Understanding pyrocumulonimbi, aka 'fire clouds'

Caused by wildfires, they can trigger lightning, create fire tornadoes and turbocharge winds that accelerate the spread of a conflagration

September 7, 2021

Fire Clouds

By Jon Reisner and Manvendra Dubey

As the Bootleg Fire rages across south-central Oregon and the Dixie Fire scorches Northern California, obliterating hundreds of thousands of acres of forest, the roiling flames have launched massive smoke plumes into the air. Typically, the smoke is lofted to a few kilometers above ground and then picked up and carried by wind currents across the country—much like weather fronts that carry moisture and bring rain—to blanket New York City and the Eastern Seaboard in a thick haze.

But the Bootleg has sent up a towering vertical plume, lofting smoke up to a record-breaking 16 kilometers. Intense hot fires from dense fuels; local weather conditions and dry surface; and high-altitude atmospheric clouds, conspired together causing the plume to rise far above ordinary clouds. When you see these pyrocumulonimbus clouds, or pyrocumulonimbi, you know you’ve got big trouble below. Known also by the abbreviation pyroCb, these clouds are are so intense that they modify local weather, producing intense downbursts, for example and creating amazing lightning storms that can ignite more fires.

In recent years, megafires and their blanketing haze have become an increasingly familiar sight, along with the towering thunderheads of smoke that form above them. Yet we’re only beginning to learn what causes those awe-inspiring “fire clouds,” what’s in them and what effects they have on weather on climate. Through a combination of field observations, experimental work in the laboratory and computer modeling at local to global scales, our team at Los Alamos National Laboratory is making progress in understanding the mechanisms and climate impacts of pyrocumulonimbus from recent megafires in British Columbia (2017) and Australia (2019-2020).

Read the rest of the story as it appeared in Scientific American.

Topics