William Gurstelle — Forget fireworks, celebrate this 4th of July with a towering inferno of science. Practical Pyromaniac William Gurstelle explains the physics of the phenomenon and shows us how to build our own.
Fire tornadoes, also known as fire whirls, were a key factor in the extreme deadliness and destructiveness of the October 1871 fires. They form only rarely, when a “perfect storm” of wind, low humidity, abundance of fuel, and bad luck come together. But such were the conditions in Chicago and Peshtigo.
In Chicago, fire crews had a chance at containing the fire early on, but the moment the fire whirls kicked in-carrying burning planks aloft and depositing them three-eighths of a mile ahead of the firefighters-the city was lost. In Peshtigo, the fire tornados were even larger and more destructive. Forming quickly from great heaps of dry organic material in the dried-up swamps on the west side of the area, the fire whirls swooped into the town.
According to the U.S. Forest Service, fire whirls are frequently observed in wild land fires. They are most often small, but large ones may form, depending on atmospheric and fuel conditions. Scientists estimate that the biggest whirls have rotating air speeds that exceed an F-5 tornado, reaching a whopping 300 miles per hour.
Meteorologists would term such a phenomenon a vortex, which is a volume of fluid possessing a rotational motion. Examples of natural vortexes abound: smoke rings from a cigarette, water going down a drain, and dust devils. In fact, a fire whirl and a dust devil are similar in many ways. Both form when a layer of hot air is covered with a layer of cooler air. The hot air, lighter and more buoyant, pokes a hole in the cool air above it and rises up through the opening.
If the conditions are right, the rising hot air begins to spin, and a whirlwind forms. The rotational trigger for the vortex could be as simple as a gust of wind rushing past a rock or small mound of dirt. The difference between a dust devil and a fire whirl is to some extent merely a matter of degree. Where the surface temperature of the dry, dusty earth that typically creates the dust devil can be as hot as 150°F, the fire whirl is created by air temperatures that may exceed 2,000°F. These extreme temperatures create massive columns of rising hot gas that shoot up erratically through the cool air layer. The hotter the fire and the more unstable the atmospheric conditions, the more erratically the hot gas rises. Some small triggering event makes the fire begin to spin as it ascends, the hot gas slipping by and spinning around with the cool air containing it, due to friction. A fire whirl is born.
In the center core of a whirl, the air is drawn down, while on the edge of the vortex there is a strong updraft. The air and hot gas rotate like a hurricane, with the greatest wind speeds closest to the core or eye of the whirl. The movement of the air works like a giant blower or bellows, providing a surfeit of oxygen, which causes the fire to burn with an intensity five to six times greater than a nonrotating fire. Little wonder, then, at the speed with which immense destruction was visited upon the unfortunate town of Peshtigo.
BUILDING THE FIRE TORNADO
The Fire Tornado dramatically illustrates the effect air has on a fire’s shape, burning rate, and fuel consumption.