  This work aims to improve the current understanding of the atmospheres of
brown dwarfs, especially cold ones with spectral type T and Y, whose modeling
is a current challenge. Silicate and iron clouds are believed to disappear at
the photosphere at the L/T transition, but cloudless models fail to reproduce
correctly the spectra of T dwarfs, advocating for the addition of more physics,
e.g. other types of clouds or internal energy transport mechanisms. We use a
one-dimensional (1D) radiative/convective equilibrium code ATMO to investigate
this issue. This code includes both equilibrium and out-of-equilibrium
chemistry and solves consistently the PT structure. Included opacity sources
are H2-H2, H2-He, H2O, CO, CO2, CH4, NH3, K, Na, and TiO, VO if they are
present in the atmosphere. We show that the spectra of Y dwarfs can be
accurately reproduced with a cloudless model if vertical mixing and NH3
quenching are taken into account. T dwarf spectra still have some reddening in
e.g. J - H compared to cloudless models. This reddening can be reproduced by
slightly reducing the temperature gradient in the atmosphere. We propose that
this reduction of the stabilizing temperature gradient in these layers, leading
to cooler structures, is due to the onset of fingering convection, triggered by
the destabilizing impact of condensation of very thin dust.
