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Encyclopedia > Atmospheric pressure

The standard atmosphere (symbol: atm) is a unit of pressure and is defined as being precisely equal to 101.325 kPa. The following non-standard units are equivalent: 760 mmHg (torr), 29.92 inHg or 14.696 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. Standard atmosphere (symbol: atm) is a unit of pressure. ... Pressure (symbol: p or P) is the measure of the force that acts on a unit area. ... For other uses, see Pascal. ... One way of defining pressure is in terms of the height of a column of fluid that may be supported by that pressure; or the height of a column of fluid that exerts that pressure at its base. ... The torr (symbol: Torr) or millimeter of mercury (mmHg) is a non-SI unit of pressure. ... Inches of mercury or inHg is a non SI unit for pressure. ... A pressure gauge reading in PSI (red scale) and kPa (black scale) The pound-force per square inch (symbol: lbf/in2) is a non-SI unit of pressure based on avoirdupois units. ...

` in sdihsd;g 1999 i layed an egg :]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 kPa (≈750.01 torr) or 29.53 inHg rather than the 101.325 kPa value of “one standard atmosphere”.[1] This value is used as the standard pressure for the compressor and the pneumatic tool industries (ISO 2787).[2] (See also Standard temperature and pressure.) In the United States, compressed air flow is often measured in "standard cubic yards" 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 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). `

IUPAC logo The International Union of Pure and Applied Chemistry (IUPAC) (Pronounced as eye-you-pack) is an international non-governmental organization established in 1919 devoted to the advancement of chemistry. ... The torr (symbol: Torr) or millimeter of mercury (mmHg) is a non-SI unit of pressure. ... Inches of mercury or inHg is a non SI unit for pressure. ... Temperature and air pressure can vary from one place to another on the Earth, and can also vary in the same place with time. ...

## Mean sea level pressure

15 year average MSLP for JJA (top) and DJF (bottom)
JJA: June July August
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. Image File history File links Download high resolution version (652x674, 23 KB) Summary Mean sea level pressure for JJA (top) and DJF (bottom) from the ERA-15 reanalysis. ... Image File history File links Download high resolution version (652x674, 23 KB) Summary Mean sea level pressure for JJA (top) and DJF (bottom) from the ERA-15 reanalysis. ...

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 (barometer vs. absolute). Diagram showing the face of a three-pointer sensitive aircraft altimeter displaying altitude in feet. ...

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 isobars on a weather map meaningful and useful tools. The word isobar derives from the two ancient Greek words, Î¹ÏƒÎ¿Ï‚ (isos), meaning equal, and Î²Î±ÏÎ¿Ï‚ (baros), meaning weight. In meteorology, thermodynamics, and similar science (and engineering), an isobar is a contour line of equal or constant pressure on a graph, plot, or map. ...

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 is a Q code. ... Diagram showing the face of a three-pointer sensitive aircraft altimeter displaying altitude in feet. ...

• 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 mnemonics "Nautical Height" (for QNH) and "Field Elevation" (for QFE) are often used by pilots to distinguish them. The Q code is a set of three-letter code signals to be used in radiotelegraphy and amateur radio communications. ... For other uses, see Mnemonic (disambiguation). ...

## Altitude atmospheric pressure variation

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. Image File history File linksMetadata Size of this preview: 272 Ã— 599 pixel Image in higher resolution (1089 Ã— 2400 pixel, file size: 588 KB, MIME type: image/jpeg) Atmospheric pressure at sea level crushing an empty bottle opened at approximately 2000m Copyright Â© 2007 David Monniaux File links The following pages on... Image File history File linksMetadata Size of this preview: 272 Ã— 599 pixel Image in higher resolution (1089 Ã— 2400 pixel, file size: 588 KB, MIME type: image/jpeg) Atmospheric pressure at sea level crushing an empty bottle opened at approximately 2000m Copyright Â© 2007 David Monniaux File links The following pages on... The mesosphere (from the Greek words mesos = middle and sphaira = ball) is the layer of the Earths atmosphere that is directly above the stratosphere and directly below the thermosphere. ...

fraction of 1 atm average altitude
(m) (ft)
1 0 0
1/2 5,486 18,000
1/3 8,376 27,480
1/10 16,132 52,926
1/100 30,901 101,381
1/1000 48,467 159,013
1/10000 69,464 227,899
1/100000 96,282 283,076

## Calculating variation with altitude

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. The barometric formula, sometimes called the exponential atmosphere or isothermal atmosphere, is a formula used to model how the pressure (or density) of the air changes with altitude. ...

Equation 1:

${P}=P_b cdot left[frac{T_b}{T_b + L_bcdot(h-h_b)}right]^frac{g_0 cdot M}{R^* cdot L_b}$

Equation 2:

${P}=P_b cdot exp left[frac{-g_0 cdot M cdot (h-h_b)}{R^* cdot T_b}right]$

where

P = Static pressure (pascals)
T = Standard temperature (kelvins)
L = Standard temperature lapse rate (kelvins per m)
h = Height above sea level (meters)
R * = Universal gas constant: 8.31432×10³ N·m / (kmol·K)
g0 = Gravitational constant (9.80665 m/s²)
M = Molar mass of Earth's air (28.9644 g/mol)

Or converted to English units:[4] For other uses, see Kelvin (disambiguation). ... Molar gas constant (also known as universal gas constant, usually denoted by symbol R) is the constant occurring in the universal gas equation, i. ...

