The Kern arc is an extremely rare atmospheric
optical phenomenon
Optical phenomena are any observable events that result from the interaction of light and matter.
All optical phenomena coincide with quantum phenomena. Common optical phenomena are often due to the interaction of light from the Sun or Moon with ...
belonging to the family of
ice crystal halos. It is a complete and faint circle around the zenith, in contrast to the related and much more common
circumzenithal arc
The circumzenithal arc, also called the circumzenith arc (CZA), the upside-down rainbow, and the Bravais arc,"Mémoire sur les halos et les phénomènes optiques qui les accompagnent", J. de l' École Royale Polytechnique 31(18), 1-270, A. Brav ...
, which is only ever a partial circle.
The Kern arc is named after H.F.A. Kern in the
Netherlands
, Terminology of the Low Countries, informally Holland, is a country in Northwestern Europe, with Caribbean Netherlands, overseas territories in the Caribbean. It is the largest of the four constituent countries of the Kingdom of the Nether ...
, who first reported it on October 8, 1895.
[S. W. Visser: ''Optische verschijnselen aan de hemel'', bladzijde 58, § 44: ''De gespiegelde circumzenitale boog. Boog van Kern''] Since then it has been reported on six occasions.
[ (Mikkilä's description of the observation.)]
It is formed by rays entering the top faces of horizontal plate crystals and leaving through a near vertical side face. Kern arc rays undergo an internal reflection off a side face inclined at 60° to the exit face. Near ''triangular'' hexagonal crystals with three alternate side faces much shorter than the others allow the Kern arc to occur.
The first photographs of the Kern arc were taken by Marko Mikkilä close to Vuokatti Ski Resort,
Sotkamo
Sotkamo is a municipality of Finland, located in the Kainuu region about east of Kajaani, the capital of Kainuu. Vuokatti, in west of Sotkamo, is the most populous village in the municipality and also a popular skiing resort. Both Hiidenportti ...
,
Finland
Finland, officially the Republic of Finland, is a Nordic country in Northern Europe. It borders Sweden to the northwest, Norway to the north, and Russia to the east, with the Gulf of Bothnia to the west and the Gulf of Finland to the south, ...
on 17 November 2007.
The photographs were in natural sunlight taken from an artificial cloud of ice crystals created by
snowguns.
At the Vuokatti Ski Resort about 100 snowguns may operate at one time. The temperature was −15 to −18 °C in clear sky and almost no wind. Nikon D70 and Nikon FM cameras were used with Sigma 8mm EX F4,
Sigma 15mm EX F2.8 lenses. Some samples of ice crystals were also collected, confirming their nearly triangular base.
Mikkilä's photographs were first published in Tähdet ja avaruus-magazine 1/2008. The magazine is published by
URSA, the Finnish association of astronomy.
See also
*
Halo (optical phenomenon)
A halo () is an optical phenomenon produced by light (typically from the Sun or Moon) interacting with ice crystals suspended in Earth's atmosphere, the atmosphere. Halos can have many forms, ranging from colored or white rings to arcs and spo ...
*
Circumzenithal arc
The circumzenithal arc, also called the circumzenith arc (CZA), the upside-down rainbow, and the Bravais arc,"Mémoire sur les halos et les phénomènes optiques qui les accompagnent", J. de l' École Royale Polytechnique 31(18), 1-270, A. Brav ...
References
Atmospheric optical phenomena
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