What happens to relative humidity when air with a given amount of water vapor cools?

When air with a certain amount of water vapor cools, the relative humidity increases due to the relationship between temperature and water vapor capacity. Relative humidity is a measure of how much water vapor is present in the air compared to the maximum amount of water vapor the air can hold at a specific temperature.

As air cools, its ability to hold water vapor decreases. This means that the same amount of water vapor will make up a larger percentage of the total capacity of the air, resulting in higher relative humidity. When air reaches its dew point—the temperature at which it becomes saturated—the relative humidity reaches 100%, indicating the air can hold no more moisture, leading to the formation of dew or condensation.

Thus, the cooling of air directly causes an increase in relative humidity as the air approaches its saturation point. This foundational understanding of how temperature affects humidity is crucial in meteorology and weather forecasting.