where

P = Static pressure (inches of mercury)
T = Standard temperature (kelvins)
L = Standard temperature lapse rate (kelvins per ft)
h = Height above sea level (feet)
R * = Universal gas constant (using feet and kelvins and gram moles: 8.9494596×104 kg·sq ft·s-2·K-1·kmol-1)
g0 = Gravitational constant (32.17405 ft/s²)
M = 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, g0, M and R* 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, L0, the tropospheric lapse rate, is negative.) It should be noted that the values used for M, g0, and R * are in accordance with the U.S. Standard Atmosphere, 1976, and that the value for R * in particular does not agree with standard values for this constant.[5] The reference value for Pb for b = 0 is the defined sea level value, P0 = 101325 pascals or 29.92126 inHg. Values of Pb 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 h = hb + 1.:[5] For other uses, see Kelvin (disambiguation). ... Molar gas constant (also known as universal gas constant, usually denoted by symbol R) is the constant occurring in the universal gas equation, i. ... The mole (symbol: mol) is the SI base unit that measures an amount of substance. ... The U.S. Standard Atmosphere is a series of models that define values for atmospheric temperature, density, pressure and other properties over a wide range of altitudes. ... For other uses, see Pascal. ... Inches of mercury or inHg is a non SI unit for pressure. ...

Subscript b Height Above Sea Level Static Pressure Standard Temperature
(K)
Temperature Lapse Rate
(m) (ft) (pascals) (inHg) (K/m) (K/ft)
0 0 0 101325 29.92126 288.15 -0.0065 -0.0019812
1 11,000 36,089 22632 6.683245 216.65 0.0 0.0
2 20,000 65,617 5474 1.616734 216.65 0.001 0.0003048
3 32,000 104,987 868 0.2563258 228.65 0.0028 0.00085344
4 47,000 154,199 110 0.0327506 270.65 0.0 0.0
5 51,000 167,323 66 0.01976704 270.65 -0.0028 -0.00085344
6 71,000 232,940 4 0.00116833 214.65 -0.002 -0.0006097

### Sample Calculation

Find the average pressure at 30,000 meters.

First note that 30,000 meters is above 20,000 but below 32,000 so it therefore falls in the range of subscript b=2 in the chart above. Also note that the temperature lapse rate for that region is not equal to zero; therefore, equation 1 is appropriate.

${P}=P_2 cdot left[frac{T_2}{T_2 + L_2cdot(h-h_2)}right]^frac{g_0 cdot M}{R^* cdot L_2}$

Or

${P}=5474.89 cdot left[frac{216.65}{216.65 + 0.001cdot(30,000-20,000)}right]^frac{9.80665 cdot 28.9644}{8314.32 cdot 0.001}$
${P}=5474.89 cdot left[frac{216.65}{226.65}right]^{34.163195}$
${P}=5474.89 cdot 0.214044$
${P} = 1171.867$ Pascals at 30,000 meters

## Local atmospheric pressure variation

Hurricane Wilma on 19 October 2005 – 88.2 kPa in eye

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 inches) 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. A barometer is an instrument used to measure atmospheric pressure. ... An inch (plural: inches; symbol or abbreviation: in or, sometimes, â€³ - a double prime) is the name of a unit of length in a number of different systems, including English units, Imperial units, and United States customary units. ... Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ...

One atmosphere (101.325 kPa or 14.7 lbf/sq 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. Suction is the creation of a partial vacuum, or region of low pressure. ...

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. For other uses, see Natural gas (disambiguation). ... A water column is a conceptual column of water from surface to bottom sediments. ...

Non-professional barometers are generally aneroid barometer (Figure 3) or strain gauge based. See Pressure measurement for a description of barometers. A barometer is an instrument used to measure atmospheric pressure. ... Typical foil strain gauge. ... The construction of manometer, construction elements are made of brass Many techniques have been developed for the measurement of pressure and vacuum. ...

## 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. [6] 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. [7] For considerations of sea level change, in particular rise associated with possible global warming, see sea level rise. ...

Plenum may refer to: the antithesis of a vacuum; in other words, completely filled space. ... NRLMSISE output The NRLMSISE-00 is an empirical, global model of the Earths atmosphere from ground to space. ... The barometric formula, sometimes called the exponential atmosphere or isothermal atmosphere, is a formula used to model how the pressure (or density) of the air changes with altitude. ... The International Standard Atmosphere (ISA) is a model of the standard variation of pressure, temperature, density, and viscosity with altitude in the Earths atmosphere. ...

## References

1. ^ IUPAC.org, Publications, Standard Pressure (20 kB PDF)
2. ^ Compressor.co.za, May 2003 Newsletter
3. ^ Sample METAR of CYVR Nav Canada
4. ^ 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.
5. ^ a b U.S. Standard Atmosphere, 1976, U.S. Government Printing Office, Washington, D.C., 1976. (Linked file is very large.)
6. ^ http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/vappre.html
7. ^ http://www.crisco.com/basics/tips/high_altitude.asp
• 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.

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 Atmospheric Pressure: force exerted by the weight of the air (257 words) Atmospheric pressure is defined as the force per unit area exerted against a surface by the weight of the air above that surface. Atmospheric pressure is measured with an instrument called a "barometer", which is why atmospheric pressure is also referred to as barometric pressure. Meteorologists use a metric unit for pressure called a millibar and the average pressure at sea level is 1013.25 millibars.
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