Moderate quantities of dirt make more rain

This release is available in German.

Not every dense cloud brings rain. Rain falls in a normal way in unpolluted air from this cloud over the Amazon (left). In contrast, no rain fell from another cloud... Click here for more information.

Drought or deluge - scientists working with Meinrat O. Andreae, Director at the Max Planck Institute for Chemistry in Mainz, have now discovered how aerosols affect the when, where and how much of rainfall. Until now, the answers to these questions have been as varied as they have been inconsistent. Andreae and his co-authors are now tracing a common theme through the sometimes contradictory effects that these tiny particles have on precipitation. Their new approach: they are observing how aerosols change the flow of energy in the atmosphere and thus air circulation, the way drops form and the way they fall. Because the role of aerosols has to date been very much a subject of dispute and a source of great uncertainty in climate predictions made by researchers, this work removes one of the largest obstacles to the development of more accurate climate forecasts.(Science, September 5, 2008)

Human beings blow vast quantities of aerosols into the air with their cars, power plants and heating systems. Fires set to clear forests also release these floating particles which, in some cases, measure just a few thousandths of a millimetre, or even less. Before humans had any impact, the aerosol load in air over land was only double that in the air over the sea. However, today, the former can amount to one hundred times more than the latter. There is no doubt that natural and human-made aerosols have an impact on our climate. But what effect do they have, exactly? Some say they lead to more clouds and more precipitation. Others say they mean fewer clouds and less precipitation. "Both sides are right," says Meinrat Andreae, Director at the Max Planck Institute for Chemistry in Mainz, "but it depends on the number of particles. This is what determines how the energy needed to evaporate water and transport air is distributed."