Rim Fire Prompts Questions About Wildfires

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, reports NBC News.

California fire officials say at least 4,500 structures are threatened. The fire is also having an impact on San Francisco, more than 100 miles away.

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.”

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.

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