Fire Behaviour Quick Notes

A field-reference summary of wildfire behaviour fundamentals. For detailed predictions, refer to BCWS S-100/S-185 course material and the Canadian Forest Fire Danger Rating System (CFFDRS) manuals (see 08_Reference/Links_to_Download_When_Online).

The Fire Behaviour Triangle

Fire behaviour is driven by the interaction of three factors. The only one firefighters can realistically influence is fuel (through fuel management, fireguards, and removing fuel ahead of the fire).

1. Fuels — type, amount, size, arrangement, moisture content, and continuity (horizontal and vertical).
2. Weather — temperature, relative humidity, wind speed/direction, and precipitation. The single biggest driver of changes in fire behaviour, especially wind.
3. Topography — slope, aspect, and terrain features (chimneys, saddles, narrow draws) that channel wind and preheat fuels upslope.

Rule of thumb: Fire spread roughly doubles for every 10° increase in slope, and fires generally spread faster upslope and with the wind.

BC Fuel Types (FBP System — Common Local Types)

The Joe Rich area is predominantly Interior Douglas-fir / Ponderosa Pine (C-7), lodgepole pine (C-3/C-4), mixedwood (M-1/M-2), and grass (O-1a/O-1b) in open range/field areas — all of which can carry fast-moving surface and grass fires, especially in spring and late summer/fall cure.

Weather Indicators to Watch

• Relative humidity below ~30%, especially combined with high temperatures, dramatically increases fire activity and ignition probability.
• Wind — sustained wind over ~20–30 km/h significantly increases rate of spread and spotting potential. Watch for wind shifts (e.g., afternoon thermal/valley winds, frontal passages, thunderstorm outflows).
• Diurnal cycle — fire activity typically peaks mid-afternoon to early evening and decreases overnight as RH recovers and temperatures drop, but dry, windy nights can still carry fire.
• Drought / fuel moisture — cumulative dry weather (high Drought Code/Buildup Index) means deeper duff layers and larger fuels are available to burn, leading to more resistant, long-duration fires and difficult mop-up.
• Thunderstorms — can bring lightning ignitions and erratic outflow winds that rapidly change fire direction and intensity.

Fire Behaviour Categories (General)

• Surface fire — Burns surface litter, duff, and low vegetation. Most common and usually the most manageable.
• Crown fire — Burns through tree canopies; can be:
  • Passive (intermittent torching of individual trees/groups)
  • Active (continuous flame front through the crowns, dependent on surface fire)
  • Independent (advances through crowns regardless of surface fire — extremely dangerous and fast)
• Spotting — Embers lofted ahead of the main fire starting new spot fires; distance increases with wind speed, fire intensity, and bark/fuel type (e.g., loose bark species). Long-range spotting (>1 km) is possible in extreme conditions.

Visual Indicators of Increasing Fire Activity / Danger

• Increasing flame lengths and rate of spread
• Increased spotting and ember production
• Smoke column transitioning from white/grey to dark grey/black and standing more vertically, or suddenly collapsing (can indicate wind shift or downdraft)
• Frequent torching/candling of individual trees
• Erratic fire behaviour or fire "running" uphill or with wind shifts
• Pyrocumulus development (fire-generated cloud) — sign of extreme fire behaviour and potential for fire-generated winds

Quick Safety Reminders

• If fire behaviour is exceeding your crew's training, equipment, or the assigned tactics — stop, reassess, and back out. Re-establish LCES (Lookouts, Communications, Escape Routes, Safety Zones) at every change of plan, terrain, or conditions.
• See 01_Wildfire/Operations/Wildfire_Response_Checklist.md for the full safety checklist (LCES, 10 Standard Orders, 18 Watch Out Situations).