The smoke is again. Massive swaths of America are as soon as once more engulfed in a poisonous haze that’s drifted down from Canada, which is experiencing its worst fire season on record. Our northern neighbor has burned via a record-breaking 8.2 million hectares to date this yr, sending smoke plumes so far as Europe. And, regardless of the very best efforts of tons of of firefighting personnel who’ve come from all over the world to pitch in, the fires don’t appear like they are going to be winding down anytime quickly.

The issue is, Canada just isn’t attempting to place out only one fireplace. Proper now, a map from the Canadian Interagency Forest Fireplace Centre exhibits a country spotted red with blazes, prefer it’s come down with a nasty case of hen pox. Remarkably, these fires aren’t clustered in a single area: Their unfold is the northern equal of New York and California burning on the identical time, with extra fires stretched in between. According to the CIFFC, greater than 509 fires are lively in Canada, 253 of that are labeled as “uncontrolled.”

Likewise, the smoke that’s been descending over America isn’t coming from one specific fireplace. It’s the cumulative impact of all these burns, David Roth, a forecaster with NOAA’s Climate Prediction Middle, instructed me, although these nearer to the border have extra of an impact. Till the fires are totally out, People will stay susceptible to extra smoke days.

When will this all be over? Normally, a hearth can burn so long as it has gasoline and oxygen and it’s heat sufficient to take action. So how lengthy do they sometimes go for? “That query doesn’t have a solution—or no less than not one which’s satisfying,” Issac Sanchez, a battalion chief for communications at Cal Fireplace, California’s firefighting company, instructed me over the telephone. Even when we take away human firefighting efforts from the equation, totally different fires burn at totally different speeds and for various lengths, relying on the place they’re positioned and what’s burning. “Each single fireplace is its personal occasion,” Sanchez defined. “It’s received its personal conduct. We are able to’t assault them precisely the identical approach.” Notably nasty fires can actually take weeks or months to resolve. California’s largest fireplace on file, the August Advanced, burned for 87 days, whereas its second-largest, the Dixie fireplace, burned for more than 100 days. In 2017, Canada’s Elephant Hill fireplace burned for well over two months.

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What’s aflame issues. Grasslands burn quickly, the identical approach a chunk of paper you throw in a hearth crumbles into ash lengthy earlier than the log beneath it does. A hillside in California can burn itself via rapidly, whereas a extra forested space, with thicker, denser brush, may linger. What vegetation is burning, how a lot, and the way dry it’s can pace up or decelerate fires. Most of Canada is classed as boreal forest—chilly, northern forest—and far of the fireplace is going on in that type of ecosystem. This kind of forest tends to burn at increased depth and over bigger areas due to the sorts of bushes and the way densely packed they’re, Piyush Jain, a analysis scientist on the Canadian Forest Service, instructed me. Some boreal forests include peat, which may gradual fireplace—if it’s moist. But when that peat is dry, it can burn underground and spread fires even farther.

Climate issues, too. Sizzling temperatures supercharge fires; the wind spreads them. Snow and rain assist dampen flames, typically ending fires altogether. Although precipitation doesn’t at all times put them out solely: In recent times, zombie fires within the Arctic have quietly smoldered under the snowpack throughout the winter, solely to reignite within the following spring.

Lastly, the place a hearth takes place can decide its life span: Fires are likely to burn uphill, and will wrestle to leap a lake or a river. The realm’s topography additionally adjustments how accessible it’s to firefighters. Distant, hard-to-access areas typically name for parachuting firefighting squads, known as smokejumpers.

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So—when will this all be over? In Canada, the imply period of a fireplace that’s greater than 1,000 hectares (or rather less than 4 sq. miles) is 23 days—or somewhat over three weeks, in response to Jain. In the meantime, a hearth that’s greater than 10,000 hectares (about 40 sq. miles) burns for a imply period of 39 days. Among the fires lively now have been burning for weeks; others are simply starting: Up to now 10 hours alone, CIFFC logged three extra fires.

And the presently entrenched fires are sufficiently big that nobody actually can say how lengthy they’ll drag on. “A few of these fires in [the] northern boreal forest of Canada proper now are monumental,” Bruce MacNab, the top of Wildland Fireplace Info Programs with Pure Sources Canada, instructed me. “And it might take some enormous rain occasions to fully cease them.” He believes that they seemingly will final “for some weeks but.” Broadly talking, Canada’s fireplace season tends to begin waning by the autumn. Karine Pelletier of SOPFEU, Quebec’s forest-firefighting company, instructed me that, this yr, barring many heavy intervals of rainfall, the company expects firefighting operations to final till September.

Within the meantime, thousands and thousands of People must brace themselves for extra excessive smoke days. For precisely how lengthy relies on various elements, together with, fairly actually, which approach the wind blows.