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atmospheric pressure
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{{short description|Static pressure exerted by the weight of the atmosphere }}{{Redirect|Air pressure|the pressure of air in other systems|Pressure}}Atmospheric pressure, sometimes also called barometric pressure (after the sensor), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as {{convert|1013.25|mbar|Pa hPa|abbr=on|lk=on}}, equivalent to 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 that atmospheric pressure decreases with increasing elevation. 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 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.MAGAZINE, atmospheric pressure (encyclopedic entry),weblink National Geographic, 28 February 2018, WEB, Q & A: Pressure - Gravity Matters?,weblink Department of Physics, University of Illinois Urbana-Champaign, 28 February 2018, It is modified by the planetary rotation and local effects such as wind velocity, density variations due to temperature and variations in composition.{{citation needed|date=March 2017}}

## Surface pressure {{anchor|Surface}}

{{redirect|Surface pressure|surface pressure in physical chemistry|Pressure#Surface pressure and surface tension}}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. Introduction to Atmospheric Chemistry. 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 1013.25 hPa, or 1 atmosphere (Atm), or 29.92 inches of mercury. Pressure (P), mass (m), and the acceleration due to gravity (g), are related by P = F/A = (m*g)/A, where A is surface area. Atmospheric pressure is thus proportional to the weight per unit area of the atmospheric mass above that location.

## Altitude variation

{{further|Barometric formula|Vertical pressure variation}}File:Storm over SnÃ¦fellsjÃ¶kull.jpg|thumb|left|A very local storm above SnÃ¦fellsjÃ¶kull, showing clouds formed on the mountain by orographic liftorographic lift(File:Atmospheric Pressure vs. Altitude.png|thumb|300px|Variation in atmospheric pressure with altitude, computed for 15 Â°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, recorded at {{convert|9000|ft|m}} and {{convert|1000|ft|m}}, as it was brought down towards sea level.)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.A quick derivation relating altitude to air pressure {{webarchive|url=https://web.archive.org/web/20110928003908weblink |date=2011-09-28 }} by Portland State Aerospace Society, 2004, accessed 05032011 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 Â°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}where the constant parameters are as described below:{| class="wikitable"! Parameter !! Description !! Value
p0 > Sea level standard atmospheric pressure style="text-align:right;"| 101325{{nbsp}}Pa
L > Temperature lapse rate, = g/cp for dry air style="text-align:right;"| ~ 0.00976{{nbsp}}K/m
cp > Constant-pressure specific heat style="text-align:right;"| 1004.68506{{nbsp}}J/(kgÂ·K)
T0> Sea level standard temperature style="text-align:right;"| 288.16{{nbsp}}K
g > Earth-surface gravitational acceleration style="text-align:right;"| 9.80665{{nbsp}}m/s2
M > Molar mass of dry air style="text-align:right;"| 0.02896968{{nbsp}}kg/mol
R0 > Universal gas constant style="text-align:right;"| 8.314462618{{nbsp}}J/(molÂ·K)

## Local variation

File:Wilma1315z-051019-1kg12.jpg|thumb|882|hPa|abbr=on}} in the storm's eyeAtmospheric 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 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.

## Measurement based on depth of water

One atmosphere (101.325 kPa or 14.7 psi) is also 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.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 1/2 psi, which is approximately 14 w.g. (3487 Pa or 34.9 millibars). 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 water

File:Kochendes wasser02.jpg|thumb|Boiling waterBoiling waterPure water 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.{{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}} 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{{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 |url-status=dead |archiveurl=https://web.archive.org/web/20120907232429weblink |archivedate=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.JOURNAL, M. N., Berberan-Santos, E. N., Bodunov, L., Pogliani, On the barometric formula, American Journal of Physics, 65, 5, 404â€“412, 1997, 10.1119/1.18555, 1997AmJPh..65..404B,

## 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 the availability of 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 accurately measure elevations on the mountain's sides. William Roy, using barometric pressure, was able to confirm Maskelyne's height determinations, the agreement being to 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 â€“ a Biography of the Ordnance Survey {{ISBN|1-84708-098-7}}

{{Div col}}
• {{annotated link|Barotrauma}} â€“ 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.
• {{annotated link|Effects of high altitude on humans}}
• {{annotated link|International Standard Atmosphere}}, a tabulation of typical variation of principal thermodynamic variables of the atmosphere (pressure, density, temperature, etc.) with altitude, at middle latitudes.
• Meteorology
• Pressure measurement
{{div col end}}

## References

{{reflist|30em}}

### Experiments

{{Meteorological variables}}{{Science of underwater diving}}{{Authority control}}

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