The nature of Saturn's atmospheric Great White Spots

Saturn's Great White Spots (GWS) are the biggest asymmetric cloud systems observable in the atmosphere of this planet, which have been detected in only four occasions in 1876, 1903, 1933 and 1960. These features emerged as compact bright clouds with sizes of about 20,000 km and were detected in a latitude range from 5 deg to 60 deg, acting as tracers of the prevailing zonal wind. Their morphology becomes irregular in less than a week due to their zonal expansion. From the horizontal velocity divergence field of the 1933 event cloud-top vertical motions of w of less than about 10 cm/s are found. The photographic photometry analysis in U, B, V and R (broad band filters) of the 1933 GWS and the yellow polarization data of the 1960 GWS, suggest that the cloud top pressure level for a GWS is situated between P = 250 mb and P = 100 mb, i.e., always above the clouds in the surrounding zone. Models for a range of parameters are presented in which GWSs are rising parcels of saturated air as a result of moist convective instability caused by the release of latent heat originating at the ammonia and at the water cloud levels. The emergence of these disturbances seems to be related to the seasonal insolation cycle, but they appear to be independent of the local zonal wind conditions. Dynamical and dissipative effects are the probable cause for the decay of a GWS event in 30 to 150 days.