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Uranus and Neptune are primarily gaseous like Jupiter and Saturn, and so where their atmospheres end (as one moves inward) depends upon arbitrary distinctions between inside and outside. Their atmospheres are mainly hydrogen (84 %) and helium (14 %). Roughly 2 % of their atmospheres is other stuff, primarily methane. The presence of the methane gives Uranus its greenish tint and Neptune its bluish tint. Methane absorbs red light, leaving the more bluish colors alone. Both Uranus and Neptune rotate at fairly high rates, spin periods of ~ 17 hours, which again has large effects on the appearance and motions of their atmospheres. Interestingly, the fact that the spin axis of Uranus lies roughly in the ecliptic plane does not alter the circulation patterns on the planet. The bands circle around the planet parallel to the latitude lines (the winds circulate around the spin axis). A major difference between Uranus and Neptune for its consequences on the appearance of the planets, is that Neptune has an internal heat source while Uranus apparently does not.
The interesting patterns and motions seen in the Uranian and Neptunian atmospheres are driven primarily by the large heat flow through the atmospheres (due to flow from the interior and heating by the Sun) and by the rapid of spins Uranus and Neptune similarly to Jupiter and Saturn.
On Uranus and Neptune, we find that the hot material near the equator of the planets is forced to flow toward the poles. This is true even for Uranus where the rotation axis lies rougnly in the ecliptic plane! The hot flows moving poleward force some of the cooler material to circulate downward toward the equator. Now recalling the Coriolis force, note that the poleward moving material deflects in the eastward direction while the equatorward moving material deflects in the westward direction. The preceding effects produce horizontal stripes which run parallel to the equator on Uranus and Neptune.
Let's look at the Great Dark Spot
Again, imagine there is a low pressure region in the atmosphere. Because of the low pressure, some atmosphere is forced to flow into the low pressure region. What happens?
In the northern hemisphere, the air circulates in the CCW sense as it flows into a low pressure region. In the southern hemisphere the air circulate in the CW sense as it flows into the low pressure region. If we have a high pressure region, the opposite occurs. In the northern hemisphere, the air circulates in the CW sense and in the southern hemisphere it circulates in the CCW sense.
