Summer thunderstorms can cool us off with welcome rain, but at the same time contribute to long-term global warming. The reason for this seeming contradiction, according to a study being led by Dr. Jiwen Fan of Pacific Northwest National Laboratory, lies in the way thunderstorm clouds are formed.
A building thunderhead pulls in strong updrafts of warm air rising from the ground, plus whatever natural and man-made pollutants are present in the local environment. In addition to the warm air, the updrafts suck in tiny aerosol droplets of everything from factory smoke to car exhaust to methane from cow manure. These pollutants mix with the water in the cloud when it cools and condenses to form the thunderhead. As the cool air in the cloud sinks, it becomes part of a convective cycle, sweeping into the warm updrafts coming from the ground and bringing more pollutants into the cloud.
The pollution serves to divide the natural water droplets within the thunderhead, making them too small to create rain. Instead, the droplets migrate to the top of the cloud where they freeze and absorb more water, increasing the overall size of the cloud. The research showed that these pollution-heavy clouds create stronger storms than those with little or no pollution. They spread out over a larger area and create larger anvils at the top, also acting as heat traps, gradually reflecting the stored heat back to earth. In general, the findings show that cleaner clouds produce more rain, while dirty clouds produce more heat.
The cloud research project, which is still ongoing, is a collaboration of scientists at Pacific Northwest National Laboratory in Richland, Washington, Hebrew University in Jerusalem, and the University of Maryland.