Slope / Topography Vectors36.7° NW
Arrows point downslope — the direction fire and embers accelerate toward the structure. Steeper slope = faster spread, longer flames, longer ember throw.
This parcel · Extreme · chimney effect~17.7× ROS
Convective column attaches to the slope. Flames double in length, embers launch upslope at the structure. Standard 100 ft defensible space is insufficient — Zone 2 must extend to 200 ft on the downhill side.
Arrow count, length & fan width scale with slope. ROS = rate of spread vs. flat ground (Rothermel surface-fire model).