Pressure altitude
Pressure altitude within the atmosphere is the altitude in the International Standard Atmosphere with the same pressure as the part of the atmosphere in question.
The National Oceanic and Atmospheric Administration has published the following formula for converting directly from pressure in millibars to altitude in feet:
In aviation, pressure altitude is the indicated altitude when an altimeter is set to an agreed baseline pressure setting under certain circumstances where the aircraft's altimeter would be unable to give a useful readout of altitude. Examples would be landing at a very high altitude or near sea level in conditions of exceptionally high air pressure. Old altimeters were typically limited to show altitude when set between 950 mbar and 1030 mbar. "Standard" pressure, the baseline used universally, is 1013.25 hPa, equivalent to 1013.25 mbar or 29.92 inHg (inches of mercury). This setting is equivalent to the air pressure at mean sea level (MSL) in the International Standard Atmosphere (ISA). Pressure altitude is primarily used in aircraft performance calculations and in high-altitude flight (above the transition altitude).
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