What is “fire weather” and why is it getting worse?

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The map above visualizes when these three variables — temperature, humidity, and wind — combine the days of fire weather, which have been shown as percentage changes since 1973. Texas also looks sad: the southern edge of the state is up 284 percent. Central California is also uneasy: in the days of fire weather, the jump was 269 percent. “The Southwest really came out on top,” Weber says. “We even see some parts of Oklahoma and Kansas, some of those places where we don’t traditionally think about fires.”

But if you’re wondering why we don’t often hear about catastrophic fires in the lowlands, as we do in California, Oregon and Colorado, it’s because “fire weather” just means the conditions are right for a fire – it’s not wrong means they are bound to occur. “We’re not talking about ignition fires, ”says Weber. “We’re talking about the number of days in the year when weather elements have prepared the landscape for these high-risk fires, which are really more dangerous to fight and really harder to fight.”

Atmospheric conditions are not the only variables that worsen the likelihood of fires. Land management decisions, for example, in California and Oregon, play a role. These coastal regions are covered with forests that once burned regularly in a healthy way: lightning would cause a relatively small fire that chewed the brush, clearing the way for new growth but leaving many mature trees alive. Historically, Native Americans have also kindled targeted fires to strategically reset ecosystems. The landscape was on fire a lot, but it also meant that it burned less intensely because the combustible brush did not have the ability to accumulate between burns.

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But in the last century, land managers have followed the opposite approach: extinguishing a fire or immediately extinguishing anything that could affect residential areas. This allowed us to accumulate dry vegetation – more fuel. And with more human communities living in the “urban wilderness interface” where the forest meets cities, people are also breeding more accidental fires, whether from cigarette butts thrown out the window, or electrical infrastructure failures.

This is one of the reasons that fires in California are much more catastrophic than in Kansas or Oklahoma: just more forests with much more fuel and more people living in danger. To adapt, land managers in the western states need to do more controlled burns that will do the job of cleaning the brush, as did frequent small fires.

Climate change has also caused some seemingly contradictory seasonal changes. Because a warmer atmosphere contains more water then amount rainfall may actually increase in the future, while length the wet season is shrinking. In California, rains usually fall in October and last until March. Now they will come at the end of the year. “The dry season will turn into a normal wet season,” says climatologist Ruby Leung of the Pacific Northwest National Laboratory. “If we look at climate models projected into the future, the fire season will lengthen.”

Firefighters are already seeing this happening. The biggest flames California received in the fall, just before the onset of seasonal rains, when the landscape was even drier in six months without water. This coincided with the fierce seasonal winds that caused huge fires. But now, because the rainy season is so short and the landscape has more time to dry, the fire season comes even earlier. “What we observe more constantly and more regularly is the fact that these fires are growing stronger and earlier than usual,” said Isaac Sanchez, chief of communications for the California Department of Forestry and Fire Protection, told WIRED earlier this month. “So when August is rolling, the end of July is rolling, we are seeing these dry conditions that are absolutely the result of climate change.”

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