  The atmospheres of the four giant planets of our Solar System share a common
and well-observed characteristic: they each display patterns of planetary
banding, with regions of different temperatures, composition, aerosol
properties and dynamics separated by strong meridional and vertical gradients
in the zonal (i.e., east-west) winds. On Jupiter, the reflective white bands of
low temperatures, elevated aerosol opacities, and enhancements of
quasi-conserved chemical tracers are referred to as 'zones.' Conversely, the
darker bands of warmer temperatures, depleted aerosols, and reductions of
chemical tracers are known as `belts.' On Saturn, we define cyclonic belts and
anticyclonic zones via their temperature and wind characteristics, although
their relation to Saturn's albedo is not as clear as on Jupiter. On distant
Uranus and Neptune, the exact relationships between the banded albedo contrasts
and the environmental properties is a topic of active study. This review is an
attempt to reconcile the observed properties of belts and zones with (i) the
meridional overturning inferred from the convergence of eddy angular momentum
into the eastward zonal jets at the cloud level on Jupiter and Saturn and the
prevalence of moist convective activity in belts; and (ii) the opposing
meridional motions inferred from the upper tropospheric temperature structure,
which implies decay and dissipation of the zonal jets with altitude above the
clouds. These two scenarios suggest meridional circulations in opposing
directions, the former suggesting upwelling in belts, the latter suggesting
upwelling in zones. This presents an unresolved paradox for our current
understanding of the banded structure of giant planet atmospheres, that could
be addressed via a multi-tiered vertical structure of 'stacked circulation
cells.' [Abridged]
