Atmosphere of Uranus - Dynamics

Dynamics

Uranus has a relatively bland appearance, lacking broad colorful bands and large clouds prevalent on Jupiter and Saturn. No discrete features were observed in Uranus's atmosphere before 1986. The most conspicuous features on Uranus observed by Voyager 2 were the dark low latitude region between −40° and −20° and bright southern polar cap. The northern boundary of the cap was located at about −45° of latitude. The brightest zonal band was located near the edge of the cap at −50° to −45° and was then called a polar collar. The southern polar cap, which existed at the time of the solstice in 1986, faded away in 1990s. After the equinox in 2007, the southern polar collar started to fade away as well, while the northern polar collar located at 45° to 50° latitude (first appeared in 2007) have grown more conspicuous since then.

The atmosphere of Uranus is calm compared to those of other giant planets. Only a limited number of small bright clouds at middle latitudes in both hemispheres and one Uranus Dark Spot have been observed since 1986. One of those bright cloud features, located at −34° of latitude and called Berg, probably existed continuously since at least 1986. Nevertheless, the Uranian atmosphere has rather strong zonal winds blowing in the retrograde (counter to the rotation) direction near the equator, but switching to the prograde direction poleward of ±20° latitude. The wind speeds are from −50 to −100 m/s at the equator increasing up to 240 m/s near 50° latitude. The wind profile measured before the equinox of 2007 was slightly asymmetric with winds stronger in the southern hemisphere, although it turned out to be a seasonal effect as this hemisphere was continuously illuminated by the Sun before 2007. After 2007 winds in the northern hemisphere accelerated while those in the southern one slowed down.

Uranus exhibits a considerable seasonal variation over its 84 year orbit. It is generally brighter near solstices and dimmer at equinoxes. The variations are largely caused by changes in the viewing geometry: a bright polar region comes into view near solstices, while the dark equator is visible near equinoxes. Still there exist some intrinsic variations of the reflectivity of the atmosphere: periodically fading and brightening polar caps as well as appearing and disappearing polar collars.