66 Scientific American, December 2019
DAVID McNEWGetty Imagestown in Wisconsin was devastat-
ed by what was probably a fire
tornado, judging by the massive
amount of debris—which includ-
ed a house—thrown around. In
1964 the Polo Fire in California
spawned one that injured four
people and destroyed two homes,
a barn, three cars and an avoca-
do orchard. One of the most hor-
rific occurred during the World
War II incendiary bombing of
Hamburg, Germany: the result-
ing firestorm generated a fire
tornado that, according to geog-
rapher Charles Ebert, was up to
two miles wide and three miles
tall. More than 40,000 civilians
died in the conflagration.
In 1923 a major earthquake
sparked an urban fire in Tokyo.
As it spread from building to
building, residents evacuated to
an open area between the struc-
tures. A large fire tornado formed over this area, killing
an estimated 38,000 people in 15 minutes. For more
than half a century the accepted explanation for this ter-
rible event was that a regular tornado happened to form
at the exact same time and location as the fire. But in the
1980s and 1990s engineers S. Soma and K. Saito of the
University of Kentucky used historical records to con-
struct a small-scale model of the actual fire, painstaking-
ly reproducing its geometry and ambient winds. Their
laboratory fire generated a vortex—proving that the
original one was not a coincidence but was caused by
the fire itself.
This research built on pioneering lab work conduct-
ed two decades earlier, when George Byram and Rob-
ert Martin of the U.S. Forest Service Southern Research
Station created small fire whirls at their facility in Ma-
con, Ga. Their apparatus consisted of a small circular
pool of burning alcohol surrounded by cylindrical
walls with vertical slits, which forced drafts into the
fire to enter in a rotating motion. Significantly, the re-
sulting fire whirl caused the fuel to burn—and its ener-
gy to be released—up to three times faster than in a
nonrotating fire. The rotating wind appears to have in-
creased the rate of burning by pushing the flames
down toward the surface of the alcohol, heating it up.
Subsequent research has found the energy-release rate
to be enhanced by up to seven times in such fires.
Something similar occurs in wildfire whirls and fire
tornadoes. A heated piece of wood generates hundreds
of different flammable gases, the further combustion of
which yields flames. The strong horizontal, rotating
winds in the fire tornado can force the flames down
into the vegetation, causing it to burn more fiercely.
In 1967 Howard Emmons and Shuh-Jing Ying of
Harvard University surrounded a stationary lab firewith a cylindrical wire screen that could be spun at var-
ious speeds, imparting rotation to the air flowing into
the flames. The researchers measured the wind veloci-
ty and temperature distribution of the fire whirl thus
generated, getting a glimpse into its inner workings.
They found that, apart from fire itself, the formation of
such a vortex requires a source of rotation and a mech-
anism to intensify it.
A fire tornado has essentially the same hydrody-
namics. Significant vorticity often exists in the atmo-
sphere—generated by wind curling around mountains
or dragging along the ground or by variations in densi-
ty and pressure. The fire itself carries out two other
crucial functions: it concentrates the rotation and
stands it up, so that a tight tube of air ends up spinning
around a vertical axis.
First the hot air rising above the fire pulls in re-
placement air at the base, thereby gathering rotating
air from the surroundings. Some of the vorticity might
originally be around a horizontal axis, but once air is
sucked up into the fire plume, its hot, buoyant upward
stream causes the axis to tilt to a vertical orientation.
Second, although the upwardly moving air starts out
slow when it is near the ground, it heats up as the gas-
es in it burn. The air pressure all around the vortex
forces the hot, light air within the core upward. The ac-
celerating air in the fire plume stretches the fire whirl
or fire tornado vertically along its axis, reducing its di-
ameter, much as pulling apart a clump of dough causes
a long, thin neck to form. The reduced diameter drives
the air to turn faster to conserve its angular momen-
tum—the same effect seen when a spinning ice skater
draws in his or her arms.
It appears that when a fire whirl or fire tornado
moves over a burning area, it stretches to a consider-CORONA FIRE in Yorba Linda, Calif., in November 2008 generated
a flaming vortex—possibly a fire tornado—that threatened homes.© 2019 Scientific American © 2019 Scientific American