Aero Academic
Aero Academic
A standard atmospheric model representing average pressure and temperature conditions at various altitudes.
The International Standard Atmosphere (ISA) is a highly defined hypothetical model of the Earth's atmosphere. It establishes a baseline against which actual atmospheric conditions can be compared. In the ISA model, the sea level pressure is defined as 1013.25 hPa (29.92 inHg) and the sea level temperature is 15°C (59°F).
The model dictates a standard temperature lapse rate: temperature decreases by 1.98°C (roughly 2°C) for every 1,000 feet of altitude gained up to the tropopause. By using this standard, aircraft manufacturers can publish performance charts that apply universally.
In exams, students frequently encounter questions asking them to calculate "ISA deviation" (e.g., ISA +10). If the actual temperature at a given altitude is warmer than the ISA temperature for that same altitude, the deviation is positive, meaning the air is less dense and performance will be degraded.