Saturn's atmosphere displays a variety of temporal changes in its cloud morphology at synoptic and planetary scales. Hemispheric long-term albedo variations, which are more pronounced at ultraviolet and blue wavelengths, have been well established and are apparently linked to the seasonal insolation cycle. Short-term changes, of the order of months, have been observed in the spectral reflectivity of the belt/zone pattern mainly at mid-latitudes. Some appear to be related to discrete spot activity, but for the majority of the changes, no such signs of spot activity were detected up to a resolution ~2000 km. In particular, the ''ribbon'' structure at 45¿N could have been affected by this activity and probably is a transient phenomenon. The polar belts at ~65¿ north and south and those at 75¿ north (hexagon) and south show a long-term stability in their location, perhaps for more than a century. The most prominent belts located in the Equator and the Equatorial Zone are persistent features of Saturn's globe; their main changes took place during the development of a rare, planetary-scale disturbance known as Great White Spots. Only five such events have been observed at repetitive regular intervals of about one Saturn year since the first report in 1876, and at different latitudes from the equator to 60¿N.

They are probably convective in origin, with the seasonal heating of the upper atmosphere acting as a trigger mechanism. During its life cycle of 2--3 years, the disturbance shows turbulent planetary-scale patterns of clouds that might be related to a wave dynamical phenomenon. Other distinct nonaxisymmetric cloud systems at synoptic scale are very scarce in Saturn's atmosphere. We observed new isolated features at several latitudes during 1990 and 1991. These spots, together with those available from historical records, are used as tracers for determining the latitude dependence and time variability of Saturn's zonal winds. The winds tend to exhibit a long-term stability in their latitudinal positions and average wind speed, although some significant departures from the zonal mean Voyager profile have been detected during the development of the equatorial disturbance in 1990 and 1991. ¿ American Geophysical Union 1992