When do down-valley winds typically occur?

Down-valley winds typically occur after midnight due to the cooling processes that take place during the night. As temperatures drop, the cooler air sinks down the valley, creating a flow that is generally denser and moves downhill. This phenomenon is primarily influenced by the earth's surface cooling, leading to variations in air density and pressure that contribute to the movement of air down the valley slopes.

Understanding the timing of down-valley winds is essential for predicting fire behavior in wildland firefighting. These winds can influence the spread of a fire by directing smoke and flames down the slopes, which may change the dynamics of fire behavior, especially in mountainous or hilly terrains. Thus, recognizing that down-valley winds are most prominent during the cooler nighttime hours helps in planning firefighting strategies and assessing fire risk.