It’s wildfire season in North America, and dozens of blazes have erupted across parts of the western United States and Canada. In the United States, some 89 large fires have torched more than 1.6 million acres so far, while in Canada, 3.7 million acres have burned.
In Oregon, the Durkee Fire has claimed 268,492 acres — about 10 times the size of Disney World. More concerning is the Park Fire burning just north of Chico, Calif., which was ignited Wednesday and already ballooned to over 164,000 acres. It’s only 3 percent contained and is exhibiting behavior that firefighters classified as “extreme.”
In Canada, a fast-moving wildfire badly damaged the resort town of Jasper, Alberta, and forced the evacuation of 25,000 people. The Jasper fire grew so ferocious that its plume began spitting out lightning strikes. So did the plume of the Fort McMurray fire, which is burning over mainly forested regions.
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Wildfires are a staple of the summer climate in the West. While the number of fires, and area burned, hasn’t been terribly anomalous in 2024, it’s likely the season will become markedly worse in the next month or two. That’s when the sunbaked landscape, largely dried out by a summer’s worth of heat and drought, overlaps with strengthening autumnal winds. The result? A breeding ground for rapidly growing and swiftly spreading wildfires that can threaten entire communities.
How do fires start?
About 85 percent of wildland fire in the United States are caused by humans. Some start as the result of carelessness — discarded cigarettes or unattended campfires. Others are sparked by less obvious negligence: Driving off-road, for example, might allow sparks from your vehicle to ignite vegetation. In 2018, the Carr Fire near Redding, Calif — which burned nearly 230,000 acres — was ignited by a spark caused by a trailer with a flat tire.
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A downed power line can do it too. Or a house fire can get out of control, with embers that spawn new fires. Sometimes the cause is arson, as suspected with the Park Fire.
The remainder of wildfires is usually caused by lightning.
What role does lightning play in fire ignition?
Lightning is roughly five times as hot as the surface of the sun. It only lasts a fraction of a second and is usually less than an inch thick, but it’s often enough to spark a fire.
“Hot lightning,” also known as continuous current lightning, is particularly efficient at generating fires. Why? Ordinarily lightning bolts “flicker,” with a rapid succession of brief pulses of current. Hot lightning features a longer-lasting channel with one solid flow of current. Since the electric channel lasts longer, it causes more heating and is more likely to start a fire.
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There’s also “dry lightning.” There’s nothing special about the lightning itself, but it forms in thunderstorms that produce very little rain. Even though rain is falling, it mostly evaporates before reaching the ground because of dry air in place. That means thunderstorms can spark new fires without delivering rains that would extinguish a fledgling fire.
The same overarching setup also can yield erratic “outflow,” or exhaust, winds that exit thunderstorms and fan flames.
On June 22, 2022, a barrage of more than 66,000 lightning strikes took aim at central California. That yielded numerous additional spot fires, several of which grew.
What weather causes fires to spread quickly?
A wildfire is a combustion reaction. That means the input has to be air. The more air entering a wildfire, the more it can grow. Strong winds are notorious for fanning fires and causing their footprint to expand. In the autumn, offshore winds in the West — from the east or northeast — heat up and dry out as they cascade down mountain slopes, often bringing about the biggest wildfire outbreaks.
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Dry weather is a simple requirement for wildfires. When the air is dry, it helps cure, or dry out, vegetation. High temperatures do the same thing. The hotter the air, the more moisture it can evaporate from the ground. That desiccates the landscape, and plants, leaving it ripe to burn.
The hot, dry conditions conducive to fast-spreading fires are expected to increase because of human-caused climate change.
What is a fire weather watch and what is a red flag warning?
When conditions support extreme fire behavior and rapid fire spread, the National Weather Service can issue alerts notifying the public of the hazard.
A fire weather watch is issued up to 72 hours before the arrival of potential fire weather. A red flag warning is issued when fire weather appears imminent and will occur within the next 24 hours.
That’s not to be confused with the fire warning, which can only be issued at the request of emergency management in most cases.
What is extreme fire behavior?
Once a wildfire grows large enough, it begins behaving much like a thunderstorm. Inflow air, or air that fuels the fire, enters from the bottom. Smoke and heat ascend to 40,000 feet or higher, much like the towering plume of a severe thunderstorm. Sometimes the plumes even emit lightning strikes. At this point, the entire system becomes a pyrocumulonimbus cloud, or a wildfire-induced thunderstorm.
Those lightning strikes spark new fires. The thunderstorm can also yield changing winds that complicate fighting the fire. Sometimes, in an environment where ambient winds change speed and direction with altitude, the plume can rotate. On rare occasions, actual fire tornadoes can form.
During the Carr Fire in 2018, a tornado of fire descended from a pyrocumulonimbus cloud. It toppled transformers, destroyed homes, uprooted trees and killed four people. Winds were estimated at 143 mph.
On Aug. 15, 2020, the National Weather Service in Reno, Nev., issued a first-of-its-kind tornado warning for a smoke plume associated with the Loyalton Fire in Lassen County in California. Multiple legitimate tornadoes touched down.
