Fire weather
Wildfire weather
A wildfire is a weather event as much as a burning event. Here is how weather makes wildfires, breaks them, and sometimes creates its own storms.
The fire triangle
Wildfires need three things: fuel, heat, and oxygen. Weather controls all three:
- Heat: temperature, solar radiation.
- Fuel: drought preconditions vegetation.
- Oxygen: wind delivers.
- Moisture: humidity determines fuel dryness.
- Ignition: lightning is the leading natural cause.
The Fire Weather Zone forecast
NWS issues Fire Weather Forecasts daily during fire season.
- Temperature and humidity forecasts.
- Wind speed and direction.
- Haines Index — stability aloft.
- Fine fuel moisture.
- 10-hour fuel moisture (twigs).
- 100-hour, 1000-hour fuel moisture (larger fuels).
- Erosivity and drying trends.
Red Flag Warning criteria
A Red Flag Warning means fire spread conditions are extreme.
- Sustained winds 15-25 mph or gusts to 35 mph (varies by zone).
- Relative humidity below 15-25% (varies).
- Temperature above 75-85°F (varies).
- Fuel moisture critical.
- Duration 3+ hours.
- When these align, ignitions grow rapidly.
The specific fire-generating patterns
Santa Ana winds
Southern California. High pressure over Great Basin sends dry offshore winds. October-January.
Diablo winds
Northern California. Similar offshore mechanism.
Chinook / Foehn
Rocky Mountain lee. Dry, warm downslope wind.
Sundowner winds
Santa Barbara. Local offshore in evening.
Wasatch winds
Utah front range downslope.
Cold front dry passage
Post-front dry air + shifting winds cause spread and rekindle.
Fire behavior in different weather
Calm morning
Slow ground creep. Firefighters can build lines.
Afternoon buildup
Instability aloft. Convection column forms.
Late afternoon
Peak flame lengths, rapid spread.
Evening
Winds calm but temperature still high. Continues.
Overnight
Humidity recovery. Fire quiets.
Dawn
Lowest activity. Windows for aerial attack.
Wind shift
Sudden direction change: firefighter fatalities. 1949 Mann Gulch, 1994 South Canyon.
Pyrocumulus formation
- Intense fire heats air below rapidly.
- Rising air cools adiabatically.
- Water vapor from combustion (and moist fuels) rises.
- Cloud forms above fire.
- If instability strong, cloud grows to cumulonimbus.
- Lightning from pyroCB can start new fires miles away.
- Rare cases: fire-generated tornadoes.
The 2018 Carr Fire tornado
July 26, 2018 — Carr Fire near Redding, CA generated an EF3-rated fire tornado.
- Winds estimated 143 mph.
- Killed 4.
- Uprooted trees 4 ft in diameter.
- First fire tornado rated on the EF scale in US history.
- Documented by radar and satellite as the fire's pyroCB developed.
How weather saves firefighting
Rain
Direct suppression. Post-rain humidity recovery.
Humidity recovery
Overnight dew brings fuel moisture up. Firefighting windows.
Wind shift
Wind change can push fire back on itself.
Marine layer
Coastal California morning fog dampens.
Cold front
Pre-frontal instability high, but post-front cooler and calmer.
Season change
First snowfall in Rockies ends fire season.
The specific historic fires and their weather
1871 Peshtigo, WI
Deadliest US wildfire, 1,500+ dead. Dry summer + high winds.
1910 Great Fires (ID, MT)
3 million acres. Cold front + dry conditions.
1988 Yellowstone
793,000 acres. Extreme drought.
2017 Tubbs (Napa)
Diablo winds. 22 dead.
2018 Camp Fire (Paradise)
85 dead. Sustained wind + drought.
2020 August Complex
1M+ acres. Lightning + heat.
2021 Dixie Fire
963,000 acres. Persistent hot dry.
2023 Maui fires (Lahaina)
Hurricane Dora offshore winds. 100+ dead.
For those in wildfire zones
- Monitor Red Flag Warnings.
- Have evacuation kit ready during fire season.
- Multiple evacuation routes identified.
- Do NOT wait for evacuation order in extreme conditions.
- Cash, documents, medications in go-bag.
- Pets in carriers ready.
- Vehicle gassed.
- Home hardening: fire-resistant roof, defensible space, ember-resistant vents.
- Chimney flue closed.
- Sprinklers on roof during red flag events.
Climate change is changing this
- Fire season is 3 months longer than in 1980.
- Mega-fires more frequent.
- Vegetation drier earlier.
- Snowpack melts earlier — earlier fuel drying.
- Nighttime humidity recovery weaker.
- Extreme fire behavior more common.
- PyroCB events more frequent.
- Firefighter fatalities from unpredictable behavior increasing.