Standard Atmosphere
The Standard Atmosphere is a hypothetical model of the atmosphere that defines standard conditions of temperature, pressure, and density at each altitude. The International Standard Atmosphere (ISA) is defined by ICAO and serves as the baseline for all aircraft performance data, altimeter calibration, and engine ratings.
ISA Conditions
- Sea level temperature: 15°C (59°F)
- Sea level pressure: 29.92 inHg (1013.25 hPa)
- Temperature lapse rate: 2°C per 1,000 ft (1.98°C/1,000 ft, or 6.5°C/km) up to the tropopause (~36,000 ft)
- Tropopause temperature: −56.5°C (constant above the tropopause)
- Sea level density: 1.225 kg/m³
ISA Deviation
ISA deviation (ISA+X or ISA-X) describes how the actual temperature differs from ISA at a given altitude. “ISA+20” means the temperature is 20°C above standard. High positive ISA deviations reduce performance significantly — always check actual conditions against ISA when planning performance.
On the TC Written Exam
Know ISA sea level conditions (15°C, 29.92 inHg) and the standard lapse rate (2°C/1,000 ft). These are used in all density altitude, performance, and altimetry calculations.
Related Terms
See also: Density Altitude, Pressure Altitude, Altimeter.