Wildfires used to ease in intensity at night, but a study says it’s not the case anymore

By Jennifer Gray, CNN meteorologist

(CNN) — The West is burning out of control. Warmer temperatures and historically dry conditions are leading to longer and more devastating wildfire seasons than we have ever seen before.

And now a new study suggests fires are becoming bigger and stronger at night, the time when firefighters used to be able to gain some ground.

“I personally saw extreme fire behavior at night on the Thomas Fire in Ventura and the Flash Fire near Saint Helena and Calistoga,” said Chief Jesse Alexander with the Yuba City, California, fire department.

He has fought some of the state’s largest fires and has seen the intensity of the nighttime fires firsthand. “Night changes everything,” Alexander explained. “Your senses are heightened due to lack of visibility, which makes it difficult to accurately determine fire size,” adding, “Fires always look bigger at night.”

In reality, nighttime fires are indeed getting bigger and more intense, making them more difficult to fight.

<p>A Colorado home burns beneath the nighttime sky.</p>

Marc Piscotty/Getty Images

A Colorado home burns beneath the nighttime sky.

Fires at night more frequent

“Nighttime fires have become more intense and more frequent in recent decades, as hot, dry nights are more commonplace,” found the study led by the Cooperative Institute for Research in Environmental Sciences’ (CIRES) Earth Lab at the University of Colorado Boulder.

While the scientists found it to be true across the globe, the western U.S. seems to be off the charts.

“There are certain places that are definitely seeing a much bigger increase than what the global picture is, and the western U.S. is one of them,” Jennifer Balch, lead author on the study, told CNN.

Scientists used satellite imagery and hourly climate data from 81,000 fires around the world to measure the Vapor Pressure Deficit, or VPD.

Weather satellites have the ability to sense different wavelengths, and one of those is fire temperature, as seen in a tweet from 2020.

“This is a really important variable for fire science,” said Balch. “It’s essentially the combination of temperature and humidity, and it indicates how fast the atmosphere is sucking moisture out of fuels.”

“When VPD is relatively low, the air is cool and moist, and fires cannot thrive,” the study outlined.

However, when the VPD is high, “the air is hot and dry, parched and primed for burning,” according to the study.

Scientists found nighttime conditions are becoming much more conducive for the spread of fires, stating “one full week of additional flammable nights per year in burnable lands across the globe in the last 40 years. And in the Western US, it increased by more: 11 nights, a 45% increase over the four decades (1979-2020).”

The study also used remote sensing and modeling techniques to track the progression of nighttime fires from tens of thousands of fires around the world.

They determined “globally, night fires have become 7.2% more intense from 2003-2020.”

However, in the western U.S., the number was four times greater at 28% more intense.

“It is related to the combination of temperature increase in the West plus the drought conditions or how dry it is,” Balch said. “Those two factors are playing a significant role in how hot and dry our nights are across the western U.S.”

There was also a 36% increase of flammable nighttime hours between 1979 and 2020 while daytime flammable hours increased by 27%.

“The fact that fires are passing from day to night means that we’re witnessing more extreme fires in terms of how fast they’re moving, how long they’re running, and how big they’re getting,” Balch said.

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