  In an atmosphere, a cloud condensation region is characterized by a strong
vertical gradient in the abundance of the related condensing species. On Earth,
the ensuing gradient of mean molecular weight has relatively few dynamical
consequences because N$_2$ is heavier than water vapor, so that only the
release of latent heat significantly impacts convection. On the contrary, in an
hydrogen dominated atmosphere (e.g. giant planets), all condensing species are
significantly heavier than the background gas. This can stabilize the
atmosphere against convection near a cloud deck if the enrichment in the given
species exceeds a critical threshold. This raises two questions. What is
transporting energy in such a stabilized layer, and how affected can the
thermal profile of giant planets be? To answer these questions, we first carry
out a linear analysis of the convective and double-diffusive instabilities in a
condensable medium showing that an efficient condensation can suppress
double-diffusive convection. This suggests that a stable radiative layer can
form near a cloud condensation level, leading to an increase in the temperature
of the deep adiabat. Then, we investigate the impact of the condensation of the
most abundant species---water---with a steady-state atmosphere model. Compared
to standard models, the temperature increase can reach several hundred degrees
at the quenching depth of key chemical tracers. Overall, this effect could have
many implications for our understanding of the dynamical and chemical state of
the atmosphere of giant planets, for their future observations (with Juno for
example), and for their internal evolution.
