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The standard atmosphere was originally defined as the pressure exerted by a 760 mm column of mercury at 0 °C (32 °F) and standard gravity (g n = 9.806 65 m/s 2). [2] It was used as a reference condition for physical and chemical properties, and the definition of the centigrade temperature scale set 100 °C as the boiling point of water at this pressure.
Atmospheric pressure, also known as air pressure or barometric pressure (after the barometer), is the pressure within the atmosphere of Earth.The standard atmosphere (symbol: atm) is a unit of pressure defined as 101,325 Pa (1,013.25 hPa), which is equivalent to 1,013.25 millibars, [1] 760 mm Hg, 29.9212 inches Hg, or 14.696 psi. [2]
The pressure of seawater at a depth of 33 feet equals one atmosphere. The absolute pressure at 33 feet depth in sea water is the sum of atmospheric and hydrostatic pressure for that depth, and is 66 fsw, or two atmospheres absolute. For every additional 33 feet of depth, another atmosphere of pressure accumulates. [6]
One bar is 100 kPa or approximately ambient pressure at sea level. Ambient pressure may in other circumstances be measured in pounds per square inch (psi) or in standard atmospheres (atm). The ambient pressure at sea level is approximately one atmosphere, which is equal to 1.01325 bars (14.6959 psi), which is close enough for bar and atm to be ...
Air pressure in an automobile tire relative to atmosphere (gauge pressure) [citation needed] +210 to +900 kPa +30 to +130 psi Air pressure in a bicycle tire relative to atmosphere (gauge pressure) [57] 300 kPa 50 psi Water pressure of a garden hose [58] 300 to 700 kPa 50–100 psi Typical water pressure of a municipal water supply in the US [59]
The International Civil Aviation Organization (ICAO) published their "ICAO Standard Atmosphere" as Doc 7488-CD in 1993. It has the same model as the ISA, but extends the altitude coverage to 80 kilometers (262,500 feet). [7] The ICAO Standard Atmosphere, like the ISA, does not contain water vapor. Some of the values defined by ICAO are:
According to the American National Center for Atmospheric Research, "The total mean mass of the atmosphere is 5.1480 × 10 18 kg with an annual range due to water vapor of 1.2 or 1.5 × 10 15 kg, depending on whether surface pressure or water vapor data are used; somewhat smaller than the previous estimate.
In aviation, pressure altitude is the height above a standard datum plane (SDP), which is a theoretical level where the weight of the atmosphere is 29.921 inches of mercury (1,013.2 mbar; 14.696 psi) as measured by a barometer. [2]