By Kasha Patel, The Washington Post

A new study showed nighttime air in the western U.S. is getting drier and warmer, potentially prolonging fire activity.

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.

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.”