What, if anything, happens to the relative humidity as the temperature warms up during the day?

As the temperature increases during the day, the relative humidity typically decreases. Relative humidity is a measure of the amount of moisture in the air compared to the maximum amount of moisture the air can hold at a given temperature. Warmer air can hold more water vapor than cooler air. As the temperature rises, the capacity for the air to hold moisture increases, but unless additional moisture is introduced into the air, the actual amount of moisture remains the same, leading to a lower relative humidity percentage.

This principle is crucial in understanding how atmospheric conditions can affect fire behavior, as lower relative humidity can contribute to increased fire spread and intensity. It is common in weather patterns that as the day heats up, and without a corresponding increase in moisture, the relative humidity will decline. This phenomenon is particularly relevant for firefighters who must consider weather conditions when planning and executing operations.