||Clouds in the atmosphere play important roles in climate processes: They transport most of the precipitations of liquid and solid water reaching the ground. Their formation and dissipation is associated with exchanges of large amounts of latent heat being supplied to or taken away from the atmosphere. They are locus of many chemical processes and, thus, participate in important geochemical cycles. Finally they alter dominantly the up- and downward radiative energy fluxes, which drive cascades of dynamical and biological processes in the climate system. Thus clouds must in all climate models be treated with appropriate accuracy.