What effect does altitude have on the density of air molecules?

At higher altitudes, the density of air molecules decreases. This occurs because as altitude increases, atmospheric pressure decreases, leading to a reduction in the number of air molecules in a given volume. The decrease in pressure means there is less force compressing the air molecules together, allowing them to spread out and occupy a larger volume.

Consequently, at higher elevations, the air is less dense, which can have important implications for various mechanical principles, such as buoyancy and lift in aviation. The understanding of how air density changes with altitude is crucial in fields like meteorology, aviation, and engineering, as it affects how aircraft perform and how we understand weather patterns.

The other options do not accurately reflect this relationship: density does not increase with altitude, is certainly affected by changes in altitude, and does not fluctuate randomly under normal atmospheric conditions. Thus, the correct response regarding the effect of altitude on air molecule density is that it decreases.