What aspect is not included in the ISA definition?

The International Standard Atmosphere (ISA) is a model that represents the average conditions of the Earth's atmosphere, providing a reference for various atmospheric parameters. The definition of ISA typically includes the pressure, temperature, and density at different altitudes, which are essential for various aviation calculations and simulations.

Humidity, while it affects weather conditions and local atmospheric behavior, is not a part of the ISA definition. The ISA assumes a standard set of conditions that simplify calculations, which means it does not account for moisture content in the air. This makes the model more straightforward and widely applicable, particularly for aviation purposes where consistency and predictability are essential. Thus, moisture variations are excluded to avoid complicating the basic atmospheric parameters defined by the ISA.