According to the ISA definition, what element is not included?

The correct answer is that humidity is not included in the ISA (International Standard Atmosphere) definition. The ISA is a model used for various aeronautical purposes, particularly aimed at establishing a standardized atmospheric condition at sea level for temperature, pressure, and altitude.

In this context, temperature is defined as 15 degrees Celsius, pressure is 1013.25 hPa (hectopascals), and altitude is referenced to mean sea level. Humidity, however, is not explicitly accounted for in the ISA standard, which is focused on the physical properties of the atmosphere relevant to aviation.

While humidity can affect aircraft performance and atmospheric behavior, it is not part of the fundamental parameters set forth by the ISA. This makes the inclusion of temperature, pressure, and altitude essential components of the model, as these elements provide critical data for calculations related to flight dynamics, such as air density and engine performance.

Thus, humidity's absence from the ISA definition highlights the framework's focus on temperature, pressure, and altitude, which are the primary variables affecting aviation operations.