The ICAO Standard Atmosphere (ISA) is an agreed model of the atmosphere. Your altimeter and airspeed indicator are calibrated to it, and the performance tables in your flight manual use it.
At sea level the ISA is 15 °C and the pressure is 1013 hPa.
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Meteorology
The standard atmosphere (ISA)
The ICAO Standard Atmosphere (ISA) is an agreed model of the atmosphere. Your altimeter and airspeed indicator are calibrated to it, and the performance tables in your flight manual use it.
At sea level the ISA is 15 °C and the pressure is 1013 hPa.