Aug. 27, 2013 at 12:27 AM ET
The California wildfire that's burning in and around Yosemite National Park, known as the Rim Fire, illustrates how wildfires can spread quickly — and what firefighters can do to block their path. It also shows how the flames can endanger cities far beyond the fire lines.
The fire is raging amid some of the country's most rugged and picturesque vistas. California fire officials say at least 4,500 structures are threatened. The blaze is also having an impact on San Francisco, more than 100 miles away.
How fast can wildfires spread?
Wildfires get their start either from natural causes (such as lightning) or human-related causes, which range from campfires that haven't been put out properly to the sparks thrown off by all-terrain vehicles and motorcycles. It's not yet known how the Rim Fire got its start on Aug. 17 — that investigation could take weeks or months.
The speed of a forest fire's spread depends on a wide range of factors: Drier wood makes for a faster, hotter fire. Fires spread more quickly if they're rising up steep terrain, or stoked by high winds. The Rim Fire made quick progress in part because it wasn't confined to the ground, but leaped from tree crown to tree crown. Gusts of up to 30 mph can carry burning debris into the air and transport it far beyond the fire's front lines to set fresh fuel alight — a phenomenon known as "spotting."
Crown fires typically advance a quarter-mile per hour if unchecked, but experts say they can move "like a freight train" in extreme conditions, at speeds of up to 19 mph. In less than 10 days, the Rim Fire has stretched out to about 15 miles from east to west, despite the efforts of thousands of firefighters. It has burned about 160,980 acres —about 252 square miles — so far and is continuing to grow.
How do you slow down the spread of a wildfire?
If the fire is burning in relatively narrow lanes, firefighters can take the direct approach, which involves constructing control lines so that there's nothing to burn when the flames reach the barrier. Fighting the Rim Fire requires a less direct strategy, however: The teams have to get ahead of the fire and build their control lines in anticipation of the flames' spread. Helicopters and air tankers are dropping fire retardants and water to slow down the fire's spread. Water drops and sprinklers have also been deployed to direct the fire away from homes and buildings, in a strategy known as "point protection."
The goal is to contain the fire: Officials estimate the percentage of containment based on the state of the control lines as well as other factors, such as wind and humidity. The Rim Fire went from being 7 percent contained on Sunday to 20 percent late Monday. That's because officials were seeing higher humidity over the weekend, which helps tamp down wildfires — and also because the wind direction has stabilized, blowing uniformly out of the southwest rather than erratically. As a result, firefighters can concentrate their efforts on the fire's northeastern edges.
How serious is the threat to San Francisco's water and power supply?
California Gov. Jerry Brown has declared a state of emergency for San Francisco due to the potential fire threat to the city's power and water supply. So far, the city has not been directly affected — but it is incurring costs due to the need to replace lost electricity-generating capacity.
Two of the three powerhouses that provide electricity for San Francisco were damaged by the fire and have been taken offline. Charles Sheehan, a spokesman for the San Francisco Public Utilities Commission, said assessment teams are gauging just how serious the damage is. To close the power gap, San Francisco has been purchasing electricity from other utilities via the regional electrical grid. The cost has mounted to $700,000 since the fire began, Sheehan told NBC News.
The flames have come within a mile of the Hetch Hetchy Reservoir, which provides about 85 percent of the municipal water for 2.6 million people in the San Francisco area.
Officials have been worried that ash and debris from the fire would fall into the reservoir, creating problems for the water supply. As a precaution, some of the water normally stored at Hetch Hetchy has been pumped to other reservoirs.
Some ash indeed reached the reservoir, but so far it hasn't sunk far enough to reach the intake pumps. Sheehan said no change has been noted in water turbidity — that is, cloudiness. If the sensors in San Francisco's water system show a significant rise in turbidity, the Hetch Hetchy Reservoir would be closed off, and the water utility would switch to other supplies. "There are several months of local storage," Sheehan said.
Wildfire debris can change the character of a community's water supply, as residents of The Dalles, Ore., found out this summer. The Dalles Chronicle reported last week that heavy smoke and burning debris imparted a "smoky" taste and smell to the city's surface water supply, which comes from the South Fork Mill Creek drainage basin. The city's Public Works Department says there's no adverse health effect, but it is looking into ways to eliminate the aesthetic effect by treating the water or blending it with well water from city reservoirs.
How hot can wildfires get?
Wood bursts into flames at about 572 degrees Fahrenheit, but in a wildfire, the temperature rises as the area of the burn increases. Typical wildfire temperatures are around 1,500 degrees F, but an intense fire can get hotter than 2,100 degrees F.
How big can they get?
At more than 150,000 acres, California's Rim Fire is the largest wildfire so far this year in the Lower 48 U.S. states. (Earlier this summer, Alaska's Lime Hills Fire burned more than 200,000 acres.) The Rim Fire ranks among the 20 largest California wildfires in the last 80 years. But in the national scheme of things, it doesn't even rank in the top 50. The largest wildfire recorded by federal officials is 2004's 1.3-million-acre Taylor Complex Fire in Alaska, followed by the 907,245-acre East Amarillo Complex Fire in Texas (2006) and the 652,016-acre Murphy Complex Fire in Idaho and Nevada (2007).
How much has been spent fighting wildfires?
The Rim Fire is likely to add millions of dollars to a tally of firefighting costs that already amounts to $44 million for the state of California during the current fire season. "We're eight weeks in [to the state government's budget year] and we've spent roughly a quarter of what's budgeted," The Sacramento Bee quoted H.D. Palmer, a spokesman for the California Department of Finance, as saying.
The state has more than $1 billion in reserve, in case costs exceed the $172 million that's been budgeted. And there's also the prospect of reimbursement from the federal government: The Federal Emergency Management Agency announced on Monday that it would reimburse California for up to 75 percent of the eligible costs of fighting the Rim Fire.
So far this year, more than $1 billion in federal funds has been spent fighting wildfires, according to the National Interagency Fire Center. That compares with a 10-year average of $1.4 billion. Statistics kept since 1985 show that the biggest years for federal firefighting expenses were 2006 ($1.925 billion) and 2012 ($1.902 billion).
More about this summer's Western fires:
Thanks to Robyn Broyles, communications specialist at the Bureau of Indian Affairs, for explaining the ins and outs of wildfire science.
Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the NBC News Science Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with NBCNews.com's stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.