A new NASA-led study helps answer decades-old questions about the role of smoke and human-caused air pollution on clouds and rainfall. Looking specifically at deep convective clouds—tall clouds like thunderclouds, formed by warm air rising—the study shows that smoky air makes it harder for these clouds to grow. Pollution, on the other hand, energizes their growth, but only if the pollution isn’t heavy. Extreme pollution is likely to shut down cloud growth.