University of Manchester scientists, writing in the journal Nature Geoscience, have shown that some natural emissions and man made pollutants can have an unexpected cooling effect on the world’s climate by making clouds brighter. Clouds are made of water droplets, condensed on to tiny particles suspended in the air. When the air is humid enough, the particles swell into larger cloud droplets. It has been known for some decades that the number of these particles and their size control how bright the clouds appear from the top, which affects the the efficiency with which clouds scatter sunlight back into space. A major challenge for climate science is to understand and quantify these effects which have a major impact in polluted regions of the world.