What assumption is made regarding temperature above 36,090 feet in the ISA?

In the International Standard Atmosphere (ISA), the assumption made about temperature above 36,090 feet, also known as the tropopause, is that it remains constant. This is a fundamental aspect of the ISA model, which is used to standardize atmospheric parameters for aviation and meteorological purposes.

Above the tropopause, the temperature stabilizes and generally holds at approximately -56.5 degrees Celsius. This assumption simplifies calculations and provides a consistent reference for understanding atmospheric behavior at high altitudes. It contrasts with the behavior in layers below the tropopause, where temperature typically decreases with altitude. By maintaining a constant temperature at these high altitudes, the ISA model allows for clearer predictions related to flight dynamics and weather patterns.