African Halos

Anna Grove
photographed these halos on September 8, 2013 in Cameroon, West Africa.   

The innermost ice crystal halo is a weak circular 22° halo from randomly oriented hexagonal prisms.

Quite precisely aligned horizontal column crystals dominate the rest of the display. They made the intense circumscribed halo. Reflections from their hexagonal end faces generated the white parhelic circle crossing the sun.   They also made the coloured arc near to the horizon – an infralateral arc.

A particularly unusual feature is the faint Wegener arc (horizontal columns again!) curving out across the sky from beneath the sun.

Another rarity is the high sun Parry arc, this from horizontal columns with two side faces also aligned horizontal.

Images ©Anna Grove, shown with permission

Circumscribed halos & Wegener arcs
Rays forming the circumscribed halo enter a column side face and leave through another inclined 60° to the first. When the sun is lower than 29° the halo splits into upper and lower tangent arcs.

The Wegener arc, so different in appearance, has the same ray path except that the ray reflects internally from a crystal end face.

The lower coloured arc could be a circumhorizon arc from plate crystals rather than an infralateral.

However, given that it appears to curve upwards to the Wegener arc and that high concentrations of column crystals are present, an infralateral arc is more probable.

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A big thank you to Alan Clark of Alberta, Canada for bringing this display to OPOD and for many helpful discussions.
Below:   Anna has caught the region opposite the sun. The Wegener arc forms a cross on the parhelic circle.     The unsharp masked and embossed version also shows the upper Parry arc. A HaloSim ray computation is at right
The Wegener arc was discovered by Alfred Wegener (1880 - 1930), best known for proposing continental drift the basis of plate tectonics.

It is tangent to the top of the circumscribed halo. It loops through the anthelic point opposite the sun and curves back to touch the circumscribed halo again, this time beneath the sun.

HaloSim ray tracing
Singly oriented columns  - Aerodynamic drag as the ice crystals drift downwards relative to cloud air currents aligns them with their long axes almost precisely horizontal. The crystals can rotate around their long axis.