Pyrocumulonimbus clouds (or pyroCB for short), are thunderstorms that are created over the top of large wildfires. PyroCB’s act like giant chimneys, funneling massive amounts of smoke into thunderstorms, creating the conditions for dangerous and volatile lightning. These clouds are a form of severe weather known as fire weather.
The lightning that appears from the clouds can potentially worsen the wildfires on the ground. There is not enough data on the Bootleg fire to say if this is currently happening, however, early research does give some insight into this possibility.
“We do have very strong evidence that lightning is abundant and particularly energetic in these pyroCBs and they do start new fires,” says Michael Fromm, a meteorologist at the US Naval Research Lab.
Aside from just problems with lightning, these clouds also push smoke to higher altitudes, where it can travel further and affect more people. They also do not produce rain like a normal nimbus cloud, which means it is more likely to start additional wildfires.
“You can imagine this extremely dirty thunderstorm, with all these smoke particles for water to condense on. And that creates differences in the properties of the cloud,” says David Peterson, another meteorologist at the US Naval Research Lab. “It’s not very efficient at generating precipitation because of that. The droplets don't get large enough to fall as rain. But, it is a cloud that can produce a lot of lightning.”
The Bootleg fire in Oregon created pyroCB clouds multiple days in a row.
Because regularly occurring pyroCB clouds are a relatively new phenomenon, agencies are still scrambling to figure out how to predict, detect and warn residents about them. These weather patterns make wildfire activity even more unpredictable and puts residents and firefighters in greater danger.
