In this Friday, Aug. 30, 2013 photo provided by the U.S. Forest Service, a member of the Bureau of Land Management Silver State Hotshot crew from Elko, Nevada, walks through a burn operation on the southern flank of the Rim Fire near Yosemite National Park in Calif. The wildfire burning in and around Yosemite National Park has become the fourth-largest conflagration in California history. AP Photo/U.S. Forest Service, Mike McMillan In this Friday, Aug. 30, 2013 photo provided by the U.S. Forest Service, a member of the Bureau of Land Management Silver State Hotshot crew from Elko, Nevada, stands by a burn operation on the southern flank of the Rim Fire near Yosemite National Park in California. In this Friday, Aug. 30, 2013 photo provided by the U.S. Forest Service, a member of the Monterey Hotshots carries a gas can near a burn operation on the southern flank of the Rim Fire near Yosemite National Park in California. In this Friday, Aug. 30, 2013 photo provided by the U.S. Forest Service, a fire crew stands watch along a fire break near a burn operation on the southern flank of the Rim Fire near Yosemite National Park in California. (AP) - The wildfire burning in and around Yosemite National Park has become the fourth-largest conflagration in modern California history, fire officials said Sunday as clouds and higher humidity helped crews further contain the biggest blaze in the United States this year. The 2-week-old Rim Fire moved up a spot on the state's list of large wildfires dating back to 1932 when it grew to 348 square miles - an area larger than the cities of San Francisco, Oakland and San Jose combined - on Saturday, California Department of Forestry and Fire Protection spokesman Daniel Berlant said. Although the fire still is growing, it was 40 percent contained as of Sunday, up from 35 percent a day earlier. Moister air was expected to slow flames from advancing through brush and trees, giving firefighters room to set backfires, dig containment lines and to strengthen lines around threatened communities, U.S. Forest Service spokeswoman Pam Baltimore said. Full containment is not expected until Sept. The blaze started Aug. 17 in the Stanislaus National Forest and two-thirds of the land burned since then is located there as well. In Yosemite, 94 square miles have burned. The cause remains under investigation, Baltimore said. "Either way, if it was lightning or human-caused, they have not released any findings and we are not sure if and when that will be released," she said. Meanwhile, the dense smoke that obscured Yosemite's majestic views for the first time on Saturday and prompted air quality warnings was starting to ease, park spokeswoman Kari Cobb said. Although park officials advised visitors to avoid heavy exertion, Cobb said she has seen people outside running "and enjoying Yosemite, despite the smoke." "The park was actually busier than I thought it would be," she said. A 427-square-mile fire in San Diego County that killed 14 people and destroyed more than 2,800 structures a decade ago tops the list of California's largest wildfires. The Rim Fire has claimed 111 structures, 11 of them homes, but no lives.