What happens to air as it cools down?

As air cools down, its molecules lose energy and come closer together, resulting in an increase in density. This is due to the fundamental principle of gas behavior, which indicates that temperature and density are inversely related in a closed system. The cooler air has less kinetic energy, which means the air molecules are less active and tend to occupy a smaller volume. Consequently, as the density of air increases, it can lead to various atmospheric effects, such as the sinking of cooler air in comparison to warmer air, which tends to rise due to its lower density.

The other options do not appropriately describe the behavior of air as it cools. Rising air indicates that it is warm and less dense, while expansion occurs as air is heated rather than cooled. Evaporation pertains to the transformation of liquids to gases and does not apply directly to the cooling of air itself. Thus, the correct understanding of how cooling affects air points directly to its increased density.