Sketch of the formation mechanism of ice particles in cirrus clouds. As moist air cools along an ascending air parcel trajectory, ice crystals nucleate homogeneously in small aqueous aerosol droplets. Subsequently, the ice crystals grow by condensation of water vapor. To know more: T. Peter, C. Marcolli, P. Spichtinger, T. Corti, M.B. Baker, T. Koop, Science 314, 1399-1402 (2006).