Atmospheric pressure, density, temperature, and lapse rate vary significantly and are constantly changing across different parts of the globe. To provide a consistent reference, the International Civil Aviation Organization (ICAO) has defined a standard average atmosphere known as the International Standard Atmosphere (ISA). This model represents atmospheric conditions from just below mean sea level up to 32 km. At mean sea level, ISA specifies the following conditions:

• MSL pressure: 1013.25 hPa
• MSL temperature: 15°C
• MSL density: 1.225 kg/m³
• MSL temperature lapse rate: 0.65°C per 100 m or 1.98°C per 1000 ft up to 11 km, where the temperature decreases to -56.5°C
• The tropopause is at 11 km (36 000 ft), with a temperature of -56.5°C
• From 11 km to 20 km, the temperature lapse rate is zero (isothermal), remaining at -56.5°C