Editing Module:Uranus (section)

== Climate ==
{{main|Climate of Uranus}}

At ultraviolet and visible wavelengths, Uranus's atmosphere is bland in comparison to the other giant planets, even to Neptune, which it otherwise closely resembles.<ref name="Sromovsky & Fry 2005" /> When ''Voyager&nbsp;2'' flew by Uranus in 1986, it observed a total of 10 [[cloud]] features across the entire planet.<ref name="Smith Soderblom et al. 1986" /><ref name="planetary" /> One proposed explanation for this dearth of features is that Uranus's [[internal heat]] is markedly lower than that of the other giant planets, being the coldest planet in the Solar System.<ref name="Lunine 1993" /><ref name="Pearl Conrath et al. 1990" />

=== Banded structure, winds and clouds ===
[[File:Uranus-timelapse.gif|thumb|''Voyager 2''<nowiki/>'s timelapse of Uranus's dynamic atmosphere]]
In 1986, ''Voyager&nbsp;2'' found that the visible southern hemisphere of Uranus can be subdivided into two regions: a bright polar cap and dark equatorial bands.<ref name="Smith Soderblom et al. 1986" /> Their boundary is located at about −45° of [[latitude]]. A narrow band straddling the latitudinal range from −45 to −50° is the brightest large feature on its visible surface.<ref name="Smith Soderblom et al. 1986" /><ref name="Hammel de Pater et al. Uranus in 2003, 2005" /> It is called a southern "collar". The cap and collar are thought to be a dense region of methane clouds located within the pressure range of 1.3 to 2&nbsp;bar.<ref name="Rages Hammel et al. 2004" /> Besides the large-scale banded structure, ''Voyager&nbsp;2'' observed ten small bright clouds, most lying several degrees to the north from the collar.<ref name="Smith Soderblom et al. 1986" /> In all other respects, Uranus looked like a dynamically dead planet in 1986.

''Voyager&nbsp;2'' arrived during the height of Uranus's southern summer and could not observe the northern hemisphere. At the beginning of the 21st century, when the northern polar region came into view, the Hubble Space Telescope (HST) and [[Keck telescopes|Keck]] telescope initially observed neither a collar nor a polar cap in the northern hemisphere.<ref name="Hammel de Pater et al. Uranus in 2003, 2005" /> So Uranus appeared to be asymmetric: bright near the south pole and uniformly dark in the region north of the southern collar.<ref name="Hammel de Pater et al. Uranus in 2003, 2005" /> In 2007, when Uranus passed its equinox, the southern collar almost disappeared, and a faint northern collar emerged near 45° of latitude.<ref name="Sromovsky Fry et al. 2009" /> In 2023, a team employing the [[Very Large Array]] observed a dark collar at 80° latitude, and a bright spot at the north pole, indicating the presence of a [[polar vortex]].<ref>{{Cite journal |last1=Akins |first1=Alex |last2=Hofstadter |first2=Mark |last3=Butler |first3=Bryan |last4=Friedson |first4=A. James |last5=Molter |first5=Edward |last6=Parisi |first6=Marzia |last7=de Pater |first7=Imke |date=28 May 2023 |title=Evidence of a Polar Cyclone on Uranus From VLA Observations |journal=[[Geophysical Research Letters]] |volume=50 |issue=10 |arxiv=2305.15521 |bibcode=2023GeoRL..5002872A |doi=10.1029/2023GL102872 |issn=0094-8276 |s2cid=258883726}}</ref>
[[File:Uranus Dark spot.jpg|thumb|The first dark spot observed on Uranus. Image obtained by the HST [[Advanced Camera for Surveys|ACS]] in 2006.|left]]
In the 1990s, the number of the observed bright cloud features grew considerably, partly because new high-resolution imaging techniques became available.<ref name="Sromovsky & Fry 2005" /> Most were found in the northern hemisphere as it started to become visible.<ref name="Sromovsky & Fry 2005" /> An early explanation—that bright clouds are easier to identify in its dark part, whereas in the southern hemisphere the bright collar masks them—was shown to be incorrect.<ref name="Karkoschka ('Uranus') 2001" /><ref name="Hammel de Pater et al. Uranus in 2004, 2005" /> Nevertheless, there are differences between the clouds of each hemisphere. The northern clouds are smaller, sharper and brighter.<ref name="Hammel de Pater et al. Uranus in 2004, 2005" /> They appear to lie at a higher altitude.<ref name="Hammel de Pater et al. Uranus in 2004, 2005" /> The lifetime of clouds spans several orders of magnitude. Some small clouds live for hours; at least one southern cloud may have persisted since the ''Voyager&nbsp;2'' flyby.<ref name="Sromovsky & Fry 2005" /><ref name="planetary" /> Recent observation also discovered that cloud features on Uranus have a lot in common with those on Neptune.<ref name="Sromovsky & Fry 2005" /> For example, the dark spots common on Neptune had never been observed on Uranus before 2006, when the first such feature dubbed [[Climate of Uranus#Uranus Dark Spot|Uranus Dark Spot]] was imaged.<ref name="DarkSpot" /> The speculation is that Uranus is becoming more Neptune-like during its equinoctial season.<ref name="Hammel2007" />

