Don't forget the surface

Surface effects play a key role in cloud droplet formation [Also see Report by Ruehl et al. ] Clouds are an essential source of fresh water to continents and all ecosystems ( 1 ) and a major cooling factor in the climate budget ( 2 ). Yet, predicting their formation remains a challenge ( 2 ). In the atmosphere, cloud droplets form not from water vapor alone but through condensation of water on aerosol particles called cloud condensation nuclei (CCN) ( 3 ). On page 1447 of this issue, Ruehl et al. ( 4 ) show experimentally that surface effects play a central role in cloud droplet formation from CCN.