Extreme weather

Fire tornadoes science

Wildfires do more than burn — they generate their own weather. Here is the physics behind fire tornadoes, and why they are becoming more common in a warming climate.

The distinction that matters

Fire whirl

Small vortex from local wildfire heat. Common in any large fire. 30-100 ft tall. Not a tornado in a technical sense.

Firenado

Popular term. Refers to larger fire-driven vortices.

Fire tornado

True tornado that forms from a pyrocumulonimbus supercell. Extremely rare. Physics same as ordinary tornado but ignition and intensification driven by fire.

Firestorm

Regional inferno with self-sustaining wind pattern. Historic fire behavior. Can include multiple vortices.

How fire tornadoes form

Wildfire generates intense localized heat.
Heated air rises rapidly in an updraft.
Water vapor from combustion (and any moisture) rises with it.
At the condensation level, a pyrocumulus cloud forms.
If the updraft continues, cloud grows to pyrocumulonimbus.
Wind shear organizes updraft into a rotating column.
The rotating column may descend to the ground as a tornado.
Fire acts as the persistent heat source that maintains rotation.

The 2003 Canberra firestorm

January 18, 2003 — an EF3-equivalent fire tornado formed during Australia's Canberra bushfires. It killed 4 people, destroyed a subdivision, and became the first officially documented fire tornado in weather records.

Winds estimated at 155-160 mph. Path length 15 miles.

The 2018 Carr Fire tornado

July 26, 2018 — during the Carr Fire in California, a fire-generated tornado developed near Redding. NWS Sacramento assessed it as EF3, the first fire tornado in US history to receive an official EF rating.

Winds estimated at 143 mph. Killed 4 (one firefighter, three civilians). Uprooted trees 4 ft in diameter.

The 2020 Loyalton, CA fire tornado warning

August 15, 2020 — NWS Reno issued the first-ever Fire Tornado Warning in the US. During the Loyalton Fire, satellite and Doppler showed a rotating pyrocumulonimbus.

Why fire tornadoes are becoming more common

Larger wildfires — more heat = more updraft.
Longer fire season — more days at risk.
Drier fuel — hotter fire.
Warmer air — reduced convective inhibition.
Fire-atmosphere coupling research is accelerating.
2020s wildfire seasons have produced more pyroCBs than any previous decade.

The peak: pyrocumulonimbus

PyroCB clouds are Category 6 fires in atmospheric terms. They can reach 45,000 ft, generate lightning, produce hail, spawn tornadoes, and inject smoke into the stratosphere where it lingers for months.

The 2019-20 Australian bushfires generated over 20 pyroCBs.
The 2020 California fires similar.
Smoke from these events was detected globally.
Some events create stratospheric aerosol impacts comparable to small volcanic eruptions.

Safety if you're near one

Fire tornadoes carry burning debris — the danger zone extends far beyond the visible funnel.
Ember shower can start new fires miles away.
Evacuation zones need to be larger than for normal fires.
Firefighting aircraft cannot operate near pyroCBs.
If you see a rotating column of fire — leave immediately, do not stop for possessions.