf A quick derivation relating altitude to air pressure] {{webarchive|url=https://web.archive.org/web/20110928003908/http://psas.pdx.edu/RocketScience/PressureAltitude_Derived.pdf |archive-url=https://web.archive.org/web/20100614221022/http://psas.pdx.edu/RocketScience/PressureAltitude_Derived.pdf |archive-date=2010-06-14 |url-status=live |date=2011-09-28 }} by Portland State Aerospace Society, 2004, accessed 05032011 Temperature and humidity also affect the atmospheric pressure. Pressure is proportional to temperature and inversely related to humidity, and both of these are necessary to compute an accurate figure. The graph {{if mobile|above|on the right}} was developed for a temperature of 15 °C and a relative humidity of 0%.

At low altitudes above sea level, the pressure decreases by about {{convert|1.2|kPa|hPa|abbr=on}} for every 100 metres. For higher altitudes within the troposphere, the following equation (the barometric formula) relates atmospheric pressure p to altitude h:

$\begin{align}
p &= p_0 \cdot \left(1 - \frac{L \cdot h}{T_0} \right)^\frac{g \cdot M}{R_0 \cdot L} \\
&= p_0 \cdot \left(1 - \frac{g \cdot h}{c_\text{p} \cdot T_0} \right)^{\frac{c_\text{p} \cdot M}{R_0}}
\approx p_0 \cdot \exp \left(-\frac{g \cdot h \cdot M}{T_0 \cdot R_0} \right)
\end{align}$

The values in these equations are:

Parameter	Description	Value
class="wikitable sortable"
h	style="text-align:left;"\| Height above mean sea level	style="text-align:right;" \| {{nbsp}}m
p₀	style="text-align:left;"\| Sea level standard atmospheric pressure	style="text-align:right;"\| 101,325{{nbsp}}Pa
L	style="text-align:left;"\| Temperature lapse rate, = g/c_p for dry air	style="text-align:right;"\| ~ 0.00976{{nbsp}}K/m
c_p	style="text-align:left;"\| Constant-pressure specific heat	style="text-align:right;"\| 1,004.68506{{nbsp}}J/(kg·K)
T₀	style="text-align:left;"\| Sea level standard temperature	style="text-align:right;"\| 288.15{{nbsp}}K
g	style="text-align:left;"\| Earth-surface gravitational acceleration	style="text-align:right;"\| 9.80665{{nbsp}}m/s²
M	style="text-align:left;"\| Molar mass of dry air	style="text-align:right;"\| 0.02896968{{nbsp}}kg/mol
R₀	style="text-align:left;"\| Universal gas constant	style="text-align:right;"\| 8.314462618{{nbsp}}J/(mol·K)

Local variation

File:Wilma 2005-10-19 1315Z.jpg on 19 October 2005. The pressure in the eye of the storm was {{convert|882|hPa|abbr=on}} at the time the image was taken.]]

Atmospheric pressure varies widely on Earth, and these changes are important in studying weather and climate. 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 hectopascals, and almost zero in polar areas. These variations have two superimposed cycles, a circadian (24 h) cycle, and a semi-circadian (12 h) cycle.

Records

The highest adjusted-to-sea level barometric pressure ever recorded on Earth (above 750 meters) was {{convert|1084.8|hPa|inHg|abbr=on}} measured in Tosontsengel, Mongolia on 19 December 2001.{{citation|url=http://wmo.asu.edu/highest-sea-lvl-air-pressure-above-700m |title=World: Highest Sea Level Air Pressure Above 750 m |publisher=World Meteorological Organization's World Weather & Climate Extremes Archive |date=2001-12-19 |access-date=2013-04-15 |url-status=dead |archive-url=https://web.archive.org/web/20121017130834/http://wmo.asu.edu/highest-sea-lvl-air-pressure-above-700m |archive-date=2012-10-17 }} The highest adjusted-to-sea level barometric pressure ever recorded (below 750 meters) was at Agata in Evenk Autonomous Okrug, Russia (66°53'{{nbsp}}N, 93°28'{{nbsp}}E, elevation: {{convert|261|m|abbr=on|disp=comma}}) on 31 December 1968 of {{convert|1083.8|hPa|inHg|3|abbr=on}}.{{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=World Meteorological Organization's World Weather & Climate Extremes Archive |date=1968-12-31 |access-date=2013-04-15 |url-status=dead |archive-url=https://web.archive.org/web/20130514175318/http://wmo.asu.edu/world-highest-sea-level-air-pressure-below-700m |archive-date=2013-05-14 }} The discrimination is due to the problematic assumptions (assuming a standard lapse rate) associated with reduction of sea level from high elevations.

