Wildland Fire Behavior S-190 – Exam Review Notes

Parts of a fire: perimeter, point of origin, head (fastest spread), flanks, rear/heel (slow), fingers, pockets, islands, spot fires.
Suppression terms: anchor point, control line, fireline (to mineral soil), mop-up (final extinguish).
Status: contained → expected to hold; controlled → perimeter halt confirmed.
Key behavior terms: smoldering, creeping, running, spotting, torching, flare-up, fire whirl, backing, flaming front, crowning.
Fire triangle: oxygen, heat, fuel – remove one to stop fire.
Heat transfer: conduction (minor in wood), convection (rising hot gases), radiation (straight-line heating).

Six wildland fuel types: grass, shrub, grass-shrub, timber litter, timber-understory, slash-blowdown; plus artificial fuels.
Fuel loading = dry weight per area; high load ≠ always high intensity.
Chemical content (oils, resins, wax, pitch) raises spread/intensity.
Arrangement
- Horizontal: uniform vs patchy.
- Vertical: ground, surface, ladder, aerial fuels (ladder → crown fire potential).
Moisture
- Live vs dead fuels; live peaks mid-growing season, dead varies daily.
- Size classes: $0!\text{–}!\tfrac14\,\text{in},\; \tfrac14!\text{–}!1\,\text{in},\; 1!\text{–}!3\,\text{in},\; 3!\text{–}!8\,\text{in}$ .
- Timelag: $1\,\text{h},\;10\,\text{h},\;100\,\text{h},\;1000\,\text{h}$ for the above classes.

Dry-bulb = ambient air temp; wet-bulb lowest temp via evaporation; dew point = saturation temp (best humidity metric).
Relative humidity (RH) = actual / max moisture (percent). Temp ↑ ⇒ RH ↓.
Critical RH: western US ≈ $15\%$ ; Florida ≈ $30\text{–}35\%$ .
Daily cycle: mid-afternoon = max T, min RH; sunrise = min T, max RH.
Monitoring: fixed/remote automated weather stations (RAWS).
Influences: elevation, aspect (south/west hotter, drier), cloud cover, wind mixing, proximity to water.

Key features: canyons (narrow, box, wide), ridges (dominant, spur, flat, knife), saddles & chutes, natural/constructed barriers.
Slope: steeper ⇒ faster upslope spread (convective/radiant pre-heat). Grade = vertical / horizontal (expressed %).
Aspect: south / southwest = hotter, drier, lighter fuels; north = cooler, moister, heavier fuels.
Position on slope: lower slopes dry & ignite sooner; upper slopes cooler/moister.

Stability: parcel cooler/equal ⇒ stable; warmer ⇒ unstable.
- Stable signs: smoke layers, stratus/fog, low-intensity fire.
- Unstable signs: towering plumes, cumulus/Cb, gusty winds, fire whirls.
Inversions: cap vertical motion; dissipation marked by rising temps, falling RH, wind increase.
Winds
- General (synoptic) vs local.
- Local types: downslope $2\text{–}5\,\text{mph}$ nights; down-valley $5\text{–}10\,\text{mph}$ ; sea/land breezes.
- Mid-flame/Eye-level wind drives flame angle, spotting.
Cloud cues: cumulonimbus ⇒ lightning, gust fronts; altocumulus castellanus/floccus ⇒ mid-level instability; towering cumulus over fire = pyro-cumulus.

Ingredients for extreme behavior: dry lightning, strong/shifting winds, warm T, low RH, unstable air, receptive fuels.
Cold front sequence
- $150\,\text{mi}$ ahead: light SE winds.
- $50!\text{–}!100\,\text{mi}$ : SW $15!\text{–}!30\,\text{mph}$ , fire escalation.
- Passage: NW winds remain strong then ease; temps ↓, RH ↑.
Thunderstorms: in-draft $10!\text{–}!20\,\text{mph}$ ; outflow $25!\text{–}!35\,\text{mph}$ (gusts >60\,\text{mph}); watch for virga, rain shafts, dust fronts.
Foehn winds (e.g., Santa Ana): lee-side warming/drying, $40!\text{–}!60\,\text{mph}$ (gust $90\,\text{mph}$ ), extreme ROS.
Other locals: glacier winds (spring), nocturnal low-level jets.
Pyro-Cb: large fires generate cumulus; can evolve to thunderstorm with sudden outflows/lightning.

Extreme fire behavior occurs when fuel, weather, topography align:
- Dry, continuous fuels.
- Wind + high temperature.
- Slopes/aspects oriented with wind (chimney, chute, saddle).
Constantly reassess interactions; margin for error narrows when conditions trend toward alignment.