  Clouds seem like an every-day experience. But -- do we know how clouds form
on brown dwarfs and extra-solar planets? How do they look like? Can we see
them? What are they composed of? Cloud formation is an old-fashioned but still
outstanding problem for the Earth atmosphere, and it has turned into a
challenge for the modelling of brown dwarf and exo-planetary atmospheres. Cloud
formation imposes strong feedbacks on the atmospheric structure, not only due
to the clouds own opacity, but also due to the depletion of the gas phase,
possibly leaving behind a dynamic and still supersaturated atmosphere. I
summarise the different approaches taken to model cloud formation in substellar
atmospheres and workout their differences. Focusing on the
phase-non-equilibrium approach to cloud formation, I demonstrate the inside we
gain from detailed micro-physical modelling on for instance the material
composition and grain size distribution inside the cloud layer on a Brown Dwarf
atmosphere. A comparison study on four different cloud approaches in Brown
Dwarf atmosphere simulations demonstrates possible uncertainties in
interpretation of observational data.