The Dead Sea, the lowest place on Earth at {{convert|430|m}} below sea level, has a correspondingly high typical atmospheric pressure of 1,065{{nbsp}}hPa.{{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 |url=http://journal.publications.chestnet.org/data/Journals/CHEST/21761/571.pdf |doi=10.1378/chest.113.3.571 |pmid=9515826 |url-status=dead |archive-url=https://web.archive.org/web/20131029195349/http://journal.publications.chestnet.org/data/Journals/CHEST/21761/571.pdf |archive-date=2013-10-29 }} A below-sea-level surface pressure record of {{convert|1081.8|hPa|inHg|abbr=on}} was set on 21 February 1961.{{cite journal |last=Court |first=Arnold |year=1969 |title=Improbable Pressure Extreme: 1070 Mb. |journal=Bulletin of the American Meteorological Society |volume=50 |issue=4 |pages=248–50 |jstor=26252600 }}

The lowest non-tornadic atmospheric pressure ever measured was 870 hPa (0.858 atm; 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.{{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 |access-date=2010-11-23|author-link=Chris Landsea| archive-url= https://web.archive.org/web/20101206200600/http://www.aoml.noaa.gov/hrd/tcfaq/E1.html| archive-date= 6 December 2010 | url-status= live}}

Measurement based on the depth of water

One atmosphere ({{cvt|101.325|kPa|psi|disp=or|1}}) is also the pressure caused by the weight of a column of freshwater of approximately {{cvt|10.3|m|ft|1}}. Thus, a diver 10.3 m under water 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.

Low pressures, such as natural gas lines, are sometimes specified in inches of water, typically written as w.c. (water column) gauge or w.g. (inches water) gauge. A typical gas-using residential appliance in the US is rated for a maximum of {{cvt|1/2|psi|kPa mbar}}, which is approximately 14 w.g. Similar metric units with a wide variety of names and notation based on millimetres, centimetres or metres are now less commonly used.

Boiling point of liquids

File:Kochendes wasser02.jpg]]

Pure water boils at {{convert|100|C}} at earth's standard atmospheric pressure. The boiling point is the temperature at which the vapour pressure is equal to the atmospheric pressure around the liquid.{{citation |url=http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/vappre.html |title=Vapour Pressure |publisher=Hyperphysics.phy-astr.gsu.edu |access-date=2012-10-17 |archive-date=2017-09-14 |archive-url=https://web.archive.org/web/20170914100414/http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/vappre.html |url-status=live }} Because of this, the boiling point of liquids is lower at lower pressure and higher at higher pressure. Cooking at high elevations, therefore, requires adjustments to recipes{{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 |access-date=2012-10-17 |url-status=dead |archive-url=https://web.archive.org/web/20120907232429/http://www.crisco.com/Cooking_Central/Cooking_Tips/Prep_High_Alt.aspx |archive-date=2012-09-07 }} or pressure cooking. 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.{{cite journal |first1=M. N. |last1=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 }} Conversely, if one wishes to evaporate a liquid at a lower temperature, for example in distillation, the atmospheric pressure may be lowered by using a vacuum pump, as in a rotary evaporator.

Measurement and maps

An important application of the knowledge that atmospheric pressure varies directly with altitude was in determining the height of hills and mountains, thanks to reliable pressure measurement devices. In 1774, Maskelyne was confirming Newton's theory of gravitation at and on Schiehallion mountain in Scotland, and he needed to measure elevations on the mountain's sides accurately. William Roy, using barometric pressure, was able to confirm Maskelyne's height determinations; the agreement was within one meter (3.28 feet). This method became and continues to be useful for survey work and map making.Hewitt, Rachel, Map of a Nation – a Biography of the Ordnance Survey {{ISBN|1-84708-098-7}}

References

External links

[https://earth.nullschool.net/#current/wind/surface/level/overlay=mean_sea_level_pressure/winkel3 Current map of global mean sea-level pressure]
[https://web.archive.org/web/20060513194709/http://modelweb.gsfc.nasa.gov/atmos/us_standard.html 1976 Standard Atmosphere] from NASA
[http://www.pdas.com/atmos.html Source code and equations for the 1976 Standard Atmosphere]
[https://web.archive.org/web/20070310223946/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]
[https://www.herramientasingenieria.com/onlinecalc/altitude/altitude.html Calculate pressure from altitude and vice versa]

= Experiments =

[http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/patm.html#atm Movies on atmospheric pressure experiments from] Georgia State University's 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.]

Parameter	Description	Value
class="wikitable sortable"
h	style="text-align:left;"\| Height above mean sea level	style="text-align:right;" \| {{nbsp}}m
p₀	style="text-align:left;"\| Sea level standard atmospheric pressure	style="text-align:right;"\| 101,325{{nbsp}}Pa
L	style="text-align:left;"\| Temperature lapse rate, = g/c_p for dry air	style="text-align:right;"\| ~ 0.00976{{nbsp}}K/m
c_p	style="text-align:left;"\| Constant-pressure specific heat	style="text-align:right;"\| 1,004.68506{{nbsp}}J/(kg·K)
T₀	style="text-align:left;"\| Sea level standard temperature	style="text-align:right;"\| 288.15{{nbsp}}K
g	style="text-align:left;"\| Earth-surface gravitational acceleration	style="text-align:right;"\| 9.80665{{nbsp}}m/s²
M	style="text-align:left;"\| Molar mass of dry air	style="text-align:right;"\| 0.02896968{{nbsp}}kg/mol
R₀	style="text-align:left;"\| Universal gas constant	style="text-align:right;"\| 8.314462618{{nbsp}}J/(mol·K)