Extreme temperatures and a torrent of wildfires, many reaching hellish proportions, marked this summer in the northern hemisphere. An unprecedented number of these have burned through the threshold separating regular blazes from the far more serious class known as firestorms, or pyrocumulonimbus (pyroCbs).
Generating their own weather and puncturing the tropopause with monstrous plumes of smoke, pyroCbs are incredibly scary, and their sudden spike in prevalence sparks fears that they could become more frequent in the future.
“We’ve seen 75 to 80 pyroCbs this year in the northern hemisphere summer season, and this does seem to be a number which is exceeding previous seasons for which we have data,” Mike Fromm, a meteorologist with the US Naval Research Laboratory (NRL), explained to IFLScience. “I have to state pretty emphatically though that we can’t call this a trend. We don’t know if we have enough data to give trend information.”
This lack of data is largely due to the fact that, despite their staggering intensity, identifying pyroCbs is not easy. It’s only in the last two decades or so that researchers have been able to detect them from satellite measurements of smoke in the stratosphere.
“When I started at NRL in the mid-90s the term pyroCB didn’t even exist,” says Fromm. “In the work that we did, we stumbled upon smoke in the stratosphere. We now have to get good enough at identifying and quantifying pyroCbs to see if they are increasing in frequency.”
What Is A Pyrocumulonimbus?
As their other name "firestorm" suggests, a pyroCb is essentially a thunderstorm generated by a wildfire. When extremely large fires send hot air up into the atmosphere, it condenses into clouds. If the atmosphere is unstable, the smoke plume can continue to rise, generating powerful updrafts as cold air is sucked in to fill the void below. These winds fan the flames, creating a feedback loop resulting in more hot air being thrust skywards, causing the whole system to snowball.
If the smoke plume reaches the stratosphere, it can trigger a lightning storm. At this point, the fire can be said to be generating weather rather than the other way round. Once this occurs, a wildfire earns the title of pyroCb and becomes incredibly difficult to predict or control.
The highest wildfire smoke plume ever recorded belonged to the Australian New Year Super Outbreak (ANYSO) that raged between December 2019 and January 2020, sending smoke 34 kilometers (21.1 miles) into the atmosphere – well into the ozone layer. Winds generated by pyroCbs of this size can exceed 160 kilometers per hour (100 miles per hour), throwing firebrands far beyond the front of the blaze, where they can ignite new fires.
What Makes PyroCbs So Dangerous?
Once an unremarkable route passing through the sparsely populated Portuguese interior, Highway N 236-1 was dubbed the "Road of Death" following a pyroCb in June 2017 – the first ever recorded in Western Europe. As evening fell on the day of the fire, a downdraft caused a section of the road and the surrounding forest to become incinerated in the blink of an eye – along with many motorists attempting to flee.