Which factor affects density altitude by decreasing air density as altitude increases?

The concept of density altitude is critical for understanding how aircraft perform at different altitudes. Density altitude is the pressure altitude corrected for non-standard temperature and humidity. As altitude increases, the air becomes less dense; therefore, density altitude increases. This means that aircraft performances—such as lift, engine power, and propeller efficiency—diminish at higher altitudes due to the reduced air density.

In this case, the choice indicating altitude as a factor is related to the intrinsic nature of how air density changes with elevation. As you gain altitude, the atmospheric pressure drops and, consequently, the density of the air decreases. This reduction in air density can impact various aspects of flight, including takeoff requirements and climb rates.

While other factors like humidity and temperature also influence air density, the question specifically asks for the primary factor that directly correlates with increasing altitude causing a decrease in air density. Wind speed, on the other hand, does not significantly affect density itself but can influence aircraft performance in other ways, such as ground speed and control effectiveness. Thus, the choice related to altitude accurately identifies the foundational characteristic affecting air density as elevation changes.