Editing Module:Uranus (section)

=== Seasonal variation ===
[[File:Uranus clouds.jpg|thumb|upright|Uranus in 2005. Rings, southern collar and a bright cloud in the northern hemisphere are visible (HST ACS image).]]

For a short period from March to May 2004, large clouds appeared in the Uranian atmosphere, giving it a Neptune-like appearance.<ref name="NYT-20240104">{{cite news |last=Ferreira |first=Becky |title=Uranus and Neptune Reveal Their True Colors - Neptune is not as blue as you've been led to believe, and Uranus's shifting colors are better explained, in new research. |url=https://www.nytimes.com/2024/01/04/science/uranus-neptune-colors-blue.html |date=4 January 2024 |work=[[The New York Times]] |url-status=live |archiveurl=https://archive.today/20240105004738/https://www.nytimes.com/2024/01/04/science/uranus-neptune-colors-blue.html |archivedate=5 January 2024 |accessdate=5 January 2024 }}</ref><ref name="Hammel de Pater et al. Uranus in 2004, 2005" /><ref>{{cite web |last=Devitt |first=Terry |url=http://www.news.wisc.edu/10402 |title=Keck zooms in on the weird weather of Uranus |publisher=University of Wisconsin-Madison |date=2004 |access-date=24 December 2006 |archive-url=https://web.archive.org/web/20110813072359/http://www.news.wisc.edu/10402 |archive-date=13 August 2011 |url-status=dead }}</ref> Observations included record-breaking wind speeds of {{convert|229|m/s|km/h mph|order=out|abbr=on}} and a persistent thunderstorm referred to as "Fourth of July fireworks".<ref name="planetary" /> On 23 August 2006, researchers at the Space Science Institute (Boulder, Colorado) and the University of Wisconsin observed a dark spot on Uranus's surface, giving scientists more insight into Uranus atmospheric activity.<ref name="DarkSpot" /> Why this sudden upsurge in activity occurred is not fully known, but it appears that Uranus's extreme axial tilt results in extreme seasonal variations in its weather.<ref name="weather" /><ref name="Hammel2007" /> Determining the nature of this seasonal variation is difficult because good data on Uranus's atmosphere has existed for less than 84 years, or one full Uranian year. [[Photometry (astronomy)|Photometry]] over the course of half a Uranian year (beginning in the 1950s) has shown regular variation in the brightness in two [[spectral band]]s, with maxima occurring at the solstices and minima occurring at the equinoxes.<ref name="Lockwood & Jerzykiewicz 2006" /> A similar periodic variation, with maxima at the solstices, has been noted in [[microwave]] measurements of the deep troposphere begun in the 1960s.<ref name="Klein & Hofstadter 2006" /> [[Stratosphere|Stratospheric]] temperature measurements beginning in the 1970s also showed maximum values near the 1986 solstice.<ref name="Young et al. 2001" /> The majority of this variability is thought to occur owing to changes in viewing geometry.<ref name="Karkoschka ('Uranus') 2001" />

There are some indications that physical seasonal changes are happening in Uranus. Although Uranus 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.<ref name="Hammel2007" /> During its previous northern solstice in 1944, Uranus displayed elevated levels of brightness, which suggests that the north pole was not always so dim.<ref name="Lockwood & Jerzykiewicz 2006" /> This information implies that the visible pole brightens some time before the solstice and darkens after the equinox.<ref name="Hammel2007" /> Detailed analysis of the visible and microwave data revealed that the periodical changes in brightness are not completely symmetrical around the solstices, which also indicates a change in the [[meridional]] albedo patterns.<ref name="Hammel2007" />

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),<ref name="Rages Hammel et al. 2004" /> whereas the northern hemisphere demonstrated increasing activity,<ref name="planetary" /> such as cloud formations and stronger winds, bolstering expectations that it should brighten soon.<ref name="Hammel de Pater et al. Uranus in 2004, 2005" /> This indeed happened in 2007 when it passed an equinox: a faint northern polar collar arose, and the southern collar became nearly invisible, although the zonal wind profile remained slightly asymmetric, with northern winds being somewhat slower than southern.<ref name="Sromovsky Fry et al. 2009" />

The mechanism of these physical changes is still not clear.<ref name="Hammel2007" /> Near the summer and winter solstices, Uranus's 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.<ref name="Rages Hammel et al. 2004" /> The bright collar at −45° latitude is also connected with methane clouds.<ref name="Rages Hammel et al. 2004" /> Other changes in the southern polar region can be explained by changes in the lower cloud layers.<ref name="Rages Hammel et al. 2004" /> The variation of the microwave [[Emission (electromagnetic radiation)|emission]] from Uranus is probably caused by changes in the deep tropospheric [[Circulation (fluid dynamics)|circulation]], because thick polar clouds and haze may inhibit convection.<ref name="Hofstadter & Butler 2003" /> Now that the spring and autumn equinoxes are arriving on Uranus, the dynamics are changing and convection can occur again.<ref name="planetary" /><ref name="Hofstadter & Butler 2003" />