French firefighters face 'pyrocumulonimbus' for first time

There was none like it since Canada became a nation, either: the exodus of May 3 was the largest, most rapid displacement of people due to fire in North American history. It took the form of an unbroken ribbon of vehicles crawling in ranks, like army ants, northward and southward out of the city while fire raged along the highway, in some cases right up to the breakdown lanes. Visible in every rearview mirror was a monstrous plume where their city should have been, as if the city itself had erupted. Many who saw this sight speculated that the entire city was lost. The fire plume, which was growing steadily larger, was actively changing the region’s meteorology. No longer simply a ground-level interface fire, it had become a force of Nature. As temperatures rose past 1,000°F, the air at the smoke column’s center rose ever more rapidly, driving upward, like smoke up a hot chimney. As this superheated air rose higher and faster, it created a vacuum into which cooler air was drawn from all sides at greater and greater velocity. Operating like a recirculating fountain, storm systems this large also generate powerful downdrafts along their outer edges, which, in the case of a wildfire, can cause it to burn even more intensely, like an atmospheric turbocharger.

Smoke columns behave like fountains in other ways, too: suffused within that swirling vortex, inconceivable in the face of so much fire, was a colossal amount of water—not just from moisture bound up in the forest, but also from melting ice, broken water lines, and fire hoses. In order for fuels to burn as explosively as they did in Fort McMurray, any residual moisture had to be removed by evaporation. All that water has to go somewhere, and it does: what looks from a distance like “smoke” is really a combination of soot, combustive gases, toxic chemicals, and steam. Hundreds of thousands of gallons of water vapor were being carried skyward though the smoke column, ten, twenty, thirty thousand feet above the fire, where it condensed and then froze. There, miles above the city, hurricane-force downdrafts hurled fusillades of black hail back to earth, just as they had done in ancient Egypt. Reduced to their most elementary ingredients, these carbon-infused ice pellets were all that remained of the trees and houses so recently devoured by the fire.

As the fire intensified, ash and glowing spruce needles grew into firebrands the size of work boots, and then branches, treetops, fence panels, and entire garden sheds—all flying through the air, on fire. Some of these were carried thousands of feet into the smoke column, just as they would in a tornado. Pilots flying over large wildfires have reported charred tree branches bouncing off their windshields at twenty thousand feet. A photo taken from an airplane window late on the night of May 3 shows a vast and luminous smoke cloud where the city had been while, high above, the northern lights blaze across the sky. In another age, this might have been an omen worthy of formal record, but that night, it was just one more illumination from the twenty-first century, captured in this smartphone-crowdsourced record of apocalyptic visions.

Other anomalies appeared as well, and, from this vantage, they sound more like details from the Old Testament or Greek mythology than events reported from one of the twenty-first century’s wealthiest industrial centers. Among them was a fire-borne thunderhead. Known to meteorologists as a pyrocumulonimbus cloud, or pyroCb, these massive formations can be two hundred miles wide and reach into the stratosphere. A fully developed pyroCb, like the one shrouding Fort McMurray on May 3, is so huge and energetic that its behavior is influenced by the coriolis effect—the rotation of the earth. In the Northern Hemisphere this will cause such a system to spin counterclockwise, just like a hurricane. Because of their size, particularly their height, pyroCbs are Nature’s most efficient delivery system for high-altitude pollutants, including carbon monoxide, hydrogen cyanide, ammonia, and vast amounts of carbon and other particulates. Once these smoke columns reach the lower stratosphere, between thirty thousand and forty thousand feet above the earth, the aerosols and particulates within them can be carried around the world on the jet stream, which circles the poles like a high-speed conveyor belt.

Breakthroughs in aerosol-sensing satellite technology have revolutionized scientists’ understanding of these phenomena. As recently as the 1990s, hemisphere-spanning aerosol clouds generated by enormous wildfires were mistakenly attributed to volcanic activity. In part because they were so rare, wildfire-generated pyroCbs have only been formally identified and studied as such since 1998, the dawn of this new era of twenty-first-century fire. One of the most exhaustively studied pyroCb events to date occurred during the Chisholm Fire, the same one American satellite data analysts initially suspected might be a nuclear bomb test. The plume it generated obscured an area of more than fifty thousand square miles, roughly the size of Greece. While they remain an atmospheric rarity, pyroCbs have become significantly more common over the past two decades, occurring around the world, in places they have never been observed before.

In addition to hail, pyroCbs can also generate their own lightning. “Pyrogenic lightning” has been described since ancient times, but almost exclusively in the context of large volcanic eruptions. While ember-generated fires are relatively easy to predict (they appear downwind, typically less than five miles from their source), fires caused by lightning can be ignited virtually anywhere within a fifty-mile radius of a pyroCb, where they are accompanied by all the hazards associated with electrical storms—tower strikes, power outages, and electrocution. By 4:00 p.m., as tens of thousands of citizens were making their slow escape, Fort McMurray was experiencing the same “darkness at noon” phenomenon associated with apocalyptic events recounted throughout the world’s histories and mythologies. With the forest already primed to burn, a pyroCb, combined with wind-driven embers and lightning, changed this fire from a localized conflagration into a perpetual motion machine of destruction operating on a regional scale. Given the long-term forecast, this fire could burn as long as the fuel held out, and, in these conditions, the boreal forest was nothing but fuel.