Wildfires that ravaged Northern California in October led to 44 deaths and the destruction of over 8,400 structures, including homes and businesses, making their mark as the most destructive wildfires in state history. On Dec. 5, the fast-moving Thomas fire swept into the city of Ventura, burning 50,000 acres and forcing 27,000 people to evacuate.
Here’s why California’s fall fire season has become so destructive — and why it could get worse.
Powerful winds, greater devastation
California is susceptible to fires year-round, but fires that occur in fall can be especially dangerous. The effects of hot, dry temperatures during the summer worsen in later months due to winds from the Great Basin. Known as Santa Ana winds in Southern California and Diablo winds in the north, these powerful warm winds cause fires to spread faster and quicker.
A wet winter leading into a long, dry summer
Historic amounts of rain and snow in the winter kept large fires from burning in California until April. The 2016-2017 water year set records in the northern Sierra Nevada, which recorded a total of 94.7 inches of rain throughout the year.
Average amount of precipitation per month in California
While wildland vegetation grows every year during the wetter months, the heavy rains led to a larger amount of growth in areas like Santa Rosa and Napa, which hadn’t seen large fires in several years. New brush growth is very flammable and can create embers that can travel a considerable distance.
This summer was the hottest ever recorded in California, allowing for new vegetation to dry up.
Composite images from the Landsat 8 satellite show just how dry the area around Santa Rosa had become after a recordbreaking rainy winter. In the first months of the year, record-setting rainfall led to increased vegetation in the area, but by summer, much of it had dried up.
“You kind of have this perfect storm of weather conditions,” said Yufang Jin, an assistant professor of remote sensing and ecosystem change at UC Davis, who co-authored the study on Southern California wildfires. Given these hot, dry conditions, Jin said, it’s not unusual to see more than 10 wildfires burning in a close area, much like the clusters of fires in Northern California this week.
As urbanization grows, so does the risk of wildfire
Over time, the edges of cities have encroached on wild spaces. The close proximity between private property and wildlands allows fires to spread more rapidly and damage or destroy more property in the process.
That allows fires during this part of the year to spread more rapidly into urbanized areas, Jin said.
Many of these at-risk areas are in wildland-urban interface areas, or WUIs — where housing and vegetation intermix or come within close proximity of each other.