Atmospheric pressure

Diurnal (daily) rhythm of air pressure in northern Germany (black curve is 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. Low pressure areas have less atmospheric mass above their location, whereas high pressure areas have more atmospheric mass above their location. Similarly, as elevation increases there is less overlying atmospheric mass, so that pressure decreases with increasing elevation. 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 lbf. A 1 m (11 sq ft) column of air would weigh about 100 kilonewtons (equivalent to a mass of 10.2 tonnes at the surface).

Standard atmospheric pressure

The standard atmosphere (symbol: atm) is a unit of pressure and is defined as being precisely equal to 101.325 kPa. It is equivalent to 760 mmHg (torr) or 29.92 inHg. 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 1982, 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 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 = 0.0765 lb/cu ft (1.225 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), pressure 14.73 psia.

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.

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).

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 atmospheric pressure (Q) at nautical (N) height (H) or sea-level pressure. 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 101.325 kPa (1013.25 mbar) or 29.921 inches of mercury (inHg) or 760 millimeters (mmHg). In aviation weather reports (METAR), QNH is transmitted around the world in millibars or hectopascals (1 millibar = 1 hectopascal), except in the United States and in Canada where it is reported in inches (or hundredths of inches) of mercury. (The United States and Canada also report 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 [3]. However, in Canada's public weather reports, sea level pressure is instead reported in kilopascals [1], while Environment Canada's standard unit of pressure is the same [2] [3].) 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

This plastic bottle was closed at approximately 2000 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.

fraction of 1 atm (m) (ft) average altitude 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 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.

Equation 1:

Equation 2:

where
= Static pressure (pascals)
= Standard temperature (kelvins)
= Standard temperature lapse rate (kelvins per m)
= Height above sea level (meters)
= Universal gas constant: 8.31432×10³ N·m / (kmol·K)
= Gravitational constant (9.80665 m/s²)
= Molar mass of Earth's air (28.9644 g/mol)

Or converted to English units:[4]

where
= Static pressure (inches of mercury)
= Standard temperature (kelvins)
= Standard temperature lapse rate (kelvins per ft)
= Height above sea level (feet)
= Universal gas constant (using feet and kelvins and gram moles: 8.9494596×104 kg·sq ft·s-2·K-1·kmol-1)
= Gravitational constant (32.17405 ft/s²)
= 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. It should be noted that the values used for M, g0, and are in accordance with the U.S. Standard Atmosphere, 1976, and that the value for 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 .:[5]

Subscript b Height Above Sea Level Static Pressure Standard Temperature
(K)
Temperature Lapse Rate
(m) (ft) (pascals) (inHg) (K/m) (K/ft)
00010132529.92126288.15-0.0065-0.0019812
111,00036,08922632.16.683245216.650.00.0
220,00065,6175474.891.616734216.650.0010.0003048
332,000104,987868.0190.2563258228.650.00280.00085344
447,000154,199110.9060.0327506270.650.00.0
551,000167,32366.93890.01976704270.65-0.0028-0.00085344
671,000232,9403.956420.00116833214.65-0.002-0.0006096

Sample Calculation:

Find the 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.

Or

Pascals at 30,000 meters

Local atmospheric pressure variation

Hurricane Wilma on 19 October 2005 – 88.2 kPa in eye

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.

The highest recorded atmospheric pressure, 108.6 kPa (1,086 mbar or 32.06 inches of mercury), occurred at Tosontsengel, Khövsgöl Province, Mongolia, 19 December, 2001.2

The lowest recorded non-tornadic atmospheric pressure, 87.0 kPa (870 mbar or 25.69 inches of mercury), occurred in the Western Pacific during Typhoon Tip on 12 October, 1979.2 The record for the Atlantic ocean was 88.2 kPa (882 mbar or 26.04 inches of mercury) during Hurricane Wilma on 19 October 2005.

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. A graph on the top of this page shows these rhythmic variations in northern Europe. 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.

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.

Non-professional barometers are generally aneroid barometer (Figure 3) or strain gauge based. See Pressure measurement for a description of barometers.

