PYROCUMULONIMBUS CLOUDS HAVE been in the news this past week as wildfires rage across Canada and Europe. These fire-generated storm clouds that create their own weather systems have been forming over wildfires in British Columbia, and one was recorded for the first time ever in France. But our understanding of this phenomenon — and how certain fire-mitigation activities might be making conditions worse — traces back to the 2013 Rim Fire in California.

On August 17, 2013, an illegal campfire sparked the Rim Fire in Stanislaus National Forest near Yosemite. In just two days, the blaze scorched 10,000 acres. Then a column of superheated air rushed upward, carrying ash and embers deep into the atmosphere. Far above the canopy, a rare and destructive atmospheric phenomenon was taking shape.

Fire crews were witnessing the birth of a pyrocumulonimbus, or pyroCb — a fire-generated thunderstorm. These atmospheric plumes punch skyward on 140-mile-per-hour updrafts, supercharged by hot temperatures below and capped by an anvil cloud of ice and soot. A pyroCb generates its own lightning and severe downbursts of rain, while hurling burning embers miles ahead of the flame front. They can even spawn fire tornadoes. And they are loud: On the ground, the air vibrates with a low, muscular thrum that swells into a roar resembling a jet engine, punctuated by a sharp, high-pitched hiss.

A hundred miles north of the Rim Fire, inside a National Weather Service operations center in Sacramento, meteorologists watched the blaze pulse across their satellite displays. Around 4 p.m. on August 19, the plume punctured the upper troposphere. The column topped out at 46,900 feet — more than 10,000 feet above the cruising altitude of commercial jets…

PyroCbs require two primary ingredients. One is an unstable air mass characterized by a dry lower atmosphere beneath an upper layer of cold air. As the plume rises through the unstable air, moisture within the plume condenses rapidly, releasing bursts of latent heat — accelerating the plume like a booster rocket as it penetrates deep into the upper atmosphere.

The other ingredient is a massive, highly concentrated explosion of energy. This is typically generated by the burning biomass in a landscape desiccated by severe drought. But such bursts of energy can also come from a direct human source: slash piles of downed timber that people have built up through logging or other types of forest clearing.

Journalist Paul Koberstein offers an explainer on pyrocumulonimbus storm clouds, and investigates how the slash piles left behind from US Forest Service thinning operations could be making this destructive and once rare atmospheric phenomenon more frequent in the US.

Photo courtesy of Acacia England / US Forest Service

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