Firefighters have reported that Western wildfires are starting earlier in the morning and dying down later at night, hampering their ability to recover and regroup before the next day鈥檚 flareup.
A study by 91爆料 and U.S. Forest Service scientists shows why: The drying power of nighttime air over much of the Western U.S. has increased dramatically in the past 40 years. The was published online in July in Geophysical Research Letters, a journal of the American Geophysical Union.
鈥淣ighttime is an important time in fire management. When fires die down at night it gives firefighters a chance to rest, move equipment and strategize. The problem firefighters are reporting is an unexpected increase in nighttime fire activity,鈥� said lead author , a 91爆料 research scientist at the Cooperative Institute for Climate, Ocean & Ecosystem Studies, a joint center with the National Oceanic and Atmospheric Administration. 鈥淥ur findings support that this has been going on over the last 40 years over much, but not all, of the Western U.S.鈥�
Earth鈥檚 atmosphere is warming due to climate change, and warming in many places has been greater at night. Warmer night air had been suspected as the culprit , with burns continuing later into the night.
The new study, however, shows it鈥檚 not just that the night air is warmer, but also a dramatic shift from 1980 to 2019 in its drying power 鈥� how much moisture the nighttime air can carry away from the fuels 鈥� over much of the Western U.S. This shift is not captured in climate models, and the authors say it could be related to natural long-term cycles rather than to climate change.
鈥淲e paid special attention to the change in recent years compared to the conditions seen in the 鈥�80s and 鈥�90s, which is when many of the current firefighters started their careers, and presumably formed their ideas about what normal fire behavior should look like,鈥� Chiodi said. 鈥淲e tried to quantify the changes that we were hearing about from firefighters.鈥�
The study looks at the 鈥渧apor pressure deficit,鈥� or the difference between the moisture in the air and the saturation moisture level at that air temperature. This difference is a measure of the air鈥檚 drying power.
鈥淚n the southern Sierra Nevada, the average summer nighttime vapor pressure deficit for the recent decade was 50% higher than the average in the 鈥�80s and 鈥�90s,鈥� Chiodi said. 鈥淚 was surprised 鈥� it鈥檚 unusual to see geophysical data change that dramatically.鈥�
Some of this shift in vapor pressure deficit is happening because warmer nighttime air, caused by climate change, produce higher saturation values. But part of the drying power is happening because the nighttime air in some regions has less moisture, and that effect is not predicted by climate change models, at least this much or in this pattern. The authors find a possible connection to the Pacific Decadal Oscillation, a long-term cycle that can influence inland weather.
The increased drying power of nighttime air is especially pronounced in California鈥檚 San Joaquin Valley and in the Bitterroot-Blue Mountain Region 鈥� including parts of the Idaho Panhandle, southeast Washington, northeast Oregon and western Montana.
鈥淔irefighters had been saying for several years that they feel some fires burn later into the evening than they used to,鈥� said co-author at the U.S. Forest Service鈥檚 Pacific Wildland Fire Sciences Laboratory. 鈥淲e found that in some areas, the amount of water in the air is decreasing, sort of doubling up on the warmer nights. These areas, including where the Snake River Complex and Lick Creek fires are burning right now, are much more likely to have fires burn late into the night.鈥�
The analysis used hourly weather outputs from the European Centre for Medium-Range Weather Forecasts. The recently released hourly reconstructions of historical weather allowed investigation of daily cycles.
The next step, Chiodi said, is to further explore the causes of these changes in nighttime vapor pressure deficit. After that, he hopes to connect the atmospheric conditions more directly to fuel moisture and fire behavior.
The other co-author is at the U.S. Forest Service鈥檚 Pacific Wildland Fire Sciences Laboratory in Seattle. The research was funded by the U.S. Forest Service through its research team and by NOAA.
###
For more information contact Chiodi at chiodi@uw.edu or Potter at brian.potter@usda.gov.