What increases air density as altitude decreases?

The correct answer is gravity. As altitude decreases, gravity exerts a stronger force on air molecules, bringing them closer together. This increase in the density of air occurs because the weight of the air above compresses the lower layers more significantly, resulting in higher air density at lower elevations. The gravitational pull effectively pulls more air molecules closer to the Earth, leading to a more densely packed atmosphere as altitude decreases.

While temperature, humidity, and pressure also influence air density, they do so in different contexts. For instance, as temperature increases, air tends to expand and become less dense, while increased humidity can decrease density due to the lighter nature of water vapor compared to nitrogen and oxygen. Pressure, though inversely related to altitude, is not an independent factor in increasing density but rather a result of how gravity compresses the air. Thus, gravity is the key factor that directly influences air density as altitude decreases.