  We developed a simple, physical and self-consistent cloud model for brown
dwarfs and young giant exoplanets. We compared different parametrisations for
the cloud particle size, by either fixing particle radii, or fixing the mixing
efficiency (parameter fsed) or estimating particle radii from simple
microphysics. The cloud scheme with simple microphysics appears as the best
parametrisation by successfully reproducing the observed photometry and spectra
of brown dwarfs and young giant exoplanets. In particular, it reproduces the
L-T transition, due to the condensation of silicate and iron clouds below the
visible/near-IR photosphere. It also reproduces the reddening observed for
low-gravity objects, due to an increase of cloud optical depth for low gravity.
In addition, we found that the cloud greenhouse effect shifts chemical
equilibriums, increasing the abundances of species stable at high temperature.
This effect should significantly contribute to the strong variation of methane
abundance at the L-T transition and to the methane depletion observed on young
exoplanets. Finally, we predict the existence of a continuum of brown dwarfs
and exoplanets for absolute J magnitude=15-18 and J-K color=0-3, due to the
evolution of the L-T transition with gravity. This self-consistent model
therefore provides a general framework to understand the effects of clouds and
appears well-suited for atmospheric retrievals.
