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In chemistry, the term standard temperature and pressure (abbreviated STP) denotes an exact reference temperature of 0° C (273.15 K) and pressure of 1 atm (defined as 101.325 kPa). These values approximate freezing temperature of water and atmospheric pressure at sea level.
Also in chemistry, the term Standard Ambient Temperature and Pressure (abbreviated SATP) denotes a reference temperature of 25° C (298.15 K) and pressure of 100 kPa. Although there are many variations of the definition, the most accepted one is the temperature and pressure where the equilibrium constant for the autoionization of water is 1.0x10-14.
The Army Standard Metro atmosphere, now used only in ballistics, defines sea-level conditions as 29.5275 inHg of pressure (99.9916 kPa), 59°F (15°C), and 78% humidity. (Ref: U.S. Army Ballistic Research Laboratory, U.S. Army Aberdeen Proving Ground)
The International Civil Aviation Organisation (ICAO) defines the sea-level International Standard Atmosphere (ISA) as 101.325 kPa, 15°C and 0% humidity. These values provide a reference for calculating various aircraft performance figures, such as endurance, range, airspeedAirspeed refers to the speed of an aircraft in relation to the air which may be different from groundspeed which is the speed of the aircraft in relation to the ground. The difference between airspeed and groundspeed is a function of the velocity and dire, and fuel consumption. When used to calculate performance at any pressure altitudePressure altitude an aviation term, is the indicated altitude when an altimeter is set to 1013 hPa (29. 92 inHg US and Canada). It is primarily used in aircraft performance calculations, and in high-altitude flight (Class A airspace in the United States). other than sea level, the temperature is adjusted using the prescribed lapse rate which is -6.5 °C/km for the first 11 km.
(Ref: Manual of the ICAO Standard Atmosphere (extended to 80 kilometres (262 500 feet)), Doc 7488 / Third Edition, 1993)