Why Pollution Causes Thunderstorms to Leave Bigger, Longer-lasting Clouds Behind

By discovering how pollution leads thunderstorms to leave larger, longer-lasting clouds in their wake, scientists have not only solved a long-standing debate, but uncovered a new clue as to how pollution relates to climate warming.

Previously, scientists believed pollution led to bigger, more persistent clouds because they made thunderheads draftier via convection. According to researchers from the Pacific Northwest National Laboratory, the real reason has to do with pollution's role in decreasing the size and increasing the lifespan of cloud and ice particles.

"This study reconciles what we see in real life to what computer models show us," Jiwen Fan said in a statement. "Observations consistently show taller and bigger anvil-shaped clouds in storm systems with pollution, but the models don't always show stronger convection. Now we know why."

Starting with cloud data from three locations differing in levels of pollution, humidity and wind, the team ran simulations on a supercomputer of a month of storms. In doing so, they found that pollution triggered an increase in size, thickness and duration of the anvil-shaped clouds. However, only two locations showed stronger convection, suggesting this was not the reason.

The researchers turned to the properties of water droplets and ice crystals inside the clouds and found that, regardless of the location, pollution meant smaller droplets and ice crystals. Where pollution was limited, heavier particles fell out of the clouds faster, causing them to disintegrate.

"Most models don't simulate convection well, take into account the microphysical processes of storm clouds, nor address how pollution interacts with those processes," PNNL officials wrote. "Accounting for pollution effects on storm clouds in this way could affect the ultimate amount of warming predicted for the earth in the next few decades. Accurately representing clouds in climate models is key to improving the accuracy of predicted changes to the climate."