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Encyclopedia > Sea level pressure

Atmospheric pressure is the pressure caused by the weight of air above any area in the Earth's atmosphere. Standard atmospheric pressure (atm) is discussed in the next section.


Air masses are affected by the general atmospheric pressure within the mass, creating areas of high and low pressure.


As elevation increases, fewer air molecules are above. Therefore, atmospheric pressure decreases with increasing height. 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 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 m2 column of air would weigh about 100 kilonewtons. See density of air.

Contents

Standard atmospheric pressure

Standard atmospheric pressure or "the standard atmosphere" (1 atm) is defined as 101.325 kilopascals (kPa). (see also Standard temperature and pressure)


This can also be stated as:

This "standard pressure" is a purely arbitrary representative value for pressure at sea level, and real atmospheric pressures vary from place to place and moment to moment everywhere in the world.


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 = 68F (20C), air density = 0.075 lb/cu ft, altitude = sea level, and relative humidity = 0%. In the air conditioning industry, the standard is often temperature = 32F (0C) instead. For natural gas, the petroleum industry uses a standard temperature of 60F (15.6C).


Mean sea level pressure (MSLP or SLP)

Mean sea level pressure (MSLP or SLP) is the pressure at sea level or (when measured at a given height on land) the station pressure reduced to sea level by an appropriate formula.


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.


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, with two meanings either QNH or QFE (maybe one of these is MSLP above), 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 altitude above mean sea level in the vicinity of an airfield.


QFE barometric altimeter setting which will cause an altimeter to read height above a particular runway threshold.


Atmospheric pressure variation

Atmospheric pressure varies widely on the 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, 1085.7 millibars (32.06 inches of mercury), occurred at Tonsontsengel, Mongolia, 19 December 20012.


The lowest recorded non- tornadic atmospheric pressure, 869.96 millibars (25.69 inches of mercury), occurred in the Western Pacific during Typhoon Tip on 12 October 19792.


Intuitive feeling for atmospheric pressure based on height of water

Atmospheric pressure is often measured with a mercury barometer, and a height of approximately 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.


1 atmosphere (14.7 PSI) is the amount of pressure that can lift water approximately 33.90 feet (approx. 10.333 meters). Thus when dumping water on someone from a height of approximately 10 meters (or, equivalently, when someone enters a pool from a 10-meter platform as is commonly found in olympic swimming pools), the potential energy approximately equivalent to water at a pressure of one atmosphere. At this height of water, the person and water collide at slightly more than 50 km/h (slightly more than 30 mi/h), as potential energy is converted to kinetic energy. Thus, many swimmers wear ear plugs to avoid barotrauma from water pressure transiently (surge at impact) in the one atmosphere range, where there is little or no time to equalize the ears as can be done when scuba diving.


In terms of city water pressure, one atmosphere is approximately one half to one quarter the pressure of typical city water mains (i.e. water pressure is around 2 to 4 atmospheres).


See also

References

  1. US Department of Defense Military Standard 810E
  2. Burt, Christopher C., (2004). Extreme Weather, A Guide & Record Book. W. W. Norton & Company

External links

  • An exercise in air pressure (http://avc.comm.nsdlib.org/cgi-bin/wiki_print.pl?)

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