Firefighters in the western U.S. have noticed a disturbing trend in recent years: fires are intensifying earlier in the morning and burning longer into the night.

“Firefighters are still fighting the fire at 10:00 or 11:00 at night when historically they thought they could stop at 8:00,” said Brian Potter, a research meteorologist with the U.S. Forest Service at the Pacific Northwest Research Station. “What that means is the fire managers don’t get a break.”

Satellite data and ground reports indicate wildfire activity has increased at night in recent decades, meaning firefighters have less time to rest and regroup overnight. This year, the Caldor fire southwest of Lake Tahoe, which has consumed more than 220,000 acres as of Sept. 24, more than doubled overnight early on. The Windy fire also experienced significant overnight growth as it burned in the Sequoia National Forest.

Potter and his colleagues investigated why firefighters are seeing more nighttime fire activity now than at the beginning of their careers. In a recent study, they found air over most of the western U.S. has become drier and warmer at night over the past 40 years, influencing the rate at which vegetation and other fuels for fire will dry out and burn.

“As long as the air is still dry relative to the fuels, the fuels will continue to dry out and the fire will continue to burn more energetically,” said Potter.

Multiple fires were ablaze at once in parts of California and Oregon in early September 2020. Here's how those wildfires can diminish air quality. (Allie Caren/The Washington Post)

The team assessed the night air’s drying power — or how much moisture the air could uptake from the fuels — through a quantity known as the vapor pressure deficit. It measures the difference between how much water vapor the air would have when completely saturated and how much it actually has (i.e. vapor pressure of saturated air minus vapor pressure of the current air). A high vapor pressure deficit signifies a very dry and thirsty atmosphere conducive to the spread of fires whereas a low deficit means the air is close to saturated and a hindrance to fires.

“We’re interested in this variable vapor pressure deficit because we’re interested in better understanding the rate at which moisture in fuels can dry to the atmosphere,” said Andy Chiodi, the lead author of the paper and climate scientist at University of Washington and NOAA. “If the atmosphere is saturated, it’s very hard for the fuels to dry.”

The team compared vapor pressure deficit for the core of fire season for most of the western U.S. (July to September) for the period of 2010-2019 to 1980-1999, when many current firefighters began their careers.

The team found that drying power increased from 1980-2019 over most of the western U.S., shown in the map above. The greatest increases, around 50 percent higher, were in California’s central valley near the southern Sierra Nevada and the Bitterroot-Blue Mountain region of Idaho and surrounding states — by foothills of mountains near arid plateaus.

The study’s results hold consistent in 2021 as well. The map below shows the vapor pressure deficit for July 2021, compared to the previous 40 July months. The shaded region shows where July 2021 had a higher nighttime average than any other July from 1980-2019.

Fires in this shaded region could be more prone to a drier nighttime than they would have been previously, said Chiodi. In July 2021, the Snake River Complex and Lick Creek fires were burning around Idaho, Oregon and Washington.

The Caldor fire, which began on Aug. 14, is located in the central Sierra Nevada (adjacent to shaded areas in northern California). The Windy fire, which began on Sept. 9, is located on the eastern edge of the San Joaquin Valley, by the southern Sierra Nevada.

Chiodi said there are two potential factors that increased nighttime drying power. First, the saturated vapor pressure increased with rising temperatures brought on by global warming.

Second, the actual amount of water in the air decreased at night in some areas. The team did not explore mechanisms of why actual water vapor pressure went down.

“It has less moisture in the air, meaning more potential for drying at night of the fuels, then [vapor pressure deficit] goes up,” said Chiodi. “So all other things equal, drying of fuels at night goes up.”

Perhaps more surprising is the deficit increased more than climate models predicted. Climate models showed an increase in water vapor deficit, but projections were much lower than observations and not in the same pattern. Chiodi said the discrepancy suggests another possible factor beyond climate change, perhaps more natural atmospheric processes, are playing a part in affecting the moisture content of nighttime air.

He said it is important to understand “what process is responsible for this additional increase in vapor pressure deficit and then try to understand what’s the predictability associated with them.” Then, the team can better inform fire fighting conditions in the future.

The study’s findings are just one clue in parsing out why wildfire behavior can rage overnight. Fire behavior is influenced by hot, dry and windy conditions, said Potter. Vapor pressure deficit partly informs the hot and dry conditions of fire weather.

“A whole bunch of factors change from day to night, but humidity recovery is the most direct, discernible influence on the fuels, and the fuels are the most direct influence on the fire behavior,” said Potter. “It’s just another piece of the puzzle — the puzzle that has no edges.”