What happens to relative humidity as the temperature cools off toward evening?

As the temperature cools toward evening, the relative humidity tends to increase. This occurs because relative humidity is the ratio of the current amount of water vapor in the air to the maximum amount of water vapor the air can hold at a given temperature. Cooler air holds less moisture than warmer air. Therefore, as temperatures drop, the air reaches a point where it can hold less water vapor, leading to an increase in relative humidity.

As the temperature decreases, if the amount of moisture in the air remains constant, the relative humidity will rise because the air is now closer to saturation—the point at which it can no longer hold additional moisture. This increase in relative humidity can often lead to the formation of dew or fog, as the air may become saturated with moisture. Understanding this relationship is crucial for predicting weather changes and for various firefighting operations where humidity can greatly affect fire behavior.