The same phenomenon recurs every year. During the dry season, in winter, burning fossil fuels and biomass in South Asia creates a huge pollution haze: the Atmospheric Brown Cloud. How and why it disappears as soon as the rainy season starts in spring has now been clarified by an international team of scientists led by the Max Planck Institute for Chemistry. The result is that thunderstorm updrafts, lightning and chemical reactions enhance the self-cleansing power of the atmosphere, allowing atmospheric pollutants to be efficiently washed out of the air. However, the pollutants that are not eliminated are transported into the upper troposphere by the monsoon and then spread worldwide.