What primarily influences fire spread on slopes?

The degree of incline of the hillside is the primary factor influencing fire spread on slopes due to the physics of fire behavior. As a fire burns uphill, the flames reach the unburned fuel ahead more quickly because of the natural convection currents created by the heat rising. This results in the fire spreading faster on steeper slopes compared to gentler inclines.

When a slope increases, the heat from the flames rises rapidly, preheating the fuels above and causing them to ignite sooner. Effective fire spread can be significantly affected by how steep the slope is; therefore, understanding the topography is crucial for predicting fire behavior and making firefighting decisions.

While moisture content, type of vegetation, and ambient temperature are all important in influencing fire behavior and its intensity, none have the direct and immediate impact on fire spread as the degree of incline does. Each of these other factors contributes to the overall fire environment but does not influence the mechanics of how quickly a fire will move upslope.