Hotter, drier night air may be making wildfire containment more difficult

In response to anecdotal reports from fire managers that wildfires were burning hotter and longer into the night, making them more difficult to contain, Forest Service researchers and partners investigated historical data on nighttime atmospheric vapor pressure deficits across the United States. The vapor pressure deficit is a measure of the drying power of air. Increases in the nighttime vapor pressure deficit are suspected to be a result of human-caused increases in nighttime temperatures.   

The researchers found that nighttime atmospheric vapor pressure deficits are higher across the western United States compared to 40 years ago – meaning that nights in this region are now drier. In areas where the drying is greatest, including the San Joaquin Valley and Bitterroot-Blue Mountains, the drying is 2 to 3 times more than anticipated predictions based on climate models. The trend appears to be driven by a combination of higher temperatures and lower humidity at night.

Watch Brian Potter, research meteorologist with the PNW Research Station explain this study in the following 2-minute video: Air temperature, moisture content, and nighttime fire behavior.