What Caused Napa, California Wildfires?

More evacuations were ordered by Tuesday, Oct. 10 as the number of wildfires sweeping through Northern California grew to more than a dozen. The more widespread one already killed at least 17 people, driven by winds and dry conditions in the region.

REUTERS/Stephen LamA burning structure is seen at the Hilton Sonoma Wine Country during the Tubbs Fire in Santa Rosa.

The largest of the wildfires were draped over the Sonoma, Napa and Mendocino counties in the state, as fires drove people to flee their homes. Officials in the Sonoma County and the nearby city of Santa Rosa have called for more evacuations, with more than 20,000 people having been ordered to leave the affected parts of Northern California, according to CNN.

Most of the dozen and more fires now raging in Northern California sprang up on Sunday night, according to Ken Pimlott, director of Cal Fire. The cause is still being determined, with the possibility that the fires were started by people also being considered.

"All these fires are under investigation," Pimlott said, noting that for now, their focus remains on combatting the fires and conducting rescues.

He does note that there's a "fairly minimal" chance that lightning strikes started the fires, which are being whipped into infernos by winds with speeds reaching up to more than 50 miles per hour.

A weather condition known in the area as the "Diablo winds" is making it difficult to keep the wildfires under control, according to Business Insider. The month of October is a particularly difficult time to rein in wildfires, as well.

Usually, humid winds originate from the Pacific Ocean and end up inland. The fall season, however, gives rise to high-pressure areas in the Great Basin, the region covering much of the inland continent of western U.S., including Nevada and Utah.

Winds come down from these areas down to Northern California, being compressed and warmed up as they descend to sea levels. This process gives rise to "Diablo winds," dry gusts that can go over 50 miles per hour, according to the Los Angeles Times.

California Governor Jerry Brown emphasized just how quickly these winds can spread the wildfires in his statement putting the Napa, Sonoma, and Yuba counties under a state of emergency on Monday.

"This is really serious. It's moving fast. The heat, the lack of humidity, and the winds are all driving a very dangerous situation and making it worse," the governor said.