Which atmospheric condition generally lowers air density?

The correct response highlights that high humidity and elevation both contribute to a decrease in air density. When the altitude increases, the air becomes less dense because there are fewer air molecules per unit volume. As for humidity, its effect on air density stems from the fact that water vapor is lighter than the nitrogen and oxygen that dominate dry air. When humidity increases, the proportion of water vapor in the air rises, displacing heavier air molecules and effectively lowering the overall density of the air.

This combination of higher altitude, which inherently reduces density due to the lower pressure, and increased humidity—which replaces denser nitrogen and oxygen with lighter water vapor—typically results in a significant lowering of air density, making this answer the most accurate choice.

In contrast, the other options describe conditions that either have effects that are not aligned with decreasing air density or mix factors that would lead to an increase in density. For instance, high pressure and low altitude generally result in denser air, while decreased humidity and cold temperatures can lead to variations in density but do not typically produce lower density outcomes when considered in the broader context.