Atmospheric pressure

"Air pressure" redirects here. For the pressure of air in other systems, see pressure.

I think its gravity and stuff

Standard atmospheric pressure

The standard atmosphere (symbol: atm) is a unit of pressure and is defined as being equal to 101,325 Pa or 101.325 kPa. (Thermal engineering By Ajoy Kumar, G N Sah, 2004, P.40) These other units are equivalent: 760 mmHg (torr), 29.92 inHg, 14.696 PSI, 1013.25 millibars. 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 1999, the International Union of Pure and Applied Chemistry (IUPAC) recommended that for the purposes of specifying the 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”. This value is used as the standard pressure for the compressor and the pneumatic tool industries (ISO 2787). (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 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 (60.1 °F), pressure 101.56 kPa (14.730 psi). (air pressure)

Mean sea level pressure

15 year average mean sea level pressure for June, July, and August (top) and December, January, and February (bottom).

Mean sea level pressure (MSLP) 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.

Kollsman-type barometric aircraft altimeter as used in North America displaying an altitude of 80 ft (24 m).

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.

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.

Average sea-level pressure is 101.325 kPa (1013.25 mbar, or hPa) 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. 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 1087.0 mbar. The lowest measurable sea-level pressure is found at the centers of tropical cyclones.

Altitude atmospheric pressure variation

This plastic bottle was closed at approximately 2,000 m (6,600 ft) 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	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	86,282	283,076

Calculating variation with altitude

See also: Barometric formula

There are two different equations for computing the average pressure at various height regimes below 86 km (53 mi; 282,000 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:

${P}=P_{b}\cdot \left[{\frac {T_{b}}{T_{b}+L_{b}\cdot (h-h_{b})}}\right]^{\textstyle {\frac {g_{0}\cdot M}{R^{*}\cdot L_{b}}}}$

Equation 2:

${P}=P_{b}\cdot \exp \left[{\frac {-g_{0}\cdot M\cdot (h-h_{b})}{R^{*}\cdot T_{b}}}\right]$

where

$P_{b}$ = Static pressure (pascals, Pa)

$T_{b}$ = Standard temperature (kelvins, K)

$L_{b}$ = Standard temperature lapse rate (kelvins per meter, K•m^-1)

$h$ = Height above sea level (meters, m)

$h_{b}$ = Height at bottom of layer b (meters, e.g., $h_{1}$ = 11,000 m)

$R^{*}$ = Universal gas constant: 8.31432 N•m / (mol•K)

$g_{0}$ = Standard gravity (9.80665 m•s^-2)

$M$ = Molar mass of Earth's air (0.0289644 kg•mol^-1)

Or converted to Imperial units:

where

$P_{b}$ = Static pressure (inches of mercury, in Hg)

$T_{b}$ = Standard temperature (kelvins, K)

$L_{b}$ = Standard temperature lapse rate (kelvins per foot, K•ft^-1)

$h$ = Height above sea level (feet, ft)

$h_{b}$ = Height at bottom of layer b (feet, e.g., $h_{1}$ = 36,089 ft)

$R^{*}$ = Universal gas constant (using feet and kelvins and gram moles: 8.9494596×10⁴ kg•ft²•s^-2•K^-1•kmol^-1)

$g_{0}$ = Standard gravity (32.17405 ft•s^-2)

$M$ = Molar mass of Earth's air (0.0289644 kg•mol^-1)

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. The reference value for P_b for b = 0 is the defined sea level value, P₀ = 101325 pascals or 29.92126 inHg. Values of P_b 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 $h=h_{b+1}$.:

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

Local atmospheric pressure variation

Hurricane Wilma on 19 October 2005 – 88.2 kPa (12.79 psi) 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.

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 millimetres (30 in) of mercury is often used to 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 visualize the pressure of 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 (34 ft). Thus, a diver 10.3 m underwater experiences a pressure of about 2 atmospheres (1 atm of air plus 1 atm of water). This is also the maximum height to which a column of water can be drawn up by suction.

Low pressures such as natural gas lines are sometimes specified in inches of water, typically written as w.c (water column) or W.G (inches water gauge). A typical gas using residential appliance is rated for a maximum of 14 w.c. which is approximately 0.034 atmosphere.

Non-professional barometers are generally aneroid barometers or strain gauge based. See pressure measurement for a description of barometers.

Water's boiling point

Boiling water

By definition water boils at 100 °C (212 °F) at one atmosphere. The boiling point is the temperature at which the vapor pressure is equal to the atmospheric pressure around the water. Because of this, the boiling point of water is decreased in lower pressure and raised at higher pressure. This is why baking at elevations more than 3,500 ft (1,100 m) above sea level requires adjustments to recipes. 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.

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.
• Barotrauma 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.
• Subtropical high belts

References

• 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.

External links

• How Atmospheric Pressure Affects Objects (Audio slideshow from the National High Magnetic Field Laboratory)
• 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
• Calculator using multiple units and properties for the 1976 Standard Atmosphere
• Calculator giving standard air pressure at a specified altitude, or altitude at which a pressure would be standard
• Some of the effects or air pressure

Experiments

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