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Pressure as a function of the height above the sea level. There are two equations for computing pressure as a function of height. The first equation is applicable to the atmospheric layers in which the temperature is assumed to vary with altitude at a non null lapse rate of : = [,, ()] ′, The second equation is applicable to the atmospheric layers in which the temperature is assumed not to ...
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 ]
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When atmospheric pressure is measured by a barometer, the pressure is also referred to as the "barometric pressure". Assume a barometer with a cross-sectional area A, a height h, filled with mercury from the bottom at Point B to the top at Point C. The pressure at the bottom of the barometer, Point B, is equal to the atmospheric pressure.
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]
The basic assumptions made for the 1962 version were: [3] air is a clean, dry, perfect gas mixture (c p /c v = 1.40) molecular weight to 90 km of 28.9644 (C-12 scale); principal sea-level constituents are assumed to be (in mole percent):
The word bar has its origin in the Ancient Greek word βάρος (baros), meaning weight. The unit's official symbol is bar ; [ citation needed ] the earlier symbol b is now deprecated and conflicts with the uses of b denoting the unit barn or bit , but it is still encountered, especially as mb (rather than the proper mbar ) to denote the millibar.
The pilot may choose to reduce the gross weight of the aircraft by carrying less fuel or reducing the amount/weight of the cargo, or even reducing the number of passengers (usually the last option). Not carrying sufficient fuel to complete the flight to the destination, the pilot would plan an intermediate fuel stop, which would likely delay ...