The circumscribed halo is caused by sunlight being refracted by horizontally floating hexagonal ice crystals, shaped like a pencil. The appearance of the circumscribed halo varies greatly with the height of the sun. If the sun is in the zenit, it is a complete circle. When the sun is lower, about 60 degrees high, it is shaped like an ellipsoid. With still lower sun elevations, the circumscribed halo becomes wider until the lower and upper half do not touch anymore. With low sun elevations like 10 to 20 degrees, the halo attains the shape of a V above the sun, referred to as the upper tangent arc. The upper and the lower tangent arc are part of the circumscribed halo.
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