Editing Ultraviolet (section)

==Solar ultraviolet==
[[File:Ozone altitude UV graph.svg|thumb|upright=1.35|Levels of ozone at various altitudes ([[w:Dobson unit|DU/km]]) and blocking of different bands of ultraviolet radiation: In essence, all UVC is blocked by diatomic oxygen (100–200&nbsp;nm) or by ozone (triatomic oxygen) (200–280&nbsp;nm) in the atmosphere. The ozone layer then blocks most UVB. Meanwhile, UVA is hardly affected by ozone, and most of it reaches the ground. UVA makes up almost all UV light that penetrates the Earth's atmosphere.]]
Very hot objects emit UV radiation (see [[black-body radiation]]). The [[Sun]] emits ultraviolet radiation at all wavelengths, including the extreme ultraviolet where it crosses into X-rays at 10&nbsp;nm. Extremely hot [[star]]s (such as O- and B-type) emit proportionally more UV radiation than the Sun. [[Sunlight]] in space at the top of Earth's atmosphere (see [[solar constant]]) is composed of about 50% infrared light, 40% visible light, and 10% ultraviolet light, for a total intensity of about 1400 W/m<sup>2</sup> in vacuum.<ref>{{cite web|title=Solar radiation |url=http://curry.eas.gatech.edu/Courses/6140/ency/Chapter3/Ency_Atmos/Radiation_Solar.pdf |archive-url=https://web.archive.org/web/20121101070344/http://curry.eas.gatech.edu/Courses/6140/ency/Chapter3/Ency_Atmos/Radiation_Solar.pdf|archive-date=1 November 2012|url-status=live}}</ref>

The atmosphere blocks about 77% of the Sun's UV, when the Sun is highest in the sky (at zenith), with absorption increasing at shorter UV wavelengths. At ground level with the sun at zenith, sunlight is 44% visible light, 3% ultraviolet, and the remainder infrared.<ref>{{cite web|title=Introduction to Solar Radiation|url=http://www.newport.com/Introduction-to-Solar-Radiation/411919/1033/content.aspx |website=newport.com|url-status=live|archive-date=29 October 2013|archive-url=https://web.archive.org/web/20131029234117/http://www.newport.com/Introduction-to-Solar-Radiation/411919/1033/content.aspx}}</ref><ref>{{cite web|title=Reference Solar Spectral Irradiance: Air Mass 1.5|url=http://rredc.nrel.gov/solar/spectra/am1.5/|access-date=2009-11-12|url-status=live|archive-url=https://web.archive.org/web/20130928011257/http://rredc.nrel.gov/solar/spectra/am1.5/ASTMG173/ASTMG173.xls|archive-date=28 September 2013}}</ref> Of the ultraviolet radiation that reaches the Earth's surface, more than 95% is the longer wavelengths of UVA, with the small remainder UVB. Almost no UVC reaches the Earth's surface.<ref name="Skin Cancer Foundation">{{citation|url=http://www.skincancer.org/prevention/uva-and-uvb/understanding-uva-and-uvb|title=Understanding UVA and UVB|access-date=2012-04-30|url-status=live|archive-url=https://web.archive.org/web/20120501231522/http://www.skincancer.org/prevention/uva-and-uvb/understanding-uva-and-uvb|archive-date=1 May 2012}}</ref> The fraction of UVA and UVB which remains in UV radiation after passing through the atmosphere is heavily dependent on cloud cover and atmospheric conditions. On "partly cloudy" days, patches of blue sky showing between clouds are also sources of (scattered) UVA and UVB, which are produced by [[Rayleigh scattering]] in the same way as the visible blue light from those parts of the sky. UVB also plays a major role in plant development, as it affects most of the plant hormones.<ref name="Vanhaelewyn et al. 2016">{{citation|url=https://academic.oup.com/jxb/article/67/15/4469/1750169|archive-url=https://web.archive.org/web/20160708010101/https://academic.oup.com/jxb/article/67/15/4469/1750169|title=Hormone-controlled UV-B responses in plants|bibcode=2016JEBot..67.4469V |url-status=dead|archive-date=8 July 2016 |last1=Vanhaelewyn |first1=Lucas |last2=Prinsen |first2=Els |last3=Van Der Straeten |first3=Dominique |last4=Vandenbussche |first4=Filip |journal=Journal of Experimental Botany |date=2016 |volume=67 |issue=15 |pages=4469–4482 |doi=10.1093/jxb/erw261 |pmid=27401912 |hdl=10067/1348620151162165141 |hdl-access=free }}</ref> During total overcast, the amount of absorption due to clouds is heavily dependent on the thickness of the clouds and latitude, with no clear measurements correlating specific thickness and absorption of UVA and UVB.<ref>{{Cite journal|last1=Calbó |first1=Josep|last2=Pagès|first2=David|last3=González|first3=Josep-Abel|date=2005|title=Empirical studies of cloud effects on UV radiation: A review |journal=Reviews of Geophysics|language=en|volume=43|issue=2|at=RG2002 |doi=10.1029/2004RG000155|bibcode=2005RvGeo..43.2002C |issn=1944-9208|hdl=10256/8464|s2cid=26285358 |hdl-access=free}}</ref>

The shorter bands of UVC, as well as even more-energetic UV radiation produced by the Sun, are absorbed by oxygen and generate the ozone in the [[ozone layer]] when single oxygen atoms produced by UV [[photolysis]] of dioxygen react with more dioxygen. The ozone layer is especially important in blocking most UVB and the remaining part of UVC not already blocked by ordinary oxygen in air.{{cn|date=September 2024}}