What’s in a cloud? Water, certainly, but also tiny particles known as “aerosols”. Whether they originate from wildfires, volcanoes, sandstorms, ship emissions or power stations, aerosols alter the properties of a cloud. By providing a surface for water to condense on to, they can help water droplets to form. But exactly how aerosols alter clouds is not clear. Now a new satellite study is helping scientists to see inside a cloud.

The same image as above of clouds over Vanavara, central Russia, taken using a near-infrared wavelength. Brighter clouds are made of smaller droplets. Photograph: Nasa Worldview

Nicolas Bellouin, from the University of Reading, and colleagues compared ordinary satellite images of clouds with infrared images. Clouds that appear plain white in ordinary images contain different shades and patterns when viewed in infrared. From wildfires in Siberia to volcanoes erupting on the Pacific islands of Vanuatu, and the trail of smoke from power stations in northern England, Bellouin and his colleagues show that polluted clouds tend to redistribute the same amount of water over a greater number of droplets. Their findings are published in the journal Geophysical Research Letters.

This observation contrasts with some climate models, which indicate that increasing aerosols should help clouds to become “wetter” by increasing their capacity to hold water. The findings also suggest that climate models are overestimating the ability of aerosols to offset global warming.