62 Scientific American, December 2019
A
s the plane began its descent into Medford,
we dropped into the blanket of smoke that
covered southwestern Oregon and northern
California. It was late July 2018, and several
major fires were burning in the region. I was
en route to join a Cal Fire (California Department
of Forestry and Fire Protection) team investigat-
ing a fatal incident that had taken place two days earlier. What the group leader
told me over the phone had sent chills up my spine: “A firefighter has been killed
in a fire tornado. His vehicle was thrown hundreds of feet across the ground.”I, perhaps more than anyone, had known that this
might happen someday. Ten years earlier I had gotten
my first look at the aftermath of a fire tornado. The ob-
ject, almost 1,000 feet in diameter, had moved out of
the Indians Fire in California and overrun a group of
firefighters. So strong was the wind that trying to get to
safety felt like running through chest-deep water, one
of the survivors told me. Fortunately, the men were
standing on a paved two-lane highway, which probably
saved their lives: had they been even 10 feet away and
among the trees and grass, they would have died. When
I reached the site, massive oak branches lay all around,
and the ground had been scoured of pebbles.
The scene left me impressed and worried. A fire tor-
nado could evidently harm firefighters taking refuge in
areas usually thought to be safe. It had been a close call.
Many of us had seen fire whirls, dust-devil-sized rotat-
ing columns of fire, and did not regard them as particu-
larly dangerous. In contrast, fire tornadoes—which
combine the destructive power of fire with that of winds
as ferocious as in an actual tornado—were so rare as to
be almost mythical. Even I, a firefighter since 1996 and
a fire-behavior researcher for eight years, had heard of
only one, from a story a veteran firefighter told me.
On returning to my home base at the Missoula Fire
Sciences Laboratory in Montana, I conducted a litera-
ture survey. It turned up reports, most rather sketchy, of
several fire tornadoes that had occurred around the
world in the near and distant past. So scant was the in-
formation on the subject that scientists did not even
agree on what qualified as a fire tornado. Massive forest
fires can generate so-called pyrocumulonimbus (pyroCb)
clouds at high altitudes. These are ice-capped thunder-
clouds that condense from the moisture released above
a fire—from the vegetation it consumed, from the water
vapor in the atmosphere and as a by-product of combus-
tion itself. A few researchers held that only those fire vor-tices that connect to overhead pyroCb clouds are true
fire tornadoes. By that definition, only one had ever been
documented, in a 2003 firestorm near Canberra, Austra-
lia. It had left a damage path almost 15 miles long.
That framework seemed far too restrictive to be of
much use to firefighters, however. Using the working
definition of a fire tornado as a fire whirl with tornado-
like wind speeds, my colleague Bret Butler and I had
gathered up whatever documentation we could find
and consolidated it into firefighter-training manuals
and classes. But now I found myself driving south to-
ward the Carr Fire just outside Redding, Calif., to inves-
tigate the death of a firefighter in a fire tornado—a trag-
edy I had long sought to avert.THE CARR FIRE TORNADO
the site looked like a war zone. Neither the famous tor-
nado researcher Josh Wurman, whom I had recruited
for the investigation, nor I had ever seen anything like
this. Entire blocks of homes had been leveled, with only
the foundations remaining. Roofing and other debris
littered the area, and vehicles had been rolled multiple
times over the ground. Trees were uprooted or broken
off, and flying particles of sand and rock had stripped
them of their bark. Three power-line towers built of
metal lattice, each roughly 100 feet tall, had been blown
down, with one of them having been lifted off its base
and carried 1,000 feet through the air. A 40-foot ship-
ping container had been torn apart, and a steel pipe
was wrapped around downed power poles.
We estimated that the winds could have reached 165
miles per hour, a speed that occurs in class 3 tornadoes
on the Enhanced Fujita scale. (This scale rates torna-
does on a scale ranging from 0 to 5, with 5 indicating
the fastest and most destructive winds.) In California,
only two regular tornadoes of this strength had ever
been recorded. Peak temperatures of the burning gasesJason M. Forthofer
is a firefighter and
mechanical engineer
at the U.S. Forest
Service’s Missoula
Fire Sciences
Laboratory in
Montana. His
research involves
field, laboratory
and comp utational
studies of heat
transfer and fluid
flow related to
wildland fires.
IN BRIEF
Fire tornadoes, vor-
tices of fire with
tornadolike wind
speeds, are exceed-
ingly rare but dead-
ly. The Carr Fire tor-
nado near Redding,
Calif., killed up to
four people.
Apart from fire
itself, generation
of a fire tornado
requires a source of
rotation in the atmo-
sphere. The fire can
concentrate this
vorticity into a
spinning tube of air
and stand it up.
Scientists under-
stand the physics of
fire tornadoes rather
well, but they can-
not yet predict when
and where one
might appear.
© 2019 Scientific American