Clouds determine precipitation and constitute the main component of the hydrological cycle. They can be very beautiful, but also emphemeral in the eyes of people, artists, but also of scientists, thereby creating a fascinating enigma.

The scientific study of clouds began with Luke Howard's classification in 1803. Throughout the 19th century, the boundary between the arts and sciences, particularly with regard to natural sciences, including meteorology, was much less rigid than it is today. For instance, the great German poet Goethe took particular interest in the scientific classification of clouds. The most original sea-and-sky scape painter of the 19th century, JWM Turner, annotated his copy of Goethe’s ‘Theory of Colors’, and referred to it directly in the title of one of his paintings (Light and color (Goethe’s theory)—The Morning After the Deluge—Moses writing the book of Genesis (The Tate Gallery, London)). And one the finest of all cloud painters, John Constable, was also well aware of the work of Luke Howard, and performed detailed cloud studies in the 1820s over Hampstead Heath.

It is now understood that, paradoxically, the global dynamics of the atmosphere and climate are very much dependent upon the microscale-level processes of clouds. In fact, in addition to convective heating due to the latent heat release associated with the condensation of water vapor, clouds control, to a large extent, the solar and thermal radiation balances of the atmosphere.

The present course mainly focuses on a few of the fundamental aspects concerning clouds:
- Latent heat release, which leads to convective or stratiform heating/cooling, that is, one of the main energy sources of atmospheric motions at spatial scales, ranging from local turbulence and single clouds to global circulation.
- The condensation of water vapor and the subsequent precipitation within clouds through microphysical processes that take place at cloud particle size scales, ranging from several micrometers to a few centimeters.
- The effects of clouds on radiation caused by cloud coverage, the altitude of the cloud top, the size of the cloud particles, the size distributions, and the phase.
- The effects associated with atmospheric aerosols, which play a key role in determining the properties of clouds and give rise to the formation of water droplets and ice crystals.
- The development of improved observational techniques to study microphysical processes and bulk cloud properties and to measure the physical and optical properties of atmospheric aerosols.
- The interplay between the continuously increasing resolution of large-scale and mesoscale atmospheric models and the treatment of the intrinsic unsteady evolution of individual clouds.

The design of this PhD course in part stems from activities associated with the Marie Sklodowska Curie Action Innovative Training Network, COMPLETE, which was a Cloud-MicroPhysics-Turbulence-Telemetry shared inter-multidisciplinary research training environment for enhancing the understanding and modeling of atmospheric clouds. The network was financed under the Horizon 2020 Framework Program (2016-2021, GA 675675) and coordinated by Daniela Tordella, www.complete-h2020network.eu.