  The longevity of Cassini's exploration of Saturn's atmosphere (a third of a
Saturnian year) means that we have been able to track the seasonal evolution of
atmospheric temperatures, chemistry and cloud opacity over almost every season,
from solstice to solstice and from perihelion to aphelion. Cassini has built
upon the decades-long ground-based record to observe seasonal shifts in
atmospheric temperature, finding a thermal response that lags behind the
seasonal insolation with a lag time that increases with depth into the
atmosphere, in agreement with radiative climate models. Seasonal hemispheric
contrasts are perturbed at smaller scales by atmospheric circulation, such as
belt/zone dynamics, the equatorial oscillations and the polar vortices.
Temperature asymmetries are largest in the middle stratosphere and become
insignificant near the radiative-convective boundary. Cassini has also measured
southern-summertime asymmetries in atmospheric composition, including ammonia
(the key species for the topmost clouds), phosphine and para-hydrogen (both
disequilibrium species) in the upper troposphere; and hydrocarbons deriving
from the UV photolysis of methane in the stratosphere (principally ethane and
acetylene). These chemical asymmetries are now altering in subtle ways due to
(i) the changing chemical efficiencies with temperature and insolation; and
(ii) vertical motions associated with large-scale overturning in response to
the seasonal temperature contrasts. Similarly, hemispheric contrasts in
tropospheric aerosol opacity and coloration that were identified during the
earliest phases of Cassini's exploration have now reversed, suggesting an
intricate link between the clouds and the temperatures. [Abridged]
