How does the relative humidity of an air mass behave as that air mass cools?

As an air mass cools, its relative humidity increases. This phenomenon occurs because relative humidity is defined as the amount of moisture present in the air relative to the maximum amount of moisture the air can hold at a given temperature. When the temperature of the air mass decreases, the capacity of the air to hold moisture also decreases, leading to a situation where the same absolute amount of moisture results in a higher relative humidity percentage.

For instance, if an air mass is warm and holds a significant amount of water vapor, when it cools down, it may reach the saturation point more quickly, causing the relative humidity to rise. If cooling continues beyond the dew point, condensation can occur, leading to phenomena such as fog or clouds, as the air cannot hold all of the moisture it contains in a liquid form.

The other options do not accurately describe the behavior of relative humidity with regards to cooling air. Relative humidity does not stay the same or decrease when air cools; it also does not exhibit a basic fluctuation without specific external influences acting upon it.