Examine individual changes

'{{No footnotes|date=May 2010}} [[File:Polar-Night_Longyearbyen.jpg|thumb|250px|right|Typical blue twilight experienced during the polar night in [[Longyearbyen]], [[Norway]] located at 78 degrees north.]] The '''polar night''' occurs when the night lasts for more than 24 hours. This only occurs inside the [[polar circle]]s. The opposite phenomenon, when the sun stays above the horizon for a long time is called the ''polar day'', or ''[[midnight sun]]''. A common misconception is that the [[shortest day]] is totally dark at each point where the midnight sun occurs inside the polar circle. In places very close to the poles this is true, but in regions that are located at the inner border of the polar circles where midnight sun is experienced, this is not true. Because of [[twilight]], these regions experience polar twilight instead of the polar night. In fact, [[polar region]]s typically get more twilight throughout the year than [[tropics|equatorial regions]]. In regions inside the polar circles, the length of the time when the sun is below the horizon varies from 20 hours at the [[Arctic Circle]] and [[Antarctic Circle]] {{Citation needed|date=October 2010}} to 179 days at the [[Geographical pole|Poles]]. However not all this time is classified as polar night, since there may be plenty of sunlight because of [[refraction]] {{Citation needed|date=February 2011}}. Also, one might notice that the time when the sun is '''above''' the horizon at the poles is said to be 186 days. The [[wiktionary:asymmetry|asymmetry]] in numbers is because the time when the sun is ''partially'' above the horizon is counted towards the "daytime". == Types of polar night == [[File:Morketidettermiddag.JPG|thumb|250px|right|Early afternoon in [[Tromsø]], [[Norway]], during the '''Polar Night''', viewed from the upper reaches of the city centre towards the mainland side.]] [[File:Nordkinnhalvoya-polar-night.jpg|thumb|right|250px|Polar night on [[Nordkinn Peninsula]], the northernmost peninsula in mainland Europe]] Since there are various kinds of twilight, there are also various kinds of polar night. This is because the polar night is defined as the period during which no [[twilight]] occurs. These latitude definitions are based on relatively clear skies, so the sky will be darker if there are thick clouds about. ===Polar twilight=== Polar twilight occurs in areas that are located at the inner border of the polar circles, where the sun will be on or below the horizon all day on the [[winter solstice]], although there may still be enough light for normal outdoor activities during civil twilight because of scattering of light by the upper atmosphere and refraction. It happens when there is no true ''[[daylight]]'' and only ''[[civil twilight]]'' occurs at the [[solar culmination]]. Street-lamps may remain on and one looking out a window from a brightly lit room may see their reflection even at noon, as the level of outdoor [[illuminance]] will remain below that of many illuminated indoor spaces. However, it is possible to conduct outdoor activities without any artificial light during the time of civil twilight. Sufferers of [[Seasonal Affective Disorder]] tend to seek out therapy with artificial light, as the psychological benefits of daylight require relatively high levels of ambient light (up to 10,000 [[lux]]) which are not present in any stage of twilight, thus, the midday twilights experienced anywhere inside the polar circles are still "polar night" in that sense. === Civil Polar Night === The ''Civil Polar Night'' is the period during which there is only a faint glow of light visible at midday. It happens when there is no ''[[civil twilight]]'' and only ''[[nautical twilight]]'' occurs at the [[solar culmination]]. Civil twilight happens when the sun is between zero and six degrees below the horizon. The civil polar night is limited to latitudes above 72° 33', which is exactly 6 degrees inside the polar circle. Nowhere in mainland Europe is this definition met. On the Norwegian Territory of [[Svalbard]], however, civil