Fires are growing faster in the western United States. The fastest of them – termed “fast fires” – often erupt near towns and account for most structural wildfire damage according to researchers who analyzed over 60,000 fires from 2001 to 2020.

Fast fires grow about 4,000 acres or more on their fastest day, at least two football fields a minute. That single-day growth threshold for a “fast fire” is even higher in California at almost four football fields a minute.

Humans and our infrastructure start nearly all of the fires that threaten our homes. Exactly why fires become fast is a developing area of research, but weather and fuel conditions can feed the speed.

Of the 20 fastest fires between 2001 and 2020, 16 destroyed structures and all but one fire came within 2.5 miles of a property.

Although many fast fires start near populated areas, the location alone doesn’t determine whether a fire will be fast.

“Probably hundreds, maybe thousands of ignitions happen around the Hollywood Hills every year. Most of those fires are able to be put out,” said Maxwell Cook, a research assistant with the Cooperative Institute for Research in Environmental Sciences and geography PhD candidate at the University of Colorado Boulder.

A wildfire not only needs the initial spark but also the optimal fuel and weather. When an ignition overlaps with poor conditions like heat, drought and wind, fires can spread faster than firefighters can contain them.

In the case of the fires in Los Angeles, a three-month drought parched the vegetation and buildings, making for piles of kindling. The seasonal high-speed winds blew embers to nearby fuel and made the pace of fire growth impossible to match.

“When fires are moving that fast, often times there’s not much to do in terms of stopping the fire. It really becomes a situation of getting people out,” Cook said.

Researchers have observed an increase in human-started fires and expect that pattern to continue as the population grows and more people move into wild areas. The conditions that lead to faster fires are also expected to worsen as the West becomes hotter and drier in line with climate projections.

The type of vegetation in an area can determine how quickly a wildfire spreads. The majority of fast fires burn through grasslands where the short, dry grass is easily ignitable. Broadleaf forests tend to burn more slowly in part due to higher moisture levels in the vegetation. The fires in LA were different, spreading by structure-to-structure ignition.

“When a house burns, it produces several million embers and flames up to 5 meters (14 feet) high for an hour, contributing to other houses catching fire,” said Guillermo Rein, professor of Fire Science at Imperial College London.

Housing density can work for or against fire suppression and potential damage. Heavily populated areas can often act as a fuel break for a wildfire because they tend to have less flammable vegetation. However, they face the risk of one burning house threatening another. More remote areas don’t have the same house-to-house risk but can be difficult for firefighters to reach.

“Pretty much any fire that starts in California is near where a lot of people live,” Cook said. “It’s its own beast.”

The frequency and size of wildfires is well documented, but Cook and his team say measurements of fire speed deserve more attention and are integral to assessing human safety in fire-prone areas.

“Incorporating understanding of fire speed into city planning and into fire management, that’s gonna give you a lot more nuanced understanding of what risk you’re actually assuming.”