The Upper Tangent Arc


Inner halo and upper tangent arc in case of lower solar elevation

Date & Time: Feb 25 2006, 16:17 JST(+0900), 1/500sec. Exp.
Optical: SIGMA 15mmF2.8 EX FISHEYE, Aperture: F11.3
Digital Camera: Fujifilm FinePix S2Pro
Location: Ooizumi, Hokuto city, Yamanashi pref.

Camera Settings: Recording Format...JPEG-FINE(3024 x 2016)
CCD sensitivity...ISO100, White Balance...Auto



Tangent arc in case of higher solar elevation

Date & Time: Apr 15 2006, 14:02 JST(+0900), 1/500sec. Exp.
Optical: SIGMA 15mmF2.8 EX FISHEYE, Aperture: F11.3
Digital Camera: Fujifilm FinePix S2Pro
Location: Kokubunji city, Tokyo

Camera Settings: Recording Format...JPEG-FINE(3024 x 2016)
CCD sensitivity...ISO100, White Balance...Auto




Schematics of Tangent Arc
Upper: Schematic of ice crystal
Lower: Sunlight path through ice crystal
You can rarely see a bright part at the top of solar inner halo (22 degrees halo). This light looks "V-shaped" when the height of sun is fairly low, and the shape changes to curve contacting with the top of inner halo as solar angle of elevation higher. This atmospheric optical phenomenon is called "The upper tangent arc". And symmetrically you can see same phenomenon at the lower edge of inner halo, in that case we call it "The lower tangent arc".

The tangent arcs appear by fine slender hexagonal ice crystals in high altitude sky shown in the right figure. This type of crystal is floating steady by turning the sides paralleled with ground. The 22 degrees refracted sunlight can be observed only from the upper and lower parts of the sun. As the solar angle of elevation higher, we can see the refracted light from off point of top and bottom parts of sun. In that case the light looks like a gentle curve along the inner halo. If the sun elevation is higher than 40 degrees, the upper and lower tangent arcs connect and circumvent; we call this state "The Circumscribed halo".





The Solar Halo

The Circumscribed Halo


Copyright(c) 2006 by Naoyuki Kurita, All rights reserved.
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