Question: With all the media coverage of wildfires I’ve seen more than a few shots of what they call “firenadoes.” Scary looking phenomenon! Could you explain just how those things form? — PB, Golden, CO
Answer: They’re similar to dust devils in that both form from a rising column of heated air. But in the case of “firenadoes” (also called “fire whirls”) the heat source is fire rather than warm ground. They are technically called pyrogenic tornadoes because the large ones can reach up to 1 km in height and produce wind speeds in excess of 200 km/hr (120 mph).
They’re extremely dangerous to fire fighters because their winds can fling burning debris at equally high speeds. And though they are generally stationary, they can move quickly and unpredictably under certain conditions.
Fortunately they’re not that common and require the right conditions to form. Not all fires spawn “firenadoes.” The graphic below shows what’s going on in a pyrogenic tornado:
Key to their formation is the vorticity of the incoming air at ground level. When air heated by the fire rises, new air will flow in from the surrounding areas. Depending on local meteorological conditions, the vorticity can be close to zero or quite high. It also depends on the local topography — open flat areas tend to preserve the vorticity better than hills and valleys.
The law of conservation of angular momentum states that, as a rotating mass decreases its radius of rotation (like a figure skater drawing in their arms), the speed of rotation will increase. The diameter of a large pyrogenic tornado can be as small as a few meters, so that will significantly concentrate any vorticity and increase the wind speed.
Interestingly, if you watch for them, you’ll see miniature “firenadoes” in the flames of even a small campfire. They never last very long, but if you can position your marshmallow about an inch above the tip of the whirl, it’ll toast faster because the swirling hot air acts like a convection oven.
If you do an internet search of “fire tornado” you’ll find plenty of great videos. Here’s one of my favorites. It features multiple examples of this fascinating phenomenon:
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