  Clouds on Titan result from the condensation of methane and ethane and, as on
other planets, are primarily structured by circulation of the atmosphere. At
present, cloud activity mainly occurs in the southern (summer) hemisphere,
arising near the pole and at mid-latitudes from cumulus updrafts triggered by
surface heating and/or local methane sources, and at the north (winter) pole,
resulting from the subsidence and condensation of ethane-rich air into the
colder troposphere. General circulation models predict that this distribution
should change with the seasons on a 15-year timescale, and that clouds should
develop under certain circumstances at temperate latitudes (~40\degree) in the
winter hemisphere. The models, however, have hitherto been poorly constrained
and their long-term predictions have not yet been observationally verified.
Here we report that the global spatial cloud coverage on Titan is in general
agreement with the models, confirming that cloud activity is mainly controlled
by the global circulation. The non-detection of clouds at latitude ~40\degree N
and the persistence of the southern clouds while the southern summer is ending
are, however, both contrary to predictions. This suggests that Titan's
equator-to-pole thermal contrast is overestimated in the models and that its
atmosphere responds to the seasonal forcing with a greater inertia than
expected.
