Snow Cannon Halos

Images by Peter Roos in Sweden on 27th December.

Rare halos dominate the scene with an unbelievably bright Parry arc, helic arc, supralateral arc and even a trace of a Parry supralateral and Moilanen arcs. See the labelled image below left.

Snow cannons were operating in two nearby ski slopes and they were responsible. The cannons spray nuclei on which snow crystals rapidly grow. But as a by-product some nuclei drift far downwind and tiny halo forming ice crystals grow slowly around them. The crystals are almost optically perfect and thus form bright diamond dust halos and arc of great rarity.

All images ©Peter Roos, shown with permission
The display's rare halos are all from Parry oriented hexagonal columns. With the exception of the Moilanen arc that is - we do not know what crystals form that one.

Parry crystals drift with their long axes horizontal. Strangely, the crystal does not rotate about this axis but instead it is locked with two prism side faces horizontal.

Lower image: Another view of the rare helic arcs formed by reflection off the side faces of Parry oriented columns.

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Left: The upper tangent and upper suncave Parry arcs form a pair. Their shapes and separation vary strongly with the sun's elevation.

The tangent arc always does what it says - form a tangent to the 22° halo.

The background image shows the individual glints of the crystals floating nearby. Halos are the collective glints of millions of crystals.