Alaska Subhorizon Halo ~ Paula Homan captured this unusual diamond dust halo seemingly hanging above the terrain of Denali National Park, Alaska.  Low level, columnar habit, hexagonal prism crystals formed the cusped arc.  The temperature was 30°F, high indeed for the formation of the large optical quality crystals that generate halos – a rare display.

Images ©Paula Homan
Parry oriented column. The upper and lowe prism side faces remain almost horizontal.

Left: A horizontal column crystal with a lower tangent arc forming ray. The column can take all rotational positions around its long axis but it does not necessarily 'spin'.



At right: Ray tracing simulations compare the arcs. The lower tangent arc (left-hand) has more widely dispersed colours. It sparkles inside its loop-shaped envelope. The Parry subset is tightly defined with little light inside its outer curve.

Parry crystals form a subsun – more usually seen from plate crystals. Subsuns are direct reflections of the sun by millions of near horizontal ice facets and the upper and lower faces of Parry oriented prisms are ideal mirrors. The Parry simulation, right-hand, shows a subsun whereas the less restricted singly oriented columns of the tangent arc produce only an elongated hint.

The Alaskan display is therefore likely a more common lower tangent arc although some Parry arc might also be present. Yet such lone perfection, particularly from low level crystals only just below freezing, is rare – maybe as rare or rarer as spotting a wolverine!

Thanks to Walter Tape & Marko Riikonen for discussion.
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Lower Tangent or Lower Parry Arc?

Column crystals form both and they have the same rays paths passing through a prism side face and leaving through another inclined 60°.

Orientational freedom makes the difference.

Tangent arc crystals can take all rotational positions around their near horizontal long axis (see below).  Parry crystals cannot. Instead they float fixed with two side faces near horizontal. Parry arcs are therefore a subset of tangent arcs and their sharp forms and colours always reside inside the ‘fuzz’ of the less well defined tangent arcs.

Parry and tangents arcs change shape dramatically as the sun rises. In the HaloSim ray tracings at left Parry arcs are coloured red. At sunrise the lower arcs are spread wide. As the sun climbs they close into a loop and then open again. At the sun altitude, 15-18°, of Paula Homan’s display the Parry and tangent arcs are almost indistinguishable.

But are they? Close-up ray tracing simulations reveal differences…