Standard Atmosphere

The International Standard Atmosphere (ISA) is a reference model of how atmospheric pressure, temperature, density, and viscosity vary with altitude under idealized average conditions. It provides a common baseline for aircraft performance calculations, altimeter calibration, and airspace design.

ISA conditions at sea level:

• Temperature: 15°C (59°F)
• Pressure: 1013.25 hPa (29.92 inHg)
• Density: 1.225 kg/m³
• Lapse rate: Temperature decreases by 2°C per 1,000 feet up to 36,089 feet (the tropopause), above which temperature remains constant at -56.5°C through the stratosphere

Real atmospheric conditions almost never match ISA exactly. Deviations are expressed as ISA+ or ISA- (e.g., ISA+15 means 15°C warmer than standard). Aircraft performance data in the AFM is calculated for ISA conditions, with corrections provided for non-standard temperatures. Understanding ISA deviations is essential for accurate performance planning — hot days (ISA+) degrade performance while cold days (ISA-) enhance it.