On Friday afternoon, a wildfire swept across Interstate 15, a busy freeway that connects Los Angeles and Las Vegas in Southern California, setting dozens of vehicles on fire.

“I have been doing this for 15 years, I have seen vehicles burn, but I have never seen anything of this magnitude,” said Justin Correll, who commanded one of the fire engines on the scene. “I have never seen so many vehicles impacted so quickly. It was pretty intense.”

The blaze, called the North fire, has been burning since July 17 and has affected almost 4,300 acres of land in and around the San Bernardino National Forest. While it is the largest fire in California currently, the North fire is small compared with many other fires burning across the state. A few of them have affected more than 100,000 acres but are in tracts where the threat to life and property is considerably less. Incidents where forest fires quickly descend on roadways and burn vehicles are rare, experts say, but they agree that what makes wildfires move needs to be better understood.

The United States is experiencing a severe fire season this year; as of yesterday, almost 5.5 million acres of land had been affected by wildfires. This is significantly more than the 10-year average up to this date (from 2005 to 2014). A recent study found that fire seasons grew longer by almost 20 percent in a span of 35 years in most parts of the world, and linked this lengthening to climate change. Correll contended that the agonizingly long drought in the state contributed to the abundance of inflammable dry fuels, an important factor that propelled the wildfire.

Mark Finney, who is a researcher at the Rocky Mountain Research Station in Missoula, Mont., has been studying flame dynamics at the Missoula Fire Sciences Laboratory to uncover the mechanism by which wildfires spread.

“In 98 to 99 percent of the cases of such fires, no modeling is done, there is just no time,” said Finney, who is also a scientist with the U.S. Forest Service. In almost all these cases, it is humans who “interpret the fire,” he said, and make decisions accordingly. When fires break out, crews are immediately dispatched and information is relayed back to a control center about the nature of the fire: how big it is, how fast it is spreading and in which direction it is headed.

Though it is widely believed that heat is transferred through radiation during wildfires, research by Finney’s team showed that it was also being transferred through convection. Radiation is a way of transferring heat that does not require a medium -- for example, sunlight, which travels through space and is felt as heat. Convection on the other hand is the movement of heated bodies themselves, like boiling water in a pot where the water at the bottom gets heated and rises to the top while the cool water from the top descends, moving closer to the flame. This allows heat to transfer to all the water molecules. During wildfires, both heat radiation and convection are happening simultaneously.

“Wildfires are very different than other fire problems that have been studied extensively,” Finney said. “To start with, there is a difference in the kinds of fuels.” In wildlands, the fuels are very fine material like grasses, leaves, pine needles and small branches, which is very different from fuels present in buildings, in cars or at industrial sites. “The wildland fuels are like the edges of a page—they get ignited very, very quickly from contact with flames,” he said.

Fires can pulsate, jump and swing with the wind
Flames from wildfires are very low density, so they expand and rise like a hot air balloon, Finney added, but they don’t rise in a steady fashion, instead pulsating and forming circulations affected by the wind. One of the main reasons that fires are so unpredictable is that they swing with the wind. It was most likely strong gusts of wind that led the fire onto the freeway, experts said.

“There are two factors that help fires spreadwinds and topography,” explained Scott L. Stephens, a professor of fire science in the ecosystem science division at the University of California, Berkeley. “The thing about wind is, it can change so quickly and the fire will change with it—it can happen in 15 seconds,” Stephens said. A fire can also race up a slope very rapidly, he added.

A wildfire can spread out like waves in moderate conditions, advancing quickly for a while, then slowing as it reaches out for more fuel and then surging forward again. But it can also jump across stretches when the conditions are right—this movement is called spotting. This is most likely what happened Friday, Finney said, when the fire burst onto the freeway and ignited blazes in many vehicles. “It wasn’t burning the asphalt, it was throwing embers,” he said, moving forward by igniting new fires ahead of it. “This is how fires get across lakes, freeways and ridges—with high winds, low humidity and spotting.”

Once officials realized the fire was headed for the freeway, Correll said, they assessed the potential damage and even anticipated multiple casualties from the incident. But on Friday, because of an evacuation of people stuck on the freeway, no lives were lost and only a few suffered minor injuries. However, in local media reports, several people recounted watching in horror as their vehicles caught fire, or being held hostage in their cars as the flames lashed at nearby vehicles.

An investigation into what caused the North fire is ongoing, but it was most likely caused by humans, experts said. “Most of the fires at this time of the year in this area are human-caused,” Finney said, “and because they are caused by humans, they are, well, quite near humans.”

Almost every fire like that is a threat because California is so densely populated, Stephens said.

What complicated firefighting efforts Friday was the presence of at least five drones that were most likely privately owned hobby drones, a fire services release said. These drones are commonly used to take aerial shots. The helicopters carrying water to douse the fire could not engage because of the drones flying close by, Correll said. The presence of the drones put the aerial fire suppression operation on hold for almost 25 minutes, the fire service release noted. Since Monday, the cost of suppressing the North fire totaled almost $2.5 million. The department hopes to contain it fully by today with a little help from timely rains.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500