A spiraling column of fire is often called a “fire tornado,” but scientists classify it as a fire whirl or pyro-vortex. This distinction highlights the difference between these fire-induced circulations and a true meteorological tornado. While the most intense and destructive examples are rare, smaller versions are common during large fires. Understanding their formation reveals the specific environmental ingredients required to transform a simple wildfire into a spinning column of destruction.
Understanding the Fire Whirl
A fire whirl is a rotating column of hot air and combustion gases that forms over a fire, drawing in flames, smoke, and debris. It is categorized as pyrogenic, meaning its energy and rotation are derived directly from the heat released by the fire, not from large-scale atmospheric dynamics like traditional tornadoes. Fire whirls exist on a spectrum of size, ranging from brief, localized “fire devils” to massive, long-lived “pyrotornadoes.”
The smallest fire devils are typically just a few feet in diameter, lasting only seconds, and pose little threat. Mid-sized fire whirls measure 33 to 100 feet wide, with rotational velocities reaching up to 67 miles per hour. These whirls significantly influence fire spread by lofting burning embers and accelerating the local burn rate.
The most extreme events can exceed 100 feet in diameter and exhibit wind speeds comparable to an EF-3 tornado, exceeding 140 miles per hour. The 2018 Carr Fire in California produced an example classified at this extreme level. These massive structures dominate the fire dynamics and represent a hazard due to their destructive wind and ability to rapidly spread the blaze.
The Physics of Vortex Formation
The formation of a fire whirl begins with the intense localized heat generated by a large fire. This heat causes the air above the blaze to become buoyant, creating a strong vertical flow called a convective updraft. This rising column of air is the initial engine for the vortex, similar to the process that forms a dust devil.
For the updraft to begin spinning, a source of rotation, or vorticity, must be introduced. This rotation often comes from horizontal wind shear—a difference in wind speed or direction near the ground. As this rotation is drawn into the vertical updraft, the columns of spinning air are stretched upward.
This stretching mechanism intensifies the vortex, following the principle of conservation of angular momentum, much like a figure skater pulling their arms in to spin faster. As the column narrows, it creates a lower-pressure core that draws in more air. This inflow of fresh oxygen and fuel increases the burning rate within the core, sustaining the vortex and driving its heat, which can exceed 1,000 degrees Celsius.
Factors Influencing Occurrence and Rarity
The extreme variability in scale relates directly to rarity: small fire whirls are common during intense wildfires, but massive, long-lasting pyrotornadoes are exceedingly rare. Achieving the largest scale requires an extraordinary combination of environmental factors to align and persist.
The most important factor is the intensity and size of the heat source, requiring a vast, uncontrolled fire consuming dense fuel over a large area. This extreme heat generates the powerful, sustained updraft capable of lofting the rotational column thousands of feet. The Carr Fire event created a pyrocumulonimbus cloud—a fire-induced thunderstorm that provides the deep atmospheric structure needed to support a tornadic vortex.
Topography plays a significant role in channeling and concentrating rotational forces. Ravines, canyons, or steep slopes can funnel surface winds, enhancing the initial wind shear that starts the rotation. Stable atmospheric conditions above the fire are necessary to prevent the vortex from immediately dissipating, allowing pyrogenic forces to organize and sustain the spinning column. When all these conditions converge, the result is the rare and destructive fire tornado.