What is the primary need for the International Standard Atmosphere (ISA)?

The primary need for the International Standard Atmosphere (ISA) is to establish a consistent set of average atmospheric conditions that can be used for performance predictions. The ISA provides a reference model of the atmosphere's temperature, pressure, and density at various altitudes, which is essential for various applications in aviation and meteorology. This standardization allows pilots, engineers, and meteorologists to make reliable calculations regarding aircraft performance, such as lift, drag, and engine efficiency, under consistent atmospheric conditions. By having a defined baseline, users can compare their specific conditions against these averages to assess how variations in the atmosphere may impact their operations or predictions. This serves as a fundamental basis for safety and effectiveness in aviation and related fields.