polar night lasts from about 11 November until 30 January. [[Dikson (urban-type settlement)|Dikson]], in Russia, experiences civil polar night for around a month. If there are thick clouds, it gets darker, and places like the coast of [[Finnmark]] (about 70°) in [[Norway]] will get a rather dark "day". === Nautical polar night === The ''nautical polar night'' is the period that no trace of light can be seen anywhere but the sky is not completely dark at midday. It happens when there is no ''[[nautical twilight]]'' and only ''[[astronomical twilight]]'' occurs at the [[solar culmination]]. Nautical twilight happens when the sun is between six and twelve degrees below the horizon, although there is still a place at the [[horizon]] with clearly more light than other places because of refraction. The nautical polar night is limited to latitudes above 78° 33', which is exactly 12 degrees within the polar circle, or eleven and a half degrees from the pole. [[Alert, Nunavut|Alert]], [[Nunavut]], the northernmost settlement in [[Canada]] and the world, experiences this from late November to mid January. === Astronomical polar night === Finally, the ''astronomical polar night'' is the period of continuous ''[[night]]'' where no ''[[astronomical twilight]]'' occurs. Astronomical twilight happens when the sun is between twelve and eighteen degrees below the horizon. Thus, the astronomical polar night is limited to latitudes above 84° 33', which is exactly 18 degrees within the polar circle, or five and a half degrees from the pole. During the astronomical polar night stars of the sixth magnitude, which are the dimmest stars visible to the naked eye, will be visible throughout the entire [[day]]. There are no permanent settlements anywhere in this range of latitude. This portion of the [[Arctic Ocean]] is generally permanently ice capped. Some scientific stations in [[Antarctica]], including the [[Amundsen-Scott South Pole Station]], experience this. ==Effects on humans== The period of polar night can trigger [[clinical depression|depression]] in some people. People who suffer from [[seasonal affective disorder]] are particularly susceptible to these conditions.{{Citation needed|date=March 2010}} The polar night may also be implicated in some instances of [[solipsism syndrome]].{{Citation needed|date=March 2010}} ==Popular culture== The concept of a night of almost one month long has been the subject of the vampire movies: [[Frostbiten]] and [[30 Days of Night (film)|30 Days of Night]]. In these films, the vampires are drawn to the long duration of darkness, allowing them to do as they please. [[Frostbiten]] portrays the polar night well, while [[30 Days of Night (film)|30 Days of Night]] portrays this in a rather unrealistic way. ==Further reading== * {{cite journal | last1 = Hansen | first1 = V. | last2 = Lund | first2 = E. | last3 = Smith-Sivertsen | first3 = T. | year = 1998 | month = March | title = Self-reported mental distress under the shifting daylight in the high north | journal = [[Psychological Medicine]] | volume = 28 | issue = 2 | pages = 447–452 | doi = 10.1017/S0033291797006326 }} ==External links== * [http://collections.ic.gc.ca/simply_science/toc.htm The polar night and polar darkness] * [http://weather.cs.uit.no/video/index-2006.html Many years' of webcam pictures from Tromsø, Norway.] These clearly show the progression into and away from winter at a latitude within the Arctic Circle. {{DEFAULTSORT:Polar Night}} [[Category:Earth phenomena]] [[Category:Arctic geography terminology]] [[be:Палярная ноч]] [[be-x-old:Палярная ноч]] [[ca:Nit polar]] [[cs:Polární noc]] [[da:Polarnat]] [[de:Polarnacht]] [[et:Polaaröö]] [[es:Noche polar]] [[eu:Gau polar]] [[fa:شب قطبی]] [[fr:Nuit polaire]] [[xal:Кииснә сө]] [[ko:극야]] [[hr:Polarna noć]] [[it:Notte polare]] [[lv:Polārā nakts]] [[nl:Poolnacht]] [[ja:極夜]] [[no:Mørketid]] [[pl:Noc polarna]] [[pt:Noite polar]] [[ru:Полярная ночь]] [[sq:Nata polare]] [[sk:Polárna noc]] [[sh:Polarna noć]] [[fi:Kaamos]] [[sv:Polarnatt]] [[uk:Полярна ніч]] [[vi:Ban đêm vùng cực]] [[zh:极夜]]'