air pressure

Atmospheric pressure is the pressure at any point in the Earth's atmosphere.

In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. Air masses are affected by the general atmospheric pressure within the mass, creating areas of high pressure (anti-cyclones) and low pressure (depressions). Low pressure areas have less atmospheric mass above their location, whereas high pressure areas have more atmospheric mass above their location.

As elevation increases, there are exponentially fewer and fewer air molecules. Therefore, atmospheric pressure decreases with increasing altitude at a decreasing rate. The following relationship is a first-order approximation:

,

where P is the pressure in pascals and h the height in metres. This shows that the pressure at an altitude of 31 km is about 10^(5-2) Pa = 1000 Pa, or 1% of that at sea level¹. A rough approximation valid for the first few kilometers above the surface is that pressure decreases by 100 hPa per kilometer.

A column of air, 1 square inch in cross section, measured from sea level to the top of the atmosphere would weigh approximately 14.7 lb. A 1 m² column of air would weigh about 100 kilonewtons. See density of air.

Standard atmospheric pressure

Standard atmosphere (symbol: atm) is a unit of pressure and is defined as being precisely equal to 101,325 Pa. This value is intended to represent the mean sea level pressure at the latitude of Paris, France, and as a practical matter, truly reflects the mean sea level pressure for many of the industrialized nations (those with latitudes similar to Paris). A pressure of 1 atm can also be stated as:

≡1.013 25 bar

≡101.325 kPa

≡1013.25 millibars (mbar, also mb) or hectopascals (hPa)

≡760 torr

≈1.033 227 452 799 886 kg-force/cm²

≈1.033 227 452 799 886 technical atmosphere

≈1033.227 452 799 886 cm–H₂O, 4 °C ^[A]

≈406.782 461 732 2385 in–H₂O, 4 °C ^[A]

≈760.001 mm-Hg, 0 °C, subject to revision as more precise measurements of mercury’s density become available ^{[B, C]}

≈29.9213 in-Hg, 0 °C, subject to revision as more precise measurements of mercury’s density become available ^[C]

≈14.695 948 775 5134 pounds-force per square inch (psi)

≈2116.216 623 673 94 pounds-force per square foot (psf)

^A This is the customarily-accepted value for cm–H₂O, 4 °C. It is precisely the product of of 1 kg-force per square centimeter (one technical atmosphere) times 1.013 25 (bar/atmosphere) divided by 0.980 665 (one gram-force). It is not accepted practice to define the value for water column based on a true physical realization of water (which would be 99.997 495% of this value because the true maximum density of Vienna Standard Mean Ocean Water is 0.999 974 95 kg/l at 3.984 °C). Also, this “physical realization” would still ignore the 8.285 cm–H₂O reduction (due to the vapor pressure over water @ 3.984 °C) that would actually occur in a true physical realization.

^B Torr and mm-Hg, 0 °C are often taken to be identical. For most practical purposes (to 5 significant digits), they are interchangeable.

^C NIST value of 13.595 078(5) g/ml assumed for the density of Hg @ 0 °C

One standard atmosphere is standard pressure used for pneumatic fluid power (ISO R554), and in the aerospace (ISO 2533) and petroleum (ISO 5024) industries.

In 1985, 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 (≅750.062 torr) rather than the 101.325 kPa value of “one standard atmosphere”. This value is used as the standard pressure for the compressor and the pneumatic tool industries (ISO 2787). [1] (see also Standard temperature and pressure)

In the United States, compressed air flow is often measured in "standard cubic feet" per unit of time, where the "standard" means the equivalent quantity of air at standard temperature and pressure. However, this standard atmosphere is defined slightly differently: temperature = 68 °F (20 °C), air density = 0.075 lb/ft³ (1.20 kg/m³), altitude = sea level, and relative humidity = 0%. In the air conditioning industry, the standard is often temperature = 32 °F (0 °C) instead. For natural gas, the petroleum industry uses a standard temperature of 60 °F (15.6 °C).

Mean sea level pressure (MSLP or QFF)

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.

This is the pressure normally given in weather reports on radio, television, and newspapers. 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).

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 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 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 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.

Average sea-level pressure is 1013.25 hPa (mbar) or 29.921 inches of mercury (inHg). In aviation weather reports (METAR), QNH is transmitted around the world in millibars or hectopascals, except in the United States and Canada where it is reported in inches (or hundredths of inches) of mercury. (The United States also reports 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. In Canada's public weather reports, sea level pressure is reported in kilopascals [2], while Environment Canada's standard unit of pressure is the same [3] [4].) 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 1032.0 mbar. The lowest measurable sea-level pressure is found at the centers of hurricanes (typhoons, baguios).

Altitude Atmospheric Pressure Variation

Pressure changes quickly but smoothly between the earth's surface and outerspace. The following is a list of air pressures (fraction of one atmosphere) with the corresponding average altitude. To find the pressure (in kPa or % atm) at any altitude (in feet or meters), you can use the eXtreme High Altitude Calculator

Local Atmospheric Pressure variation

Atmospheric pressure varies widely on Earth, and these variations are important in studying weather and climate. See pressure system for the effects of air pressure variations on weather..

The highest recorded atmospheric pressure, 108.6 kPa (1086 mbar or 32.06 inches of mercury), occurred at Tosontsengel, Mongolia, 19 December 2001².

The lowest recorded non-tornadic atmospheric pressure, 87.0 kPa (870 mbar or 25.69 inHg), occurred in the Western Pacific during Typhoon Tip on 12 October 1979². The record for the Atlantic ocean was 88.2 kPa (882 mbar or 26.04 inHg) during Hurricane Wilma on 19 October 2005.

Atmospheric pressure shows a diurnal (twice-daily) cycle caused by the tides. This effect is very strong in tropical zones, and almost zero in polar areas. A graph shows these rhythmic variations in northern Europe on the top of this page. In tropical zones it may reach above 5 mbar variation. These variations follow a circadian (24 h) and at the same time semi-circadian (12 h) rhythm.

Intuitive feeling for 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.

One atmosphere (101.325 kPa or 14.7 lbf/in²) is the amount of pressure that can lift water approximately 10.3 m (33.9 feet). Thus, a diver at a depth 10.3 metres 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).

In terms of city water pressure, one atmosphere is approximately one-half to one-fifth the pressure of typical city water mains (i.e., water pressure is around 2 to 5 atmospheres).

See also

• plenum
• NRLMSISE-00
• Barometric formula
• International Standard Atmosphere - a tabulation of typical variation of principal thermodynamic variables of the atmosphere (pressure, density, temperature etc) with altitude, at mid latitudes.

References

1. US Department of Defense Military Standard 810E
2. Burt, Christopher C., (2004). Extreme Weather, A Guide & Record Book. W. W. Norton & Company ISBN 0-393-32658-6
3. U.S. Standard Atmosphere, 1962, U.S. Government Printing Office, Washington, D.C., 1962.
4. U.S. Standard Atmosphere, 1976, U.S. Government Printing Office, Washington, D.C., 1976.

External links

• Encyclopedia of Earth: Atmospheric pressure
• NASA page on the 1976 Standard Atmosphere
• Source code and equations for the 1976 Standard Atmosphere
• A mathematical model of the 1976 U.S. Standard Atmosphere
• Online calculator for the 1976 Standard Atmosphere
• Calculator using multiple units and properties for the 1976 Standard Atmosphere

Experiments

• An exercise in air pressure
• Movies on atmospheric pressure experiments from Georgia State University's HyperPhysics website - requires QuickTime