Climate of Uranus

Banded structure, winds and clouds

The first suggestions of bands and weather on Uranus came in the 19th century, such as an observation in March and April 1884 of a white band circling partially around Uranus's equator, only two years after Uranus's "spring" equinox.

In 1986 Voyager 2 discovered 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). 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 Uranus's visible surface. 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. Unfortunately Voyager 2 arrived during the height of Uranus's southern summer and could not observe the northern hemisphere. However, at the end of 1990s and the beginning of the twenty-first century, when the northern polar region came into view, Hubble Space Telescope (HST) and Keck telescope initially observed neither a collar nor a polar cap in the northern hemisphere. Thus, Uranus appeared to be asymmetric: bright near the south pole and uniformly dark in the region north of the southern collar. In 2007, however, when Uranus passed its equinox, the southern collar almost disappeared, whereas a faint northern collar emerged near 45 degrees of latitude. The visible latitudinal structure of Uranus is different from that of Jupiter and Saturn, which demonstrate multiple narrow and colorful bands.

In addition to large-scale banded structure, Voyager 2 observed ten small bright clouds, most lying several degrees to the north from the collar. In all other respects Uranus looked like a dynamically dead planet in 1986. However, in the 1990s the number of observed bright cloud features grew considerably. The majority of them were found in the northern hemisphere as they started to become visible. The common though incorrect explanation of this fact was that bright clouds are easier to identify in its dark part, whereas in the southern hemisphere the bright collar masks them. Nevertheless, there are differences between the clouds of each hemisphere. The northern clouds are smaller, sharper and brighter. They appear to lie at a higher altitude, which is connected to fact that until 2004 (see below) no southern polar cloud had been observed at the wavelength 2.2 micrometres, which is sensitive to the methane absorption, whereas northern clouds have been regularly observed in this wavelength band. The lifetime of clouds spans several orders of magnitude. Some small clouds live for hours, whereas at least one southern cloud has persisted since the Voyager flyby. Recent observation also discovered that cloud-features on Uranus have a lot in common with those on Neptune, although the weather on Uranus is much calmer.

Uranus Dark Spot

The dark spots common on Neptune had never been observed on Uranus before 2006, when the first such feature was imaged. In that year observations from both Hubble Space Telescope and Keck Telescope revealed a small dark spot in the northern (winter) hemisphere of Uranus. It was located at the latitude of about 28 ± 1° and measured approximately 2° (1300 km) in latitude and 5° (2700 km) in longitude. The feature called Uranus Dark Spot (UDS) moved in the prograde direction relative Uranus's rotation with an average speed of 43.1 ± 0.1 m/s, which is almost 20 m/s faster than the speed of clouds at the same latitude. The latitude of UDS was approximately constant. The feature was variable in size and appearance and was often accompanied by a bright white cloud called Bright Companion (BC), which moved with nearly the same speed as UDS itself.

The behavior and appearance of UDS and its bright companion were similar to Neptunian Great Dark Spots (GDS) and their bright companions, respectively, though UDS was significantly smaller. This similarity suggests that they have the same origin. GDS were hypothesized to be anticyclonic vortices in the atmosphere of Neptune, whereas their bright companions were thought to be methane clouds formed in places, where the air is rising (orographic clouds). UDS is supposed to have a similar nature, although it looked differently from GDS at some wavelengths. Although GDS had the highest contrast at 0.47 μm, UDS was not visible at this wavelength. On the other hand, UDS demonstrated the highest contrast at 1.6 μm, where GDS were not detected. This implies that dark spots on the two ice giants are located at somewhat different pressure levels—the Uranian feature probably lies near 4 bar. The dark color of UDS (as well as GDS) may be caused by thinning of the underlying hydrogen sulfide or ammonium hydrosulfide clouds.

The emergence of a dark spot on the hemisphere of Uranus that was in darkness for many years indicates that near equinox Uranus entered a period of elevated weather activity.

Winds

The tracking of numerous cloud features allowed determination of zonal winds blowing in the upper troposphere of Uranus. At the equator winds are retrograde, which means that they blow in the reverse direction to the planetary rotation. Their speeds are from −100 to −50 m/s. Wind speeds increase with the distance from the equator, reaching zero values near ±20° latitude, where the troposphere's temperature minimum is located. Closer to the poles, the winds shift to a prograde direction, flowing with its rotation. Wind speeds continue to increase reaching maxima at ±60° latitude before falling to zero at the poles. Wind speeds at −40° latitude range from 150 to 200 m/s. Because the collar obscures all clouds below that parallel, speeds between it and the south pole are impossible to measure. In contrast, in the northern hemisphere maximum speeds as high as 240 m/s are observed near +50 degrees of latitude. These speeds sometimes lead to incorrect assertions that winds are faster in the northern hemisphere. In fact, latitude per latitude, winds are slightly slower in the northern part of Uranus, especially at the midlatitudes from ±20 to ±40 degrees. There is currently no agreement about whether any changes in wind speed have occurred since 1986, and nothing is known about much slower meridional winds.