to its surroundings. The spot is embedded within a region seeded by a ''field of bright clouds'' with characteristic size 1000 We report on a new series of ground-based CCD observations km reminiscent of a cellular convection pattern. The NPS's at visual wavelengths, covering a period of 1255 days between east-west apparent size is shorter at violet-blue (about 7000 May 1992 and November 1995, of the longest-lived asymmetric km as limited by a dark ring at these wavelengths) than at feature known in Saturn's atmosphere: the north polar spot green-orange (about 11,000 km corresponding to the large (NPS). This completes our previous analysis of this feature uniform area). Green processed images show apparent spiral during the period 1990-1991 (A. Sanchez-Lavega, J. Lecapatterns within the NPS consistent with anticyclonic vorticity. cheux, F. Colas, and P. Lagues, 1993, Science 260, 329-332). The results of ground-based photometry of the north polar Longitude measurements of the NPS indicate an averaged lonregion (NPR) and the NPS in the red methane absorption bands gitudinal drift of ؊0.030 deg/day for the whole period 1990and their adjacent continuum are consistent with a radiative 1995 corresponding to a zonal velocity of 0.11 msec ؊1 . These transfer model of the cloud vertical structure consisting of a data, when combined with previous and new measurements of clear gas layer, a haze layer, and a semi-infinite cloud. In the the NPS position on Voyager 1 and 2 images obtained in 1980 context of this model the NPS cloud tops are slightly higher and 1981, indicate a long-term drift in longitude of the NPS than neighboring clouds reaching a pressure level of 45 mbar. with a constant angular acceleration of 1.1 ؋ 10 ؊5 deg/(day) 2 . Calculations of the seasonal insolation at the north pole, to-High-resolution Voyager 2 violet, blue, green, and orange imgether with a simple linear radiative response of the atmosphere ages were used to measure the size and reflectivity of the NPS. to this heating at different altitudes, suggest temperature Its structure is characterized by a bright elliptical core surchanges at the level of the NPS cloud tops which should influrounded by a dark ring and a large uniform area. The contrast ence the NPS dynamics. Because of the long lifetime of the between all these features changes appreciably from violet to NPS, and because its motions did not vary appreciably during orange: the spot is dark in violet but bright in orange relative the long observing period, we suggest that the main properties and dynamics of the NPS are insensitive to the external solar forcing.