International Standard Atmosphere
International Civil Aviation Organisation (ICAO) defines the sea-level International Standard Atmosphere (ISA) as 101.325 kPa (29.92 inHg) of pressure, 15°C of temperature and 0% humidity. These values provide a reference for calculating various aircraft performance figures, such as endurance, range, airspeed, and fuel consumption. When used to calculate performance at any pressure altitude other than sea level, the temperature is adjusted using the prescribed dry adiabatic lapse rates (which are -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)
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)
In chemistry, the term standard temperature and pressure (abbreviated STP) denotes 0°C (273.15 K) and a pressure of 1 atm (defined as 101.325 kPa).
The term Standard Ambient Temperature and Pressure (abbreviated SATP) denotes a reference temperature of 25°C (298.15 K) and a pressure of 100 kPa.
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