Afterglow

{{Short description|Whitish or rosy light during twilight or after sunset}}

{{other uses}}

File:Abenddämmerung mit Purpurlicht und Schattenstrahlen.jpg]]

An afterglow in meteorology consists of several atmospheric optical phenomena, with a general definition as a broad arch of whitish or pinkish sunlight in the twilight sky, consisting of the bright segment and the purple light.{{cite web |title=afterglow |website=AMS Glossary |publisher=American Meteorological Society |date=2015-10-05 |url=https://glossary.ametsoc.org/wiki/Afterglow |archive-url=https://web.archive.org/web/20151005210202/https://glossary.ametsoc.org/wiki/Afterglow |archive-date=2015-10-05 |url-status=dead |access-date=2022-08-14}}{{cite web |publisher=The Formula |title=Full text of 'Weather Glossary' |website=Internet Archive |date=2022-01-14 |url=https://archive.org/stream/in.ernet.dli.2015.217458/2015.217458.Weather-Glossary_djvu.txt |access-date=2022-08-14}} Purple light mainly occurs when the Sun is 2–6° below the horizon, from civil to nautical twilight, while the bright segment lasts until the end of the nautical twilight.{{cite web |title=purple light |website=AMS Glossary |publisher=American Meteorological Society |url=https://glossary.ametsoc.org/wiki/Purple_light |access-date=2022-08-14}}{{cite web |title=bright segment |website=AMS Glossary |publisher=American Meteorological Society |url=https://glossary.ametsoc.org/wiki/Bright_segment |access-date=2022-08-14}} Afterglow is often in cases of volcanic eruptions discussed,{{cite web |title=The Colors of Twilight and Sunset |first= Stephen F.|last=Corfidi |publisher=NOAA/NWS Storm Prediction Center |url=https://spc.noaa.gov/publications/corfidi/sunset |access-date=2022-08-14}} while its purple light is discussed as a different particular volcanic purple light.{{cite journal | last1=Lee | first1=Raymond L. | last2=Hernández-Andrés | first2=Javier | title=Measuring and modeling twilight's purple light | journal=Applied Optics | publisher=The Optical Society | volume=42 | issue=3 | date=2003-01-20 | pages=445–457 | issn=0003-6935 | doi=10.1364/ao.42.000445 | pmid=12570266 | bibcode=2003ApOpt..42..445L }} Specifically in volcanic occurrences it is light scattered by fine particulates, like dust, suspended in the atmosphere. In the case of alpenglow, which is similar to the Belt of Venus,{{cite book | last=Powell | first=Jonathan | title=The Patrick Moore Practical Astronomy Series | chapter=Atmospheric Factors and Features | publisher=Springer International Publishing | publication-place=Cham | year=2018 | isbn=978-3319977003 | issn=1431-9756 | doi=10.1007/978-3-319-97701-0_8 | pages=105–130}} afterglow is used in general for the golden-red glowing light from the sunset and sunrise reflected in the sky, and in particularly for its last stage, when the purple light is reflected. The opposite of an afterglow is a foreglow, which occurs before sunrise.

Sunlight reaches Earth around civil twilight during golden hour intensely in its low-energy and low-frequency red component.

During this part of civil twilight after sunset and before sundawn the red sunlight remains visible by scattering through particles in the air. Backscattering, possibly after being reflected off clouds or high snowfields in mountain regions, furthermore creates a reddish to pinkish light. The high-energy and high-frequency components of light towards blue are scattered out broadly, producing the broader blue light of nautical twilight before or after the reddish light of civil twilight, while in combination with the reddish light producing the purple light. This period of blue dominating is referred to as the blue hour and is, like the golden hour, widely treasured by photographers and painters.

After the 1883 eruption of the volcano Krakatoa, a remarkable series of red sunsets appeared worldwide. An enormous amount of exceedingly fine dust were blown to a great height by the volcano's explosion, and then globally diffused by the high atmospheric winds. Edvard Munch's painting The Scream possibly depicts an afterglow during this period.

File:Dämmerung mit Purpurlicht und Schattenstrahlen, 10.10.2011 bei Limburg VIII.jpg|Purple light with crepuscular shadow

File:Lamma evening4.jpg|Sunset in Hong Kong after the 1991 eruption of Mount Pinatubo

File:Dresden.afterglow.700px.jpg|An afterglow in Dresden, Saxony, Germany

File:Afterglow.jpg|An afterglow in Slovenian mountains, near Triglav Lakes Valley

File:Full Image Sunset Bates College Lewiston Maine July 3 2008 8.30PM.JPG|Sunset over the Bates College track in Lewiston, Maine

File:Noarlunga Pier, Adelaide.jpg|An afterglow at a pier in Australia

File:Afterglow in Sanchong, New Taipei 20140114.jpg|An afterglow on skyscrapers in New Taipei, Taiwan

File:Afterglow Kraków.JPG|An afterglow on a Kraków housing estate

File:Optical effect march sunset - NOAA.jpg|Additional to foreglow here refracted sunlight reaches the South Pole just before actual sunrise. At Earth's poles the Sun appears at the horizon only and all day around equinox, marking the change between the half year long polar night and polar day.