PBS has published a 10-minute video (below) about how fire whirls and fire tornados form. Here is how they summarize it:
A fire tornado, or “firenado,” is exactly what it sounds like: a tornado made out of fire… and it is truly the stuff of nightmares. The most famous example occurred when the 2018 Carr Fire spawned an EF3 fire tornado with estimated wind speeds of 143 mph! And as climate change drives increasing wildfires around the planet, it only makes sense that we see more fire tornadoes as well.
Tune into this episode of Weathered to learn what the latest science can tell us about these rare but dangerous phenomena as well as about the surprising ability of burnt forests to store carbon and mitigate climate change.
Weathered is a show hosted by meteorologist Maiya May and produced by Balance Media that helps explain the most common natural disasters, what causes them, how they’re changing, and what we can do to prepare.
Fire whirls, much like dust devils, are not uncommon on a fire when the atmosphere is unstable, and are much smaller than fire tornados. In 1978 a researcher for the National Weather Service in Missoula, David W. Goens, established parameters for the two.
He said the average size of a fire whirl is usually 33 to 100 feet, with rotational velocities of 22 to 67 MPH.
But a fire tornado dominates the large scale fire dynamics. They lead to extreme hazard and control problems. In size, they average 100 to 1,000 feet in diameter and have rotational velocities up to 90 MPH.
Above: Screen capture from the video by Jon Krause.
Twitter user @JonLKrause posted videos of a large, long-lasting dust devil/fire whirl that persisted over a prescribed fire at Kramper Lake near Hubbard, Nebraska for about five minutes on Tuesday April 11.
Sometimes these are called “firenadoes” but this one did not have much fire in it. Dust devils and fire whirls can occur on days when the atmosphere is unstable. The heat from what remained of the fire and the solar heating of the blackened ground probably contributed to the phenomenon. It is interesting that after moving to the edge of the lake it still persisted for a while before dying out.
These two recently filmed videos illustrate what can happen over a fire when thermal instability and available vorticity combine.
The Cornelius (Oregon) Fire Department posted this video on August 12, 2016 showing an impressive fire whirl.
The video below was filmed by Stewart Turner August 5, 2016 on the Pioneer Fire in Idaho. You will see a definite rotation, or convection column vortex. While this one is not as violent as, say, a conventional tornado, the change in wind direction or the collapse of the column can cause a serious problem for nearby firefighters.
The news media sometimes calls any little fire whirl a “fire tornado, or even a “firenado”. We found out today that these and related terms (except for “firenado”) were, if not founded, at least documented and defined in 1978 by a researcher for the National Weather Service in Missoula, David W. Goens. He grouped fire whirls into four classes:
Fire Devils. They are a natural part of fire turbulence with little influence on fire behavior or spread. They are usually on the order of 3 to 33 feet in diameter and have rotational velocities less than 22 MPH.
Fire Whirls. A meld of the fire, topograph, and meteorological factors. These play a significant role in fire spread and hazard to control personnel. The average size of this class is usually 33 to 100 feet, with rotational velocities of 22 to 67 MPH.
Fire Tornadoes. These systems begin to dominate the large scale fire dynamics. They lead to extreme hazard and control problems. In size, they average 100 to 1,000 feet in diameter and have rotational velocities up to 90 MPH.
Fire Storm. Fire behavior is extremely violent. Diameters have been observed to be from 1,000 to 10,000 feet and winds estimated in excess of 110 MPH. This is a rare phenomenon and hopefully one that is so unlikely in the forest environment that it can be disregarded.
Mr. Goens further described three different types of fire whirls generated:
The Thermally Driven Form. This results from some type of shear in the horizontal airflow coupled with the energy release (convection) from fire activity.
The Convection Column Vortex. This form is poorly understood. It originates high in the convection column (up to 1000 feet) and extends in the ground as much as a fourth (1/4) mile on the leeward side of the fire.
The Wake-Type Whirl. This results from the generation of eddies caused by airflow around an obstacle coupled with heat released by the fire.
Mr. Gowns continued:
All three of these types can be a significant problem in the spread or control of fire. The fire whirl in its steady-state form, i.e., after it has formed and before it begins to collapse, has two sharply defined regions of differing airflow (Byron and Martin 1970). The cooler, slowly rotating zone surrounds a central core of hot gases with high horizontal and vertical velocities. This central core can have temperatures from 1,800° to 2,400°F and burning rates two to seven times normal. Flame height can be 10 to 50 times the core diameter. Fire spread occurs when burning debris entrained into the column just above the surface boundary layer is carried aloft and then cast out from the upper portion of the whirl core some time later. The path of the whirl can be quite erratic; therefore, direction and rate of spread are almost impossible to forecast.
One of the best videos of a large fire whirl or fire tornado was shot by Chris Tangey of Alice Springs Film and Television in 2012 while he was scouting locations near Curtin Springs station in Australia. It is used here with his permission.
And lastly, I filmed this next video at the USFS Fire Lab in Missoula in 2014, showing an artificially created fire whirl.