The Marshall Fire burns through the night on Dec. 31, 2021. Credit: Anthony Albidrez

Historically, the fall of dusk and maturation of night has subdued fire. As the sun no longer warms the earth and temperatures stabilize, winds that hitherto brought flames an endless supply of oxygen are reduced to breezes that don’t so much fan fire as periodically cause grunts and shiftings in its slumber.

In recent years, however, nighttime temperatures have not been dropping as they once did, and the increase in humidity that usually accompanies cooler temps has become similarly absent. This creates conditions that encourage flames to sally forth on quests begun in daylight.

“You now have 24-hour fires,” said Adam Mahood, a former post-doc at CU Boulder’s Earth Lab who recently began another post-doc at the USDA Agricultural Research Service in Fort Collins. “You don’t get that break you used to.”

Such were the findings published by Mahood and colleagues this year in Nature, one of the premier science journals. The study, “Warming weakens the night-time barrier to global fire,” found, among other things, that nighttime fires have increased in intensity by 7.2 percent between 2003 and 2020. But as is often the case, science merely confirmed what those in the field were already wary of. 

“Firefighters know this is happening,” Mahood said. “We’re just documenting it and looking at the data.”

Brian Oliver, Boulder Fire-Rescue’s Wildland Chief, said that on a smaller scale it’s harder to see broad trends, though some recent fires have certainly adhered to data shown in the study. “There are plenty of times when conditions don’t moderate at night,” Oliver said. “The Marshall Fire is a good example. But the NCAR Fire, that first night things did calm down quite a bit. The second night we got some intensity but we were already in the mop-up phase.”

Oliver noted that the recent fire near Wonderland Lake on June 17 continued to burn with vigor through the night due to high fuel loads and humidity remaining low.

An increase in nighttime fire intensity hobbles firefighters’ ability to contain blazes. Often, little can be done against a full-grown fire but get people and movable assets out of the way. Only once Mother Nature satiates her appetite can containment begin. Evenings were once when she took a breath between bites that allowed firefighters to counter. Not as much anymore.

The Marshall Fire burned homes in Louisville down to their foundations on Dec. 30, 2021. Credit: John Herrick

‘Portending things to come’

One of the factors cited by the study is an increased nighttime Vapor Pressure Deficit, or VPD: a measurement of the difference between how much water is currently in the air and how much water the air could potentially hold when fully saturated. Often used by those invested in plant communities — whether biologists or farmers — it gives an indication of the evaporation demands on plants. The higher the VPD, the greater the vehemence of evaporation. In greenhouses where the VPD is too low, a film of water sits on plants, promoting growth of fungal pathogens. When the VPD increases, however, and evaporation accelerates, the landscape in question is sucked dry and primed for fire.

Mahood said VPD might be a better tool to determine Red Flag Warning days. Currently, one factor in dictating what days earn a Red Flag Warning is a Relative Humidity, or RH, of 15% or less for several hours. A problem with RH (which measures the percent of water currently in the air compared to the amount needed for saturation) is the amount of water air can hold increases exponentially the hotter it is. Air that is 80 degrees Fahrenheit can hold roughly twice as much water as air at 60 degrees Fahrenheit. 

So depending on how warm or cool it is, RH might not give an accurate account of how much moisture plants are losing. The day of the Marshall Fire on Dec. 30, for instance, did not receive a Red Flag Warning, as the RH was just high enough to escape detection. Mahood said that had VPD been used instead — with VPD unaffected by temperature changes — the high deficit would have indicated that evaporation was taxing the landscape and turning all plant matter into kindling.

Oliver said VPD is an interesting measurement of “how thirsty the atmosphere is,” and he’s looking forward to researchers doing more work with it. He noted that the day of the NCAR Fire on March 26 was another day where the RH did not meet criteria for a Red Flag Warning, despite conditions being highly conducive to ignition. Yet he also said that with Red Flag Warnings already increasing in frequency, taking into account VPD would only elevate that trend. But maybe that isn’t a bad thing.

“We’ve had more Red Flags so far this year than we’ve ever had in previous years; what would happen if you changed the criteria?” Oliver said. “There’ve been lots of days with an RH of 16 or 17 percent, so it doesn’t trigger the criteria, but that’s dry enough.”

As a plant and fire ecologist who studies the effect of fire on plant communities, Mahood is able to provide insights that others might not consider. Further addressing the Marshall Fire, he explained that many prairie flora are adapted to snowfall blanketing them from late fall to early spring. As a tactic to survive such chilly coverings, plants in the area pull moisture from limbs protruding above ground to consolidate their efforts to root systems.

“Plants in northern Colorado are adapted to there being a winter,” Mahood said. “If there’s no precipitation until Christmas and it’s still 65 degrees out, you have all these plants that are going dormant because of the angle of the sun in the sky, and the vegetation is probably drier than it would be in mid-August.”

Because even in August, as Mahood explained, there are drought-tolerant plants that, despite looking brown, are still photosynthesizing with water in their tissues. But in winter, without snowfall, the prairie becomes an expanse of shrubbery devoid of moisture, ready to aggrandize flames.

Given the unlikelihood of the climate halting its descent (at least for the time being) into human-caused chaos, there is little hope that the Marshall Fire was an aberration. Rather, as Mahood said, it was likely “portending things to come.”

“There’s going to be more and more winters where we get less snow,” Mahood said. “And there’s always going to be wind in Boulder because of its proximity to the mountains. Wind plus dry fuels is going to be a pretty bad combination.”

Underscoring this anticipated new reality, the abstract of the nighttime fire study concludes: “We expect that continued night-time warming owing to anthropogenic climate change will promote more intense, longer-lasting and larger fires.”

What can we do to combat this bleak trend? Mahood provided the same answer others have so many times before: “The only thing that can be done is get to a carbon neutral or carbon negative environment as soon as possible.”

Tim Drugan is the climate and environment reporter for Boulder Reporting Lab, covering wildfires, water and other related topics. He is also the lead writer of BRL Today, our morning newsletter. Email:

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