A fire on a slope spreads uphill much faster than on flat ground, and the reason is geometric rather than chemical. Hot gases and flames rise, so on rising ground the flame tilts towards the slope surface and comes physically closer to the fuel above it, which radiation and convection then preheat and dry before the flame arrives. The steeper the slope the smaller the angle between flame and fuel and the stronger the effect; a common field rule of thumb is that spread rate roughly doubles for each \( 10^{\circ} \) of slope in the range fires usually run, and downslope spread is correspondingly slow and often backing. Slope and wind combine when aligned, and can cancel when opposed. The confusion this resolves is treating slope as a separate force from wind — both act through the same mechanism, tilting the flame towards or away from unburned fuel. After this Concept you can predict which way a fire on a hillside will run fastest and explain the mechanism in terms of flame geometry.
Wildfire rate of spread increases nonlinearly with upslope grade and with wind speed: on a 10-degree upward slope a fire spreads twice as fast as on flat ground under the same conditions, and on a 20…