<P> The composition of the clouds varies with depth and increasing pressure . In the upper cloud layers, with the temperature in the range 100--160 K and pressures extending between 0.5--2 bar, the clouds consist of ammonia ice . Water ice clouds begin at a level where the pressure is about 2.5 bar and extend down to 9.5 bar, where temperatures range from 185--270 K. Intermixed in this layer is a band of ammonium hydrosulfide ice, lying in the pressure range 3--6 bar with temperatures of 190--235 K. Finally, the lower layers, where pressures are between 10--20 bar and temperatures are 270--330 K, contains a region of water droplets with ammonia in aqueous solution . </P> <P> Saturn's usually bland atmosphere occasionally exhibits long - lived ovals and other features common on Jupiter . In 1990, the Hubble Space Telescope imaged an enormous white cloud near Saturn's equator that was not present during the Voyager encounters, and in 1994 another smaller storm was observed . The 1990 storm was an example of a Great White Spot, a unique but short - lived phenomenon that occurs once every Saturnian year, roughly every 30 Earth years, around the time of the northern hemisphere's summer solstice . Previous Great White Spots were observed in 1876, 1903, 1933 and 1960, with the 1933 storm being the most famous . If the periodicity is maintained, another storm will occur in about 2020 . </P> <P> The winds on Saturn are the second fastest among the Solar System's planets, after Neptune's . Voyager data indicate peak easterly winds of 500 m / s (1800 km / h). In images from the Cassini spacecraft during 2007, Saturn's northern hemisphere displayed a bright blue hue, similar to Uranus . The color was most likely caused by Rayleigh scattering . Thermography has shown that Saturn's south pole has a warm polar vortex, the only known example of such a phenomenon in the Solar System . Whereas temperatures on Saturn are normally − 185 ° C, temperatures on the vortex often reach as high as − 122 ° C, suspected to be the warmest spot on Saturn . </P> <P> A persisting hexagonal wave pattern around the north polar vortex in the atmosphere at about 78 ° N was first noted in the Voyager images . The sides of the hexagon are each about 13,800 km (8,600 mi) long, which is longer than the diameter of the Earth . The entire structure rotates with a period of 10h 39m 24s (the same period as that of the planet's radio emissions) which is assumed to be equal to the period of rotation of Saturn's interior . The hexagonal feature does not shift in longitude like the other clouds in the visible atmosphere . The pattern's origin is a matter of much speculation . Most scientists think it is a standing wave pattern in the atmosphere . Polygonal shapes have been replicated in the laboratory through differential rotation of fluids . </P>

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