Aerosol clouds cool earth.

• Position: Meteorology

Atmospheric scientists have developed simple, physics-based equations that address some of the limitations of current methods for representing cloud formation in global climate models--important because of increased aerosol pollution that gives clouds more cooling power and affects precipitation. These researchers--led by the Georgia Institute of Technology, Atlanta, National Science Foundation, National Oceanic and Atmospheric Administration, and NASA--also have developed a new instrument for measuring the conditions and time needed for a particle to become a cloud droplet. This will help scientists determine how various types of emissions affect cloud formation.

Clouds play a critical role in climate, notes Athanasios Nenes, a professor in Georgia Tech's School of Earth and Atmospheric Sciences and the School of Chemical and Biomolecular Engineering. Low, thick ones cool the Earth by reflecting solar radiation whereas high, thin clouds have warming properties by trapping infrared radiation emitted by the Earth.

Scientists have learned that human activities influence cloud formation. Airborne particles released by smokestacks, charcoal grills, and car exhausts restrict the growth of cloud droplets, causing condensing water to spread out among a larger number of smaller droplets. Known as the "indirect aerosol effect" this gives clouds more surface area and reflectivity, which translates into greater cooling power. The clouds also may have less chance of forming rain, which allows them to remain longer for cooling.

"Of all the components of climate change, the indirect aerosol effect has the greatest potential cooling effect, yet quantitative estimates are highly uncertain" Nenes notes. "We need to get more rigorous and accurate representation of how particles modify cloud properties. Until the indirect aerosol effect is well understood, society is incapable of assessing its impact on future climate."

Current computer climate models cannot predict cloud formation accurately, which, in turn, hinders their ability to forecast climate change from human activities. "Because of their coarse resolution, computer models produce values on large spatial scales [hundreds of kilometers] and can only represent large cloud systems."

Aerosol particles, however, are extremely small and are measured in micrometers. This means predictive models must address processes taking place on a very broad range of scale. "Equations that describe cloud formation...