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At ultraviolet and visible wavelengths, Uranus's atmosphere is remarkably bland in comparison to the other gas giants, even to Neptune, which it otherwise closely resembles.[17] When Voyager 2 flew by Uranus in 1986, it observed a total of ten cloud features across the entire planet.[16][98] One proposed explanation for this dearth of features is that Uranus's internal heat appears markedly lower than that of the other giant planets. The lowest temperature recorded in Uranus's tropopause is 49 K, making Uranus the coldest planet in the Solar System, colder than Neptune.[12][64]

Banded structure, winds and clouds

In 1986, Voyager 2 found that the visible southern hemisphere of Uranus can be subdivided into two regions: a bright polar cap and dark equatorial bands (see figure on the right).[16] Their boundary is located at about -45 degrees of latitude. A narrow band straddling the latitudinal range from -45 to -50 degrees is the brightest large feature on the visible surface of the planet.[16][99] It is called a southern "collar". The cap and collar are thought to be a dense region of methane clouds located within the pressure range of 1.3 to 2 bar (see above).[100] Besides the large-scale banded structure, Voyager 2 observed ten small bright clouds, most lying several degrees to the north from the collar.[16] In all other respects Uranus looked like a dynamically dead planet in 1986. Unfortunately, Voyager 2 arrived during the height of Uranus's southern summer and could not observe the northern hemisphere. At the beginning of the 21st century, when the northern polar region came into view, the Hubble Space Telescope (HST) and Keck telescope initially observed neither a collar nor a polar cap in the northern hemisphere.[99] So Uranus appeared to be asymmetric: bright near the south pole and uniformly dark in the region north of the southern collar.[99] In 2007, when Uranus passed its equinox, the southern collar almost disappeared, while a faint northern collar emerged near 45 degrees of latitude.