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. ^ U.S. Standard Atmosphere, 1976, U.S. Government Printing Office, Washington, D.C., 1976. (Linked file is very large.)
* 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.

Experiments

Pressure (symbol: p) is the force per unit area applied on a surface in a direction perpendicular to that surface.

Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.
Earth's atmosphere is a layer of gases surrounding the planet Earth and retained by the Earth's gravity. It contains roughly (by molar content/volume) 78% nitrogen, 20.95% oxygen, 0.93% argon, 0.
Fluid pressure is the pressure at some point within a fluid, such as water or air.

Fluid pressure occurs in one of two situations:
1. an open condition, such as the ocean, a swimming pool, or the atmosphere; or
2. a closed condition, such as a water line or a gas line.

weight is a measurement of the gravitational force acting on an object. Near the surface of the Earth, the acceleration due to gravity is approximately constant; this means that an object's weight is roughly proportional to its mass.
Earth's atmosphere is a layer of gases surrounding the planet Earth and retained by the Earth's gravity. It contains roughly (by molar content/volume) 78% nitrogen, 20.95% oxygen, 0.93% argon, 0.
A pound or pound-force (abbreviations: lb, lbf, or lbf) is a unit of force. Pound is also the name of a unit of mass. One pound-force is approximately equal to the gravitational force exerted on a mass of one avoirdupois pound on the
The newton (symbol: N) is the SI derived unit of force, named after Sir Isaac Newton in recognition of his work on classical mechanics.

Definition

A newton
tonne (t) or metric ton (M/T), also referred to as a metric tonne, is a measurement of mass equal to 1,000 kilograms. It is not an SI unit but is accepted for use with the SI.
Standard atmosphere is a pressure defined as 101 325 Pa and used as unit of pressure (symbol: atm). Standard atmosphere is a non-SI unit that is internationally recognized.
Pressure (symbol: p) is the force per unit area applied on a surface in a direction perpendicular to that surface.

Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.
The pascal (symbol: Pa) is the SI derived unit of pressure or stress (also: Young's modulus and tensile strength). It is a measure of perpendicular force per unit area i.e. equivalent to one newton per square meter or one Joule per cubic meter.
torr (symbol: Torr) is a non-SI unit of pressure defined as 1/760 of an atmosphere. It was named after Evangelista Torricelli, an Italian physicist and mathematician who discovered the principle of the barometer in 1644.
The International Union of Pure and Applied Chemistry (IUPAC) (IPA: [aɪ ju pæk]) is an international non-governmental organization established in 1919 devoted to the advancement of chemistry.
torr (symbol: Torr) is a non-SI unit of pressure defined as 1/760 of an atmosphere. It was named after Evangelista Torricelli, an Italian physicist and mathematician who discovered the principle of the barometer in 1644.
In chemistry and other sciences, STP or standard temperature and pressure is a standard set of conditions for experimental measurements, to enable comparisons to be made between sets of data.
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The word isobar derives from the two ancient Greek words, ισος (isos), meaning 'equal', and βαρος (baros), meaning 'weight'.
QNH is a Q code. It is a pressure setting used by pilots, air traffic control (ATC) and low frequency weather beacons to refer to the barometric altimeter setting which will cause the altimeter to read altitude above mean sea level within a certain defined region.
QFE is a three letter acronym which can have meanings in aviation, in software development, and in internet usage. It can refer to
• QFE, a Q code used by pilots and air traffic control (ATC)
• A software development acronym for Quick Fix Engineering.

altimeter is an active instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth underwater.
The pascal (symbol: Pa) is the SI derived unit of pressure or stress (also: Young's modulus and tensile strength). It is a measure of perpendicular force per unit area i.e. equivalent to one newton per square meter or one Joule per cubic meter.
Metar.
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Meitar

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Parliamentary Secretary Mark Warawa
Established 1971

National Water Research Institute
Siberia (Russian: Сиби́рь, Sibir); is a vast region on the eastern and North-Eastern part of the Russian Federation constituting almost all of Northern Asia and comprising a large part of the