  The Ice Giants Uranus and Neptune have hydrogen-based atmospheres with
several constituents that condense in their cold upper atmospheres. A small
number of bright cloud systems observed in both planets are good candidates for
moist convective storms, but their observed properties (size, temporal scales
and cycles of activity) differ from moist convective storms in the Gas Giants.
These clouds and storms are possibly due to methane condensation and
observations also suggest deeper clouds of hydrogen sulfide (H$_2$S) at depths
of a few bars. Even deeper, thermochemical models predict clouds of ammonia
hydrosulfide (NH$_4$SH) and water at pressures of tens to hundreds of bars,
forming extended deep weather layers. Because of hydrogen's small molecular
weight and the high abundance of volatiles, their condensation imposes a
strongly stabilizing vertical gradient of molecular weight larger than the
equivalent one in Jupiter and Saturn. The resulting inhibition of vertical
motions should lead to a moist convective regime that differs significantly
from the one occurring on nitrogen-based atmospheres like those of Earth or
Titan. As a consequence, the thermal structure of the deep atmospheres of
Uranus and Neptune is not well understood. Similar processes might occur at the
deep water cloud of Jupiter in Saturn, but the Ice Giants offer the possibility
to study these physical aspects in the upper methane cloud layer. A combination
of orbital and in situ data will be required to understand convection and its
role in atmospheric dynamics in the Ice Giants, and by extension, in hydrogen
atmospheres including Jupiter, Saturn and giant exoplanets.
