Why the California wildfires are spreading so quickly

At its fastest, the Camp Fire in Northern California spread at more than a football field a second, or around 80 football fields per minute. It burned through 20,000 acres in less than 14 hours on Thursday. By Friday, it had reached 90,000 acres.

The Woolsey Fire in Ventura and Los Angeles counties doubled in size in a 90-minute period Friday morning, to 8,000 acres -- by Friday evening, it was up to 35,000 acres.The Hill Fire in Ventura County torched 10,000 acres in six hours Thursday.

Why are the fires moving so quickly?

The combination of strong offshore winds that have been gusting as high as 70 mph, humidity values in the single digits and extremely dry conditions are leading to the perfect conditions for wildfires

Powerful winds are spreading the fires

In Southern California, Santa Ana winds gusting at 50-70 mph in Ventura and Los Angeles counties are expected to ease Saturday.

But a new round of those winds is forecast to start Sunday and last into early next week, bringing more dangerous conditions.

These Santa Ana winds will be a little weaker than the ones that precede them.

In Northern California, a classic offshore flow has been set up that's similar to the Santa Ana winds.

A second round of gusty winds is forecast to develop Saturday night into Sunday.

Lack of rain creates dry conditions

There's a lot of dry vegetation for the fires to burn.

Though the drought in California has eased somewhat, all of the state is still abnormally dry, CNN meteorologist Taylor Ward says.

More than half the state is in a moderate drought or worse and 18% of the state, including the area near the Woolsey fire, is in severe drought, according to the US Drought Monitor.

Over the past month, much of the state has received less than 5% of its normal rainfall.

Tim Chavez, a fire behavior analyst for Cal Fire, said the Camp Fire is "unique to be this late in northern California."

Normally, a series of storms would have dampened the woods and brush by now, he said Friday at a news conference. But it hasn't rained much this fall, he said, creating "mid-summer-like conditions."

"It kind of lined up perfectly to have the large 90,000-plus acre fire run we had yesterday," Chavez said. "Fuels will continue to get drier and drier and drier until we do finally get a season-ending rainfall event."

The terrain makes it hard for firefighters

The hills and canyons of Southern California are beautiful, but they can make it difficult for firefighters to gain access to wildfires.

The fire that broke out Friday near the Los Angeles Zoo and Botanical Gardens occurred on a steep hillside that could not be accessed by firetrucks, so firefighters had to approach the fire on foot, CNN affiliate KCBS reported.

The canyons can accelerate fires. They act as funnels for the wind, which pushes already heated air upward.

Climate change may make things worse

Because of climate change, California's forests will become more vulnerable to wildfires, according to California's fourth climate change assessment, issued last August.

The assessment says that by the year 2100, if greenhouse gas emissions continue to rise, the average area burned by wildfires would increase 77%.

In the areas that have the highest fire risk, wildfire insurance is estimated to see costs rise by 18% by 2055.

CNN's Carma Hassan, Taylor Ward and Judson Jones contributed to this report.

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