December 2019, ScientificAmerican.com 67able height and spins tight and fast, but when it moves
over an already burned area, it spreads out and slows
down into a diffuse cylinder of smoke. Sometimes the
rotating object is so wide and slow that firefighters fail
to perceive it. The direction of motion of the vortex
across the ground depends on ambient winds and de-
tails of terrain in ways that we have yet to understand.
Emmons and Ying also found that fire vortices are
remarkably efficient at conserving their rotational en-
ergy, which makes them (unfortunately) rather long-
lived. The Indians Fire tornado, for example, lasted for
about an hour. As the fire tornado spins up, two oppos-
ing forces in the radial direction strengthen: centrifu-
gal force pulling a parcel of rotating air outward and,
in opposition, low pressure in the core pulling it in-
ward. The resulting balance limits the movement of air
in the radial direction and therefore the loss of energy
from the vortex. In contrast, nonrotating fires ex-
change roughly 10 times more energy with the sur-
rounding atmosphere. This mechanism also makes fire
whirls thinner and taller than nonrotating fires be-
cause with practically no air being drawn in, except at
the base, less oxygen is available for combustion. Thus,
some of the fuel gases must travel high up the core be-
fore they encounter sufficient oxygen to burn.
Just as dangerous, the towering column of hot, low-
density gases induces very low pressure at the base of
the whirl. Drag near the ground slows the rotation, re-
ducing the centrifugal force pushing the air outward.
Because the inward force generated by pressure re-
mains the same, however, the wind near the ground
streams into the fire tornado. It ends up acting like a
giant vacuum cleaner, sucking air and, often, burning
debris into the base, forcing it vertically up the core at
extreme velocities and spitting it out from high up—
unpredictably generating spot fires.
IN THE FIELD
despite all this knowledge about the physics of fire tor-
nadoes, we still cannot predict where and when one
will occur. One thing is clear, however: given how rare
fire tornadoes are even though a large, intensely burn-
ing fire always has the capacity to concentrate rotation,
the essential factor for their appearance seems to be
the presence of a strong source of rotation.
We know from case studies, for example, that one of
the likeliest locations for fire tornadoes to form is on
the lee side of a mountain. Wind blowing around the
mountain causes swirling motions on the downwind
side, like water moving around a large rock in a river. A
fire burning there can gather and stretch this rotation
into a fire tornado. But matters are in fact more com-
plicated: Fiery vortices can also show up on flat ground
and in calm wind conditions. For example, a large fire
whirl in Kansas was likely generated by a cold front
that collided with warm ambient air as it passed over a
fire in a field. And a 2007 study by Rui Zhou and Zi-Niu
Wu of Tsinghua University in Beijing showed that mul-
tiple fires burning in certain specific configurations—
which can happen when a fire throws embers ahead of
itself, starting new fires—may even generate their own
rotation by inducing jets of air to flow along the ground
between them.
So where did the rotation that caused the deadly
Carr Fire tornado come from? Given the several fire
whirls that preceded the fire tornado, an abnormally
high amount of rotation obviously existed in the area.
On a hunch, I asked Natalie Wagenbrenner, a colleague
at the Missoula Fire Sciences Laboratory, to run some
specialized computer simulations of the weather that
day. Her studies showed that cool, dense air from the
Pacific Ocean was being pushed eastward and over the
top of a mountain range west of Redding. This cool air
was much heavier than the hot air in the Sacramento
Valley: the Redding airport reported a peak tempera-
ture that day of 113 degrees F, a record. So gravity
caused the air to accelerate as it moved down the
slopes toward the valley, much like water flowing
downhill. Oddly, these strong surface winds stopped
abruptly—right where the fire tornado formed.
What happened to the wind? Finally, I realized that
a hydraulic jump was occurring—the atmospheric
equivalent of what happens to water when it flows
down the spillway below a dam. When the fast-moving
water hits the low-speed pool below, the surface of the
water jumps upward, forming a breaking wave that
stays in place and marks the boundary between the
two flows. This region contains intense swirling mo-
tions. In much the same way, the cold, dense air speed-
ing down the mountainside hit the slow-moving pool
of air in the Sacramento Valley, most likely generating
the powerful rotation that formed the Carr Fire torna-
do [ see box on page 64 ]. N. P. Lareau of the University
of Nevada and his colleagues speculated in a 2018 pa-
per that the pyroCb clouds overhead, which reached al-
titudes of up to seven miles even as the fire tornado
formed, helped to stretch the vortex to a great height,
thereby thinning it and spinning it up even more.
If wildfires continue to become more extensive, we
may encounter such lethal objects more frequently. The
silver lining is that lessons learned from studying them
carefully might help prevent future tragedies. I am
hopeful that further research into fire tornadoes, com-
bined with advances in weather prediction and comput-
ing power, will, in the near future, give us the ability to
issue fire tornado warnings—possibly saving lives.MORE TO EXPLORE
Fire Whirls, Fire Tornadoes and Firestorms: Physical and Numerical Modeling. Robert N. Meroney
in Proceedings of PHYSMOD 2003: International Workshop on Physical Modelling of Flow and Dispersion
Phenomena. Edited by Giampaolo Manfrida and Daniele Contini. Firenze University Press, 2003.
Vortices and Wildland Fire. Jason M. Forthofer and Scott L. Goodrick in Synthesis of Knowledge
of Extreme Fire Behavior: Volume 1 for Fire Managers. Paul A. Werth et al. U.S. Forest Service Pacific
Northwest Research Station, November 2011.
FROM OUR ARCHIVES
Predicting Wildfires. Patricia Andrews, Mark Finney and Mark Fischetti; August 2007.
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