A new study from the American Geophysical Union has found that small particles in the smoke from wildfires in the western United States condense and act as nuclei, affecting how water droplets in clouds are formed. The process may result in less rain, exacerbating drought in the west and turning forests into match sticks. According to the AGU study abstract, “Clouds were strongly influenced by smoke across the western U.S. On average, sampled clouds had about 5x as many droplets, and droplets were about 1/2 the size, as in clouds not influenced by smoke. Because of their small droplet sizes, these smoky clouds are expected to reflect more light and produce less rain than clouds in clean air.”
When wildfires send smoke up into the atmosphere, tiny particles fly up with it. Water droplets can condense on the particles in clouds.
The study's authors expected an increase in the number of water droplets forming in clouds as a result of wildfires, because more particles create more droplets. But the difference between smoky and clean clouds was bigger than expected, with smoky clouds hosting about five times the number of droplets than their clean counterparts. Smoky droplets were also half the size of pristine droplets.
That size difference is what could stop the drops from falling. Because small droplets are less likely to grow and eventually fall out as rain, wildfires in the western U.S. could mean less rain during wildfire season, according to the new study published in the AGU journal Geophysical Research Letters, which publishes high-impact, short-format reports with immediate implications spanning all Earth and space sciences.
Rail tank cars filled with water are used to soak a bridge in the below Tik Tok video, so that nearby flames do not ignite the structure. This is a dystopian hellscape if I ever saw one.
"Over the past couple decades, summer precipitation is down and temperatures are up. The cloud effects are likely an important part of all this. I'm hoping these results will spur detailed regional modeling studies that will help us understand the net impact of smoke on clouds and climate in the region," said Twohy.
If wildfire smoke is making rain less likely, feedback between smoke, dry spells and more wildfires could be more common in the future. Cloud microphysics are complex, so it may be a matter of time before these relationships are clear. Regardless, in connecting wildfire smoke to cloud changes and tentatively, precipitation, Twohy's new research pushes atmospheric physics and chemistry to catch up with climate change.
"As humans have perturbed the composition of the atmosphere, there are all these feedbacks and interactions that we don't even know about," said Carlton. "This experiment we're doing on planet Earth is altering clouds and the hydrologic cycle, at least regionally. I think this paper is scratching the surface of what we don't know."
The AGU, in a separate press release, also found that dryer and warmer air at night are worsening fires.
A new study suggests why: The drying power of nighttime air over much of the Western U.S. has increased dramatically in the past 40 years. The paper was published in Geophysical Research Letters, AGU’s journal for high-impact, short-format reports with immediate implications spanning all Earth and space sciences.
“Nighttime 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,” said lead author Andy Chiodi, a University of Washington research scientist at the Cooperative Institute for Climate, Ocean & Ecosystem Studies, a joint center with the National Oceanic and Atmospheric Administration. “Our findings support that this has been going on over the last 40 years over much, but not all, of the Western U.S.”
Earth’s 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 altering the daily pattern of wildfire activity, with burns continuing later into the night.
The new study, however, shows it’s not just that the night air is warmer. The study found a dramatic shift from 1980 to 2019 in its drying power—how much moisture the nighttime air can carry away from the fuels—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.
More research is needed on both topics. Neither study is in any climate change models.
