The wildfire burning in and near Yosemite National Park in California has charred 293 square miles, destroying at least 11 houses and endangering several mountain communities. Containment was estimated at 23 percent on Tuesday. The Rim Fire, as it is known, threatens several ancient sequoias, and the smoke has reached Carson City and Reno, more than 100 miles distant:

Schoolchildren were kept inside for the second time in a week, people went to hospitals complaining of eye and throat irritation and officials urged people to avoid all physical activity outdoors.

“It’s five hours away,” said 22-year-old bartender Renee Dishman in disbelief after learning that the source of the haze was more than 150 miles away. “I can’t run. I can’t breathe. It makes me sneeze.” . . .

Everyone should avoid all physical activity outdoors when the air quality index reaches “hazardous,” considered “emergency conditions,” the Nevada Division of Environmental Protection said on its website. “People with heart or lung disease, older adults and children should remain indoors and keep activity levels low.”

Renown Regional Medical Center in Reno has experienced a “slight increase” in emergency room visits as a result of the smoke, said Jennifer Allen, the hospital’s clinical nursing supervisor.

“Patients are experiencing shortness of breath, eye and throat irritation, cough and headache due to the heavy smoke and poor air quality,” she said, adding that people with asthma and other respiratory ailments were most affected.

Associated Press

Despite difficult terrain, firefighters are doing what they can to contain the flames:

The U.S. Forest Service said Tuesday that ground crews planned to work through the night to build containment lines on the northern flank of the fire. Communities north of the blaze, along the Highway 108 corridor from Tuolumne City to Pinecrest, also remained under evacuation orders.

Officials said crews on the southeast flank in Yosemite were planning to conduct extensive backfires, a dangerous tactic in which firefighters burn vegetation inside a fire line to help contain a rapidly spreading blaze.

Nearly 4,100 firefighters are taking part in the effort. . . .

While firefighters have used the Tuolumne River and granite formations on the fire’s northern edges to set up defenses, crews have found little to work with on the blaze’s eastern front south of the Hetch Hetchy reservoir.

“They’re in scouting mode,” Dick Fleishman of the U.S. Forest Service said of fire crews. “There’s not a lot of real good areas to get out in there and do a lot of work.”

Los Angeles Times

A number of factors have contributed to the unusual intesnity of the wildfire, one of the largest in California since record-keeping began in 1932:

Unnaturally long intervals between wildfires and years of drought primed the Sierra Nevada for the explosive conflagration chewing up the rugged landscape on the edge of Yosemite National Park, forestry experts say. . . .

Federal forest ecologists say that historic policies of fire suppression to protect Sierra timber interests left a century’s worth of fuel in the fire’s path.

“That’s called making the woodpile bigger,” said Hugh Safford, an ecologist with the U.S. Forest Service in California.

Two years of drought and a constant slow warming across the Sierra Nevada also worked to turn the Rim Fire into an inferno. For years forest ecologists have warned that Western wildfires will only get worse.

“Every year the summer temperatures are a little warmer, hence the conditions for burning are a little more auspicious,” said Safford. “People can deny it all they want but it’s happening. Every year the fuels are a little bit drier.”

The Rim Fire’s exponential growth slowed only after hitting areas that had burned in the past two decades, and Safford says that shows the utility of prescribed and natural burns that clear brush and allow wildfires to move rapidly without killing trees.

“If you look at the Sierra Nevada as a whole, by far the largest portion hasn’t seen a fire since the 1910s and 1920s, which is very unnatural,” said Safford, who has authored several papers on the increasing wildlife severity across California’s mountain ranges. “This one isn’t stopping for a while.”

Since a 1988 fire impacted nearly one third of Yellowstone National Park, forestry officials have begun rethinking suppression policies. Yosemite has adopted an aggressive plan of prescribed burns while allowing backcountry fires caused by lightning strikes to burn unimpeded as long as they don’t threaten park facilities.

“Yosemite is one of the biggest experimental landscapes for prescribed fire and it’s going to pay off,” Safford said. “The Rim Fire is starting to hit all those old fire scars.”

Associated Press

The cause of the fire, which began a week and a half ago, remains under investigation. For current official information on the fire, visit this page.

Fire crews in California are gaining ground on a giant wildfire by Yosemite National Park, but flames are still spreading fast.