(CNN) - It's been an especially devastating year for wildfires in California, with four of the deadliest in the last 12 months, according to state records.
In October, fires ravaged Northern California's wine country in Napa and Sonoma counties, destroying an estimated 8,900 structures and leaving more than 40 people dead.
Now a round of wildfires has broken out in Southern California, and they may be scarier than the October fires.
The Thomas Fire in Ventura County had burned 96,000 acres as of Thursday, making it twice as big as Washington, D.C., and nearly three times bigger than the most destructive of the Napa-Sonoma fires.
And these fires are growing fast. Authorities don't know the cause of these new blazes, but several factors contribute to their terrifying speed.
Powerful winds are spreading the fires
The Santa Ana winds, an annual occurrence in Southern California, are more powerful than usual, with gusts of 80 mph expected in higher altitudes Thursday. In the lower levels, winds of 50 to 70 mph were likely.
The winds can push a fire the length of a football field in a minute, said Scott McLean, spokesman for the California Department of Forestry and Fire Protection, or Cal Fire.
And the hundreds of embers pushed in front of the fires become small spot fires themselves, he said.
High winds also dissipate fire retardant dropped by aircraft, McLean said.
The Diablo winds caused the same problems in Northern California in October.
Low humidity has dried out Southern California
The off-shore winds come from dry, desert areas. And when the winds come down the mountains, they dry out even more.
They bring dry air and low humidity, even to coastal areas, said CNN meteorologist Judson Jones, causing vegetation to become more flammable than normal.
McLean said the humidity is now around 7%, while it's usually between 20% and 30%.
Years of drought created a surplus of dry vegetation
California has been through five years of drought, leaving 102 million trees dead and dry, highly flammable vegetation upon thousands of acres, McLean said.
The state enjoyed much more rain than usual last winter and spring, but it ended up with even more vegetation that dried out in the summer heat, providing extra fuel for the wildfires.
The terrain creates other challenges
The hills and canyons of Southern California are beautiful, but they can make it difficult for firefighters to gain access to wildfires.
The canyons can accelerate fires, McLean said. They act as funnels for the wind, which pushes already heated air upward.