Seasonal Variation
Determining the nature of this seasonal variation is difficult because good data on Uranus' atmosphere has existed for less than 84 Earth years, or one full Uranian year. A number of discoveries have however been made. Photometry over the course of half a Uranian year (beginning in the 1950s) has shown regular variation in the brightness in two spectral bands, with maxima occurring at the solstices and minima occurring at the equinoxes. A similar periodic variation, with maxima at the solstices, has been noted in microwave measurements of the deep troposphere begun in the 1960s. Stratospheric temperature measurements beginning in 1970s also showed maximum values near 1986 solstice.
The majority of this variability is believed to occur due to changes in the viewing geometry. Uranus is an oblate spheroid, which causes its visible area to become larger when viewed from the poles. This explains in part its brighter appearance at solstices. Uranus is also known to exhibit strong meridional variations in albedo (see above). For instance, the south polar region of Uranus is much brighter than the equatorial bands. In addition, both poles demonstrate elevated brightness in the microwave part of the spectrum, while the polar stratosphere is known to be cooler than the equatorial one. So seasonal change seems to happen as follows: poles, which are bright both in visible and microwave spectral bands, come in to the view at solstices resulting in brighter planet, while the dark equator is visible mainly near equinoxes resulting in darker planet. In addition, occultations at solstices probe hotter equatorial stratosphere.
However there are some reasons to believe that seasonal changes are happening in Uranus. While the planet is known to have a bright south polar region, the north pole is fairly dim, which is incompatible with the model of the seasonal change outlined above. During its previous northern solstice in 1944, Uranus displayed elevated levels of brightness, which suggests that the north pole was not always so dim. This information implies that the visible pole brightens some time before the solstice and darkens after the equinox. Detailed analysis of the visible and microwave data revealed that the periodical changes of brightness are not completely symmetrical around the solstices, which also indicates a change in the albedo patterns. In addition, the microwave data showed increases in pole–equator contrast after the 1986 solstice. Finally in the 1990s, as Uranus moved away from its solstice, Hubble and ground based telescopes revealed that the south polar cap darkened noticeably (except the southern collar, which remained bright), while the northern hemisphere demonstrated increasing activity, such as cloud formations and stronger winds, having bolstered expectations that it would brighten soon. In particular an analog of the bright polar collar present in the southern hemisphere at −45° was expected to appear in the northern part of the planet. This indeed happened in 2007 when the planet passed an equinox: a faint northern polar collar arose, while the southern collar became nearly invisible, although the zonal wind profile remained asymmetric, with northern winds being slightly slower than southern.
The mechanism of physical changes is still not clear. Near the summer and winter solstices, Uranus' hemispheres lie alternately either in full glare of the Sun's rays or facing deep space. The brightening of the sunlit hemisphere is thought to result from the local thickening of the methane clouds and haze layers located in the troposphere. The bright collar at −45° latitude is also connected with methane clouds. Other changes in the southern polar region can be explained by changes in the lower cloud layers. The variation of the microwave emission from the planet is probably caused by a changes in the deep tropospheric circulation, because thick polar clouds and haze may inhibit convection.
For a short period in Autumn 2004, a number of large clouds appeared in the Uranian atmosphere, giving it a Neptune-like appearance. Observations included record-breaking wind speeds of 824 km/h and a persistent thunderstorm referred to as "Fourth of July fireworks". Why this sudden upsurge in activity should be occurring is not fully known, but it appears that Uranus' extreme axial tilt results in extreme seasonal variations in its weather.
Read more about this topic: Climate Of Uranus