Extreme Biking ~ Ed Stockard (photostream) is back at Summit Station at the centre of the nearly two mile thick Greenland Ice Cap. Here, colleague Robin shades the sun for a halo display image and gives mountain biking a whole new perspective. The temperature was -20C with 14-18kt winds.
 Images ©Ed Stockard, shown with permission
Overhead a colourful circumzenithal arc is sculpted by hexagonal plate ice crystals drifting with their large faces nearly horizontal.

Almost touching it is a rare 46�halo from poorly oriented or cluster crystals. Why a 46° halo rather than the more common supralateral arc? In the top image the 22° halo made by the same crystals is bright and there is little sign of a tangent arc that heralds the column crystals necessary for a supralateral.

Circling closer to the sun is the familiar 22� halo guarded by sundogs and crossed by the parhelic circle.

Crystal populations changed for the all sky view at right. Horizontal column crystals had formed upper and lower tangent arcs and � near the horizon � infralateral arcs of widely separated colours. There is a hint also of a Parry arc.
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Dave shades the sun. We see how the halos are formed from the individual glints of low-level diamond dust crystals.

Infralateral arcs colour the horizon.