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π International Standard Atmosphere (ISA)
The International Standard Atmosphere (ISA) is a theoretical model used as a baseline reference for atmospheric conditions in aviation. It defines a set of standard values for temperature, pressure, and lapse rates with altitude, allowing pilots, engineers, and meteorologists to compare real atmospheric conditions against a known reference. The ISA assumes a sea level temperature of +15Β°C, a sea level pressure (QNH) of 1013.25 hPa, and a temperature lapse rate of 1.98Β°C per 1,000 feet within the troposphere.
Under ISA conditions, pressure decreases at an approximate rate of 1 hPa per 30 feet, although this rate is non-linear with altitude and must be interpreted as an average near sea level. The ISA model also defines the tropopause at approximately 36,090 feet, where the temperature stabilises at β56Β°C and remains constant through the lower stratosphere.
For pilots, comparing actual flight conditions to ISA is crucial for performance calculations, especially when determining density altitude, true airspeed, and engine efficiency. Most aircraft performance charts are developed using ISA as the baseline, meaning any deviationβsuch as warmer-than-standard airβcan significantly affect climb performance, take-off distance, and fuel efficiency. ISA also serves as the reference for altimeter calibration, ensuring that pressure-based altitude indications are standardised worldwide.
Key Points:
ISA sea level temperature is +15Β°C.
ISA sea level pressure is 1013.25 hPa (hectopascals).
Temperature decreases at 1.98Β°C per 1,000 feet up to the tropopause.
Pressure decreases approximately 1 hPa per 30 feet near sea level (non-linear with altitude).
The tropopause occurs at 36,090 feet under ISA.
Temperature at the tropopause is fixed at β56Β°C.
ISA is used as the standard baseline for performance data, altimeter settings, and weather analysis.