What is the primary factor that affects the density of air at any altitude?

The primary factor affecting the density of air at any altitude is altitude itself. As altitude increases, the air pressure decreases, which directly influences air density. At higher altitudes, the weight of the air above decreases due to lower atmospheric pressure, leading to a decrease in air density.

While temperature, pressure, and humidity all impact air density, they are secondary factors influenced by altitude. For instance, at a given altitude, lower temperatures usually result in higher density, whereas higher humidity can decrease density. However, it is the increase in altitude that fundamentally drives the changes in pressure and temperature in the atmosphere. Thus, altitude is the key determinant affecting air density as one ascends through the atmosphere.