From Academic Kids
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 altitude. 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.
Standard atmospheric pressure
Standard atmospheric pressure or "the standard atmosphere" (1 atm) is defined as 101.325 kilopascals (kPa). 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 = 68?F (20?C), air density = 0.075 lb/ft³, 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 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 altitude dependant formula. Ships at sea and low-lying weather stations must add (rather than subtract) a small correction to their barometric pressure readings (called "station pressure") to obtain the SLP.
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.
Average sea-level pressure is 29.921 inches of mercury (inHg) or 1013.2 mbar. 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 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.) 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).
Atmospheric pressure variation
Atmospheric pressure shows a diurnal (daily) rhythm. This effect is very strong in tropical zones, and almost zero in polar areas. A graph shows these rhymic variations in northern Europe on the top of this page. In tropical zones it may reach up + 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 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 lbf/in²) is the amount of pressure that can lift water approximately 33.9 feet (10.3 m). 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 is 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).