The tracking of numerous cloud features allowed determination of [[Zonal and meridional|zonal]] winds blowing in the upper troposphere of Uranus.<ref name="Sromovsky & Fry 2005" /> At the equator winds are retrograde, which means that they blow in the reverse direction to the planetary rotation. Their speeds are from {{convert|-100|to|-50|m/s|km/h mph|order=out|abbr=on}}.<ref name="Sromovsky & Fry 2005" /><ref name="Hammel de Pater et al. Uranus in 2003, 2005" /> Wind speeds increase with the distance from the equator, reaching zero values near ±20° latitude, where the troposphere's temperature minimum is located.<ref name="Sromovsky & Fry 2005" /><ref name="Hanel Conrath et al. 1986" /> Closer to the poles, the winds shift to a prograde direction, flowing with Uranus's rotation. Wind speeds continue to increase reaching maxima at ±60° latitude before falling to zero at the poles.<ref name="Sromovsky & Fry 2005" /> Wind speeds at −40° latitude range from {{convert|150|to|200|m/s|km/h mph|order=out|abbr=on}}. Because the collar obscures all clouds below that parallel, speeds between it and the southern pole are impossible to measure.<ref name="Sromovsky & Fry 2005" /> In contrast, in the northern hemisphere maximum speeds as high as {{convert|240|m/s|km/h mph|order=out|abbr=on}} are observed near +50° latitude.<ref name="Sromovsky & Fry 2005" /><ref name="Hammel de Pater et al. Uranus in 2003, 2005" /><ref name="Hammel Rages et al. 2001" />

In 1986, the ''Voyager 2'' Planetary Radio Astronomy (PRA) experiment observed 140 lightning flashes, or Uranian electrostatic discharges with a frequency of 0.9-40&nbsp;MHz.<ref name="Atmospheric Electricity at the Ice">{{cite journal |last1=Aplin |first1=K.L. |last2=Fischer |first2=G. |last3=Nordheim |first3=T.A. |last4=Konovalenko | first4=A. |last5=Zakharenko |first5=V. |last6=Zarka |first6= P.|title=Atmospheric Electricity at the Ice Giants |journal=Space Science Reviews |date=2020 |volume=216 |issue=2 |page=26 |doi=10.1007/s11214-020-00647-0 |arxiv=1907.07151 |bibcode=2020SSRv..216...26A }}</ref><ref name="Radio detection of uranian lightnin">{{cite journal |last1=Zarka |first1=P. |last2=Pederson |first2=B.M. |title=Radio detection of uranian lightning by Voyager 2 |journal=Nature |date=1986 |volume=323 |issue=6089 |pages=605–608 |doi=10.1038/323605a0  |bibcode=1986Natur.323..605Z }}</ref> The UEDs were detected from 600,000&nbsp;km of Uranus over 24 hours, most of which were not visible .<ref name="Atmospheric Electricity at the Ice"/> However, microphysical modelling suggests that Uranian lightning occurs in convective storms occurring in deep troposphere water clouds.<ref name="Atmospheric Electricity at the Ice"/><ref name="ReferenceA">{{cite journal |last1=Aglyamov |first1=Y.S. |last2=Lunine |first2=J. |last3=Atreya |first3=S. |last4=Guillot | first4=T. |last5=Becker |first5=H.N. |last6=Levin |first6=S.|last7=Bolton |first7=S.J. |title=Atmospheric Electricity at the Ice Giants |journal=Space Science Reviews |date=2020 |volume=216 |issue=2 |doi=10.1007/s11214-020-00647-0 |arxiv=1907.07151 |bibcode=2020SSRv..216...26A }}</ref> If this is the case, lightning will not be visible due to the thick cloud layers above the troposphere.<ref name="Radio detection of uranian lightnin"/> The UEDs were detected from 600,000&nbsp;km of Uranus, most of which were not visible .<ref name="Atmospheric Electricity at the Ice"/> Uranian lightning has a power of around 10<sup>8</sup> W, emits 1×10^7 J - 2×10^7 J of energy, and lasts an average of 120 ms. There is a possibility that the power of Uranian lightning varies greatly with the seasons caused by changes in convection rates in the clouds<ref name="Radio detection of uranian lightnin"/> The UEDs were detected from 600,000&nbsp;km of Uranus, most of which were not visible.<ref name="Atmospheric Electricity at the Ice"/> Uranian lightning is much more powerful than lightning on Earth and comparable to Jovian lightning.<ref name="Radio detection of uranian lightnin"/> During the Ice Giant flybys, "Voyager 2" detected lightning more clearly on Uranus than on Neptune due to the planet's lower gravity and possible warmer deep atmosphere.<ref name="ReferenceA"/>

=== 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" />