ISA Temperature Lapse

This formula gives the International Standard Atmosphere (ISA) temperature at a given altitude in the lower atmosphere, where air temperature is modeled to fall almost linearly with height. The constant 15 represents the standard sea-level temperature in °C, and the factor 1.98 converts altitude from feet into an approximate temperature drop of about 2°C per 1000 ft. It is an engineering approximation derived from the standard lapse rate used in aviation performance calculations, not a local weather forecast.

Use it when you need the ISA temperature at a particular altitude for flight planning, performance calculations, density-altitude work, or comparing actual conditions with standard atmosphere values. It is most appropriate below the tropopause, where the standard lapse-rate model applies.

• Using altitude in metres instead of feet without converting it first
• Treating the result as actual outside air temperature rather than the standard atmosphere value
• Applying the formula above the tropopause, where the temperature lapse no longer follows the same linear rate