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Continuum mechanics 

Atmospheric pressure is the force per unit area exerted on a surface by the weight of air above that surface in the atmosphere of Earth (or that of another planet). In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. On a given plane, lowpressure areas have less atmospheric mass above their location, whereas highpressure areas have more atmospheric mass above their location. Likewise, as elevation increases, there is less overlying atmospheric mass, so that atmospheric pressure decreases with increasing elevation. On average, a column of air one square centimeter in crosssection, measured from sea level to the top of the atmosphere, has a mass of about 1.03 kg and weight of about 10.1 N (2.28 lb_{f}) (A column one square inch in crosssection would have a weight of about 14.7 lbs, or about 65.4 N).
The standard atmosphere (symbol: atm) is a unit of pressure equal to 101325 Pa^{[1]} or 1013.25 millibars or hectopascals. It is equivalent to 760 mmHg (torr), 29.92 inHg, 14.696 psi.
The Pascal unit is derived from Newton per squaremeter. However also Newton is derived, from kilogram meter per squaresecond. Hence, by the use of pure SI units only, the value of standard athmosperic pressure equals 101325 kg/(s^{2} m), "kilogram per squaresecond metre". By using the prefix for 10, deka or da instead of kilogram, we'll get 1013.25 dag/(s^{2} m) , which equals the same value in millibar or hektopascal. 1013.25 "dekagram per squaresecond meter". When physicians use math, then it's usually written as 101325 m ^{1}kg s^{2}. But by replacing kilogram (kg) with dekagram (dag), we see that the millibar and haktopascal units equal the same value in dekagram per squaresecond metre:
^{2} ^{[2]} Please note that the basic SIunit for mass is the kilogram, kg. However 1 kg = 10 hg = 100 dag = 1000 g = 1000000 mg etc, and for each SIunit only one prefix can be used. In the case of the kilogram the prefix is already in use, so to speak. Hence for instance prefix deci or centi cannot be used on the kilogram like 1 kilogram = 100 centikilogram insted gram is "treated" as the basic unit, and 1 hektogram = 100 gram or 0.1 kilogram and 1 dekagram = 10 gram or 0.01 kilogram.
The mean sea level pressure (MSLP) is the atmospheric pressure at sea level or (when measured at a given elevation on land) the station pressure reduced to sea level assuming that the temperature falls at a lapse rate of 6.5 K per km in the fictive layer of air between the station and sea level.
This is the atmospheric 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 atmospheric pressure is the same for everyone. The pressures that 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.
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 sealevel pressure is 101.325 kPa (1013.25 mbar, or hPa) or 29.92 inches (inHg) or 760 millimetres of mercury (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, Canada, and Colombia where it is reported in inches (to two decimal places) 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,^{[4]} while Environment Canada's standard unit of pressure is the same.^{[5]}^{[6]})
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 sealevel pressure on Earth occurs in Siberia, where the Siberian High often attains a sealevel pressure above 1050.0 mbar (105.00 kPa, 30.01 inHg), with record highs close to 1085.0 mbar (108.50 kPa, 32.04 inHg). The lowest measurable sealevel pressure is found at the centers of tropical cyclones and tornadoes, with a record low of 870 mbar (87 kPa) (see Atmospheric pressure records).
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 yearround. As altitude increases, atmospheric pressure decreases. One can calculate the atmospheric pressure at a given altitude.^{[7]} 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 °C and a relative humidity of 0%.
At low altitudes above the sea level, the pressure decreases by about 1.2 kPa for every 100 meters. For higher altitudes within the troposphere, the following equation (the Barometric formula) relates atmospheric pressure p to altitude h
where the constant parameters are as described below:
Parameter  Description  Value 

p_{0}  sea level standard atmospheric pressure  101325 Pa 
L  temperature lapse rate, = g/c_{p} for dry air  0.0065 K/m 
c_{p}  constant pressure specific heat  ~ 1007 J/(kg•K) 
T_{0}  sea level standard temperature  288.15 K 
g  Earthsurface gravitational acceleration  9.80665 m/s^{2} 
M  molar mass of dry air  0.0289644 kg/mol 
R  universal gas constant  8.31447 J/(mol•K) 
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 or semidiurnal (twicedaily) 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 semicircadian (12 h) cycle.
The highest adjustedtosea level barometric pressure ever recorded on Earth (above 750 meters) was 1,085.7 hectopascals (32.06 inHg) measured in Tosontsengel, Mongolia on 19 December 2001.^{[8]} The highest adjustedtosea level barometric pressure ever recorded (below 750 meters) was at Agata, Evenhiyskiy, Russia [66°53’N, 93°28’E, elevation: 261 m (856.3 ft)] on 31 December 1968 of 1,083.3 hectopascals (31.99 inHg).^{[9]} The discrimination is due to the problematic assumptions (assuming a standard lapse rate) associated with reduction of sea level from high elevations.^{[8]} The lowest nontornadic atmospheric pressure ever measured was 870 hPa (25.69 inHg), set on 12 October 1979, during Typhoon Tip in the western Pacific Ocean. The measurement was based on an instrumental observation made from a reconnaissance aircraft.^{[10]} The normal high barometric pressure at the Dead Sea, as measured by a standard mercury manometer and blood gas analyzer, was found to be 799 mmHg (1065 hPa).^{[11]}
Atmospheric pressure is often measured with a mercury barometer, and a height of approximately 760mm (29.9213in) of mercury is often used to illustrate (and measure) atmospheric pressure. However, since mercury is not a familiar substance to most people, water may provide a more intuitive way to visualize the pressure of one atmosphere.
One atmosphere (101 kPa or 14.7 psi) is the pressure caused by the weight of a column of fresh water of approximately 10.3m (33.79265ft). 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 the maximum height to which a column of water can be drawn up by suction at atmospheric pressure.
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 gasusing residential appliance is rated for a maximum of 14 w.c., which is approximately 35 hPa.
In general, nonprofessional barometers are aneroid barometers or strain gauge based. See pressure measurement for a description of barometers.
Clean fresh water boils at about 100 °C (212 °F) 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.^{[12]} Because of this, the boiling point of water is lower at lower pressure and higher at higher pressure. This is why cooking at elevations more than 1,100 m (3,600 ft) above sea level requires adjustments to recipes.^{[13]} A rough approximation of elevation can be obtained by measuring the temperature at which water boils; in the mid19th century, this method was used by explorers.^{[14]}
An important application of the knowledge that atmospheric pressure varies directly with altitude and the availability of reliable pressure measurement devices was in determining the height of hills and mountains. While in 1774 Maskelyne was confirming Newton's theory of gravitation at and on Schiehallion in Scotland (using plumb bob deviation to show the effect of "gravity") and accurately measure elevation, William Roy using barometric pressure was able to confirm his height determinations, the agreement being to within one meter (3.28084 feet). This was then a useful tool for survey work and map making and long has continued to be useful. It was part of the "application of science" which gave practical people the insight that applied science could easily and relatively cheaply be "useful".^{[15]}
