  As the 7 December 2007 equinox of Uranus approached, ring and atmosphere
observers produced a substantial collection of observations using the 10-m Keck
telescope and the Hubble Space Telescope. Those spanning the period from 7 June
2007 through 9 September 2007 we used to identify and track cloud features,
determine atmospheric motions, characterize cloud morphology and dynamics, and
define changes in atmospheric band structure. We confirmed the existence of the
suspected northern hemisphere prograde jet, locating its peak near 58 N, and
extended wind speed measurements to 73 N. For 28 cloud features we obtained
extremely high wind-speed accuracy through extended tracking times. The new
results confirm a small N-S asymmetry in the zonal wind profile, and the lack
of any change in the southern hemisphere between 1986 (near solstice) and 2007
(near equinox) suggests that the asymmetry may be permanent rather than
seasonally reversing. In the 2007 images we found two prominent groups of
discrete cloud features with very long lifetimes. The one near 30 S has
departed from its previous oscillatory motion and started a significant
northward drift, accompanied by substantial morphological changes. The complex
of features near 30 N remained at a nearly fixed latitude, while exhibiting
some characteristics of a dark spot accompanied by bright companion features.
Smaller and less stable features were used to track cloud motions at other
latitudes, some of which lasted over many planet rotations, though many could
not be tracked beyond a single transit. A bright band has developed near 45 N,
while the bright band near 45 S has begun to decline, both events in agreement
with the idea that the asymmetric band structure of Uranus is a delayed
response to solar forcing, but with a surprisingly short delay of only a few
years.

Earth and Planetary Astrophysics