The Rim fire continues to burn in near Yosemite Valley and the Hetch Hetchy Reservoir in Northern California. The fire has already burned about 225 square miles, destroying several structures, and was only 7 percent contained Sunday night. Officials don’t know what ignited the fire, which has been burning for more than a week:

Firefighters were hoping to advance on the flames Monday but strong winds were threatening to push the blaze closer to Tuolumne City and nearby communities.

“This fire has continued to pose every challenge that there can be on a fire...,” said Daniel Berlant, a spokesman with the California Department of Forestry and Fire Protection. “It’s a very difficult firefight.” . . .

It continues burning in the remote wilderness area of Yosemite and is edging closer to the Hetch Hetchy Reservoir, the source of San Francisco’s famously pure drinking water, park spokesman Tom Medema said.

Despite ash falling like snowflakes on the reservoir and a thick haze of smoke limiting visibility to 100 feet, the quality of the water piped to the city 150 miles away is still good, say officials with the San Francisco Public Utilities Commission.

The city’s hydroelectric power generated by the system has been interrupted by the fire, forcing the utility to spend $600,000 buying power on the open market.

Park employees are continuing their efforts to protect two groves of giant sequoias that are unique to the region by cutting brush and setting sprinklers, Medema said.

On Sunday, crews worked furiously to hold a line near Ponderosa Hills and Twain Hart, miles ahead of the blaze. But officials warned that the fire was so hot it could send sparks more than a mile and a half out that could start new hot spots.

“We’re facing difficult conditions and extremely challenging weather,” said Bjorn Frederickson, a spokesman for the U.S. Forest Service.

Associated Press

For patrons of the Iron Door Saloon in Groveland, Calif., the fire was impossible to ignore:

Outside, above the street sign circa 1852, the smoke from the Rim fire billowed three times higher than the tallest peak in nearby Yosemite National Park.

The 20 locals at the bar, and just about anyone else in town, could tell you that that cloud was a pyrocumulonimbus formed by heat. In the six days that the out-of-control Rim fire has been licking at this town of 3,000, the people here have learned a lot about clouds and fire.

They passed around fire incident maps, pointing out their houses and ranches, many of them inside the perimeter lines. They joked about putting together a blue grass band to make up for the venerable Strawberry Music Festival that won’t happen next weekend because the festival site is in the path of the fire. . .

But mostly, they worried about their friends.

“You’re not going to find anyone who doesn’t know at least one person on the fire,” said Corinna Loh, second-generation Iron Door owner. “There’s not many jobs in the country and one of them is firefighter.”

Diana Marcum, Los Angeles Times

The fire has been particularly difficult for firefighters to control because of the mountainous terrain, dry conditions and unfavorable winds. Meanwhile, this year is the second in a row that the federal government has depleted its budget for fighting wildfires before the end of the season. Wildfires are becoming more frequent and expensive in the Western states for several reasons:

Climate change has heated up and dried out the region, making forests more flammable. Certain forest management and fire-suppression techniques over the past century have also made forests more susceptible to truly gigantic blazes.

But there’s another key factor driving up costs: The number of people living in fire-prone areas has grown dramatically. Some 250,000 new residents have settled in Colorado’s “red zone” over the past two decades, for instance. Not only can that increase the odds of a fire starting in the first place, but more crucially, it increases the cost of suppression, as firefighters focus on protecting nearby homes.

There’s also reason to think that wildfires could keep getting bigger. A study in 2012 by a team of researchers at the University of California at Berkeley looked at 16 different climate models and concluded that wildfire activity was likely to become much more widespread in regions such as the Western United States if humans keep pumping carbon into the atmosphere and the world heats up.

So far, however, the federal government has been slow to react. The U.S. Forest Service, for its part, has proposed new forestry practices to reduce the risk of fires, including a greater use of smaller prescribed fires and “mechanical thinning” to clear out the tangled overgrowth in many forests. Yet these measures are expensive — the price tag to treat 4 million acres comes to about $1 billion — and the agency is already struggling with funding as is.

Brad Plumer

A dozen fires currently burning throughout California cover a total of around 400 square miles. For past coverage of the Rim fire, continue reading here.

A Northern California wildfire near Yosemite National Park grew overnight to more than three times the size of San Francisco and spread inside the border of the park. (Associated Press)