Arctic winter
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Arctic_winter".

The polar night is the night lasting more than 24 hours, usually inside the polar circles. 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 at each point inside the polar circle, or that at each place where midnight sun occurs, the shortest day is totally dark. Because of twilight, this is not the case. In places very close to the poles this is true, but in areas very close to the Arctic and Antarctic Circle, midnight sun is experienced, but polar night is never experienced. In fact, polar regions typically get more twilight throughout the year than regions located closer to the equator.

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 to 179 days at the Poles. However not all this time is classified as polar night, since there may be plenty of sunlight because of refraction. Also, one might notice that the time when the sun is above the horizon at the poles is said to be 186 days. The asymmetry in numbers is because the time when the sun is partially above the horizon is counted towards the "daytime".

Kinds of polar night

Various kinds of polar night exist. This is because polar night is the period during which no twilight occurs; but there are various kinds of twilight. These latitude definitions are based on relative clear skies. If there are thick clouds, it gets darker.

Polar twilight

Within the polar circles, the sun will be on or below the horizon all day on the winter solstice, and there will be no true daylight. 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. Sufferers of Seasonal Affective Disorder will probably resort to 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 no civil twilight occurs. Civil twilight happens when the sun is between zero and six degrees below the horizon. Because of scattering of light by the upper atmosphere and refraction, there may still be enough light for normal outdoor activities during civil twilight. Civil polar night is limited to latitudes above 72° 33′, which is exactly six degrees inside the polar circle. In mainland Europe, there are no places fulfilling this definition. On the Norwegian territory of Svalbard, however, civil polar night lasts from about 12 November until the end of January. 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 during which there is only a faint glow of light visible during midday. It happens when there is no nautical twilight. Nautical twilight happens when the sun is between six and twelve degrees below the horizon. Because of refraction, there is still a place at the horizon with clearly more light than other places. 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, the northernmost settlement in Canada and the world, experience this from later November to mid January.

Astronomical polar night

Finally, the astronomical polar night is the period that no trace of light can be seen anywhere and no astronomical twilight occurs. Astronomical twilight happens when the sun is between twelve and eighteen degrees below the horizon. Thus, astronomical polar night happens at latitudes above 84° 33′, which is exactly 18 degrees within the polar circle, or five and a half degrees from the pole.

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 Amundsen-Scott at the South Pole, experience this.

Effects on humans

The period of polar night can trigger depression in some people. Polar days may affect people as well. People who suffer from seasonal affective disorder are particularly susceptible. Polar night may also be implicated in some instances of solipsism syndrome.

External links

• Polar night and polar darkness
• 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.