Fires and thunderstorms: how are they linked?

Wildfires and thunderstorms: complex interconnections spawned by climate change

They used to call it the butterfly effect: the idea that weather patterns are so complex and hard to predict that a butterfly flapping its wings in China could perturb air motions enough to, weeks later, spark a tornado in Kansas.

At the time, it was used as an example of ‘chaos theory’: the concept that tiny changes can ripple into unexpected effects that are almost impossible to predict. But it’s also an illustration of how weather patterns across the globe are interconnected, much like the old children’s ditty about how the “toe bone’s connected to the foot bone,” and the “foot bone’s connected to the ankle bone,” until you get all the way up to the head and realize that everything ultimately connected to everything else.

The most famous example of this is the El Nino/La Nina oscillation, in which shifts in warm and cool water across the central Pacific can set up patterns of flood and drought all the way from Indonesia and The Philippines to the Americas and Africa.

But now, scientists have found a new interconnection, one not related to oceans, but to wildfires. Heat and smoke generated by these fires, says Dr Jiwen Fan, an earth scientist at America’s Pacific Northwest National Laboratory in Richland, Washington, can change atmospheric conditions enough to drive fierce, out-of-season thunderstorms thousands of kilometres away.

Firefighters descend a hill towards a fire
Californias Caldor fire moves east toward Lake Tahoe as firefighters continue to battle a blaze in August 2021. Credit: Michael Nigro/Pacific Press/LightRocket/Getty Images

Fan’s interest was sparked a few years ago when she noticed giant storms in the central US occurring simultaneously with devastating wildfires in California and Oregon.

Normally the thunderstorm season in that part of America is late spring and early summer, but these storms were happening for days on end across large parts of the region in late July (mid-summer in the Northern Hemisphere). “Flash flooding, hailstorms, and tornadoes were reported,” Fan says. It was also exactly the same time as severe fires were occurring in California and Oregon

Intrigued, Fan did a statistical assessment, comparing the total burned area in the Western US to the strength of thunderstorms in the Plains. “We found a positive correlation,” she says.

She then turned to high-resolution weather models to see if they showed the same thing.

They did. To begin with, her team found that strong wildfires on the West Coast produced a 38 percent increase in the risk of heavy rain far downwind, in the central US. There was also an effect on hail. “The frequency of hail larger than 5cm (billiard-ball-sized) was increased by 34 percent,” she says. That’s important because however much attention tornadoes, wind, and lightning get, the vast majority of damage from severe thunderstorms (about 70 percent) comes from hail.

The next step was to figure out why. To do that, she used a sensitivity test in which she varied parameters in a high-resolution weather model in order not only to isolate the effects of wildfires but to tease out exactly how they produced weather-changing conditions so many kilometres downwind.

The more we understand about the contributing factors behind storms like this, the better we’ll be able to prepare for them.

Jiwen Fan

Two factors came to the fore. One was simply the enormous amount of heat a huge wildfire pumps into the downwind air. “From our math, it can be up to four or five degrees Celsius,” Fan says.

That’s enough to have major impacts on weather patterns. “What we see is that the [late summer] high-pressure system in the western US is enhanced, while the low-pressure system in the central US gets lower,” she says.

This produces stronger winds, as air flows from high-pressure regions to lower-pressure ones. “This leads to a stronger moisture transport to the thunderstorm states [east of the Rocky Mountains], so, basically, the meteorological environment in these states becomes conducive to more severe storms,” Fan says. The wind change also produces greater wind shear, another factor in fueling large thunderstorms.

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Pyrocumulonimbus clouds are basically fire-induced thunderstorms. Credit: Eric Neitzel/ Wikimedia Commons

But fires also produce smoke, and it too plays a role.

That’s because raindrops (and hailstones) don’t form on their own. They condense onto tiny airborne particles atmospheric scientists call cloud condensation nuclei. (The whole concept of cloud seeding, once popular in Australia, is based on this principle.)

Smoke is full of tiny particles. When these mix with moist air, they provide a vast number of nuclei onto which water vapor can condense—a process that also releases heat, as the water turns from vapor to liquid. “This added heat provides energy that can invigorate thunderstorms,” Fan says.

If the storm becomes strong enough to produce hail, it makes it easier for the hail to grow large.

Fan’s work, published in the Proceedings of the National Academy of Sciences, comes on the heels of a 2021 study by other researchers in her laboratory who found a connection between melting Arctic Sea ice and catastrophic wildfires that in September 2020 forced evacuations of parts of the Portland, Oregon metropolitan area.

We know wildfire follows drought and humans, so the same patterns that lead to drought in one part of the world can lead to severe weather, flooding, and other hazards in others.

Ian Giammanco

But ferreting out details of such interconnections is a new field of study, and caution is warranted, says Dr Ian Giammanco of the Insurance Institute for Business & Home Safety. “While the results are interesting, as with many new studies, more rigorous work is needed to confirm what was found,” he says.

Still, he says, “we certainly know that climate teleconnections exist. We have a good understanding how things like the El Nino/La Nina cycle impact drought, extreme precipitation, and severe weather. We know wildfire follows drought and humans, so the same patterns that lead to drought in one part of the world can lead to severe weather, flooding, and other hazards in others.”

Credit: Daryl Pederson / Getty

Dr Yuwei Zhang, a postdoctoral researcher on Fan’s research team, adds that the storms their team studied weren’t small. “The cost of the storms we studied exceeded $100 million in damage,” Zhang says.

Fan adds: “We need to be careful, and informed. The more we understand about the contributing factors behind storms like this, the better we’ll be able to prepare for them. And as we look at the future climate, we know wildfires will increase.”

She also adds that her study only focused on a portion of the central US. “We didn’t look at the impact further east,” she says. Nor did her team look at other countries, though, she notes, “I remember a few years ago, after huge fires in Australia, there were huge hailstones.”

After all, the toe bone is connected to the foot bone. And the foot bone is connected to the ankle bone. And…

Because sometimes the things we learn in kindergarten really are the best things. Because, everything might really, truly, all be connected.

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