Smoky Clouds That Form Over Wildfires Produce Less Rain

Particles from the smoke create tiny droplets too small to fall to Earth

Rising smoke from the French Fire at the Sequoia National Forest in California.
New research suggests that clouds affected by wildfire smoke hold more water droplets but are tiny and less likely drop as rain. Associated Press

As heatwaves and wildfires continue to spread throughout the western United States, fear of dry landscapes fueling the infernos is rising. However, wildfire smoke may prevent clouds from dropping rain needed to curb fires, per a statement by the National Science Foundation.

During the 2018 wildfire season, scientists at the research organization NorthWest Research Associates flew a C-130 airplane through the smoke to understand what happens to clouds when wildfire smoke rises into the atmosphere. The team found that smoke particles created denser clouds with five times as many water droplets than clouds not influenced by smoke, reports Alejandra Borunda for National Geographic. However, more water droplets did not mean more rain. Droplets in smoke-affected clouds were too small to stick together and become heavy enough to fall as rain. Researchers suspect less rain may create a dangerous feedback loop where droughts and wildfire cycles worsen with insufficient rainfall to drench the land. The study was published in Geophysical Research Letters.

Clouds form when water vapor sticks to tiny particles floating in the air. But during the summertime in the West when the air is dry, water will stick to several particles in the air at once, combining them to form large raindrops. However, if the atmosphere contains an enormous number of specks flying around from smoke, the water spreads out, forming tiny, glittering droplets not heavy enough to fall as rain, per National Geographic.

"When the cloud droplets are too small, it sometimes doesn't rain," says atmospheric scientist Jonathan Jiang of the Jet Propulsion Laboratory in California, who was not involved in the study, to National Geographic.

To measure the droplets trapped in the clouds, scientists used probes aboard the plane and sampled clouds affected by wildfire smoke and those not. Each probe estimated how many droplets each cloud contained, the size of the water droplets, and the liquid water content each cloud held, Rachel Crowell reports for Science News. The droplets in smoke-affected clouds were about half the size of those found in smokeless clouds.

A special tube was mounted outside of the plane to collect cloud droplets for further analysis. The particulates contained carbon, oxygen, sulfur, and potassium that matched the chemical make-up of smoke samples taken from below the clouds.

Previous research on fires in the Amazon also found that smoke will make cloud droplets more numerous and smaller, ultimately reducing the amount of rain that falls to the ground. This new study is the first to show that the phenomenon is not unique to Amazon.

But the smoke in the U.S. may absorb heat and affect the atmosphere differently because it is not as dark as in other parts of the world. Darker smoke absorbs sunlight, warming the areas nearby. But dense clouds reflect more sunlight, which keeps the ground cool and prevents the formation of storm clouds, per National Geographic.

"The take-home message is that while other studies have shown wildfire smoke has an absorbing (warming) influence that can be important for cloud formation and development, these impacts may be less in the western U.S. because the smoke is not as dark," study author Cynthia Twohy, an atmospheric scientist, to Science News. "It's just another way that smoke-cloud interactions are a wild card in the region."