When smoke rises from wildfires in the West, it bombards clouds with small airborne particles. What happens next with these clouds is mostly unknown. During the 2018 wildfire season, however, researchers conducted seven research flights, including one over the Pacific Northwest, to assist fill this gap.
Concerns about drought are growing as wildfires and heatwaves impact the western United States: Dry landscapes burn more easily, and rain can help put out existing flames. However, wildfire smoke may prevent necessary rain from falling. According to a recent study, small particles in wildfire smoke influence how droplets form in clouds, perhaps resulting in less rain and increasing dry conditions that spark fires. When wildfires send smoke up into the atmosphere, tiny particles fly up with it. Water droplets can condense on the particles in clouds.
Using airborne instrumentation, the scientists discovered that small cumulus clouds damaged by smoke contained five times as many water droplets as unaffected clouds. That was not a great surprise; it is well known that organic and inorganic particles in smoke can act as small nuclei for the formation of droplets. The team was shocked by the sheer number of droplets in the damaged clouds.
Carbon, oxygen, nitrogen, sulfur, and potassium levels were measured in remnant particles evaporated from cloud droplets. These elements were present in comparable levels to those found in smoke particles collected from below the clouds, implying that cloud droplets formed on smoke particles.
Cynthia Twohy
Contrary to popular belief, the presence of so many droplets did not increase the likelihood of rain. In reality, the inverse occurred. Because the droplets were around half the size of those seen in a typical cloud, they were unlikely to hit and join with enough other droplets to form rain. The chances of rain were “virtually zero,” the researchers write in the August Geophysical Research Letters.
According to the new research, wildfires may cause clouds to produce less rain in the United States’ West, contributing to drought conditions and perhaps boosting future wildfire risk.
However, the environmental dynamics involved are complex, according to Cynthia Twohy. She works as an atmospheric scientist in San Diego for NorthWest Research Associates, a Redmond, Washington–based research firm specializing in geophysical and space sciences. Twohy and her colleagues, for example, discovered that “the ratio of light-absorbing to light-scattering particles in the smoke was somewhat lower than measured in several past studies.”
“The take-home lesson is that, while earlier studies have indicated that wildfire smoke has an absorbing (warming) influence that can be essential for cloud formation and growth, these consequences may be smaller in the western United States since the smoke is not as dark,” says Twohy. The effect of the lighter smoke is still unknown. “It’s just another example of how smoke-cloud interactions may be unpredictable in the region.”
The researchers utilized onboard probes to collect samples of wildfire smoke-affected clouds and compare them to more clean equivalents. The probes counted the number of cloud droplets in the samples, their size range, and the liquid water content of the clouds.
According to Robert Yokelson, an atmospheric chemist at the University of Montana in Missoula who was not involved in the project, an unique tube mounted on the plane’s exterior to collect and evaporate cloud droplets was used to “reveal the particles that the droplets were concentrated on.” This method allowed the researchers to confirm the composition of the original smoke particles, which Yokelson describes as “neat.”
Carbon, oxygen, nitrogen, sulfur, and potassium levels were measured in remnant particles evaporated from cloud droplets. These elements were present in comparable levels to those found in smoke particles collected from below the clouds, “implying that cloud droplets formed on smoke particles,” according to Twohy.
Previous research in the Amazon has demonstrated that “smoke will make the cloud droplets smaller and more abundant,” lowering rainfall, according to Yokelson. However, this study provides strong evidence that the phenomena is not limited to the Amazon. It echoes the findings of a much smaller 1974 study of smoke-filled clouds over the western United States, offering an important current snapshot of the region’s issues.
Wildfires in the western United States have been breaking records in recent years, growing in quantity and size as a result of climate change, a trend that scientists believe will worsen as the world warms. As a result, Twohy believes it is becoming increasingly crucial for experts to continue monitoring the impact of these fires on the atmosphere.
