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We aim at understanding the formation of cloud layers in quasi-staticsubstellar atmospheres. The time-dependent description presented in (Helling &Woitke 2006) is a kinetic model describing nucleation, growth and evaporation.It is extended to treat gravitational settling and is applied to thestatic-stationary case of substellar model atmospheres. From the solution forthe dust moments, we determine the grain size distribution function which,together with the calculated material volume fractions, provides the basis tocalculate the opacities of the composite dust grains. The cloud particles inbrown dwarfs and hot giant-gas planets are found to be small in the highatmospheric layers (0.01mum), and composed of a rich mixture of all consideredcondensates, in particular the abundant MgSiO3[s], Mg2SiO4[s] and SiO2[s]. Asthe particles settle downward, they increase in size and reach several 100mumin the deepest layers. The more volatile parts of the grains evaporate and theparticles stepwise purify to form composite particles of high-temperaturecondensates in the deeper layers, mainly Fe[s] and Al2O3[s]. The gas phaseabundances of the elements involved in the dust formation process vary byorders of magnitudes throughout the atmosphere. The grain size distribution isfound to be relatively broad in the upper atmospheric layers but often stronglypeaked in the deeper layers. The spectral appearance of the cloud layers in themid IR (7-20mum) is close to a grey body with only weak broad features on a fewpercent level, mainly caused by MgSiO3[s], and Mg2SiO4[s]. Our models predictthat the gas phase depletion is much weaker as compared to phase-equilibriumcalculations in the high atmospheric layers. [abridged]

Dust in Brown Dwarfs and Extra-solar Planets I. Chemical composition and spectral appearance of quasi-static cloud layers