How Pollution Impacts Storm Clouds in a Big Way

Pollution can affect our climate in multiple ways but now, scientists have found out that it can also affect thunderstorms. It turns out that pollution causes thunderstorms to leave behind larger, deeper, longer lasting clouds, which may play into climate warming.

Weather and climate models don't reconstruct the lives of clouds very well--especially storm clouds. Usually, these models replace storm clouds with simple equations that fail to capture the whole picture. Because of these poor reconstructions, researchers have been unable to determine exactly why pollution causes anvil-shaped storm clouds to linger longer than they would in clean skies.

More and smaller droplets can actually change things for the clouds. Scientists have long thought that smaller droplets start a chain reaction that leads to bigger, longer-lasting clouds. Instead of raining down, though, the lighter droplets carry their water higher, where they freeze. This freezing squeezes out the heat that the droplets carry with them and causes the thunder cloud to become draftier. The stronger convection then lifts more water droplets and builds up the cloud. Yet researchers don't always see stronger convection in a polluted environment.

In order to solve this mystery, the scientists compared real-life summer storm clouds to a computer model that zoomed deep into simulated clouds. The model included physical properties of the cloud particles as well as the ability to see convection.

So what did they find? It turns out that in all cases, pollution increased the size, thickness and duration of anvil-shaped clouds. However, only two locations--the tropics and China--showed stronger convection. The opposite occurred in Oklahoma; pollution created weaker convection.

So what's responsible? The researchers discovered that pollution resulted in smaller droplets and ice crystals, regardless of location. They also found that in clean skies, the heavier ice particles fall faster out of the anvil-shaped clouds, which causes the clouds to dissipate. However, the ice crystals in polluted skies were smaller and too light to fall out of the clouds, which made them last longer. In addition, it turns out that polluted clouds cooled the day and warmed the night, which decreased the daily temperature range.

The findings reveal a little bit more about how pollution can impact the weather. This, in turn, could help future climate models and weather predictions.

The findings are published in the journal Proceedings of the National Academy of Sciences.