California Halos - Seen at Oakland by Alexandra Childs (ARTSS) on January 3, 2009. ©Alexandra Childs, shown with permission.

Splendid halos do not need Polar weather, 3- 6 mile high cirrus cloud over mild California is cold enough to hold halo generating ice crystals.

Halos can be seen anywhere in the world at any season but it must be admitted that most of the very best are those in the low level diamond dust of sub-zero temperatures.

A strong upper tangent arc generated by horizontal column crystals dominates the display. Similar column crystals produced the colourful supralateral arc just in the image at top right.

A rare Parry arc - expect to see one or two times a year in temperate cirrus cloud - tops the tangent arc. Parry arcs also come from horizontally aligned column crystals but the crystals have to be additionally and improbably aligned so that a crystal side face is horizontal too. The Parry orientation is very efficient indeed at producing bright, sharp halos otherwise we would probably almost never see them.

Is there an extremely rare Lowitz arc?

There is a hint of a halo between the tangent and Parry arcs in just the position of an upper Lowitz arc. The accompanying HaloSim ray tracing shows it.

Unlike Parry arcs, Lowitz arcs are produced very inefficiently indeed which probably accounts much for their rarity. The HaloSim ray tracing needed 20% Lowitz crystals to produce the faint trace.

There is another twist. Traditionally oriented crystals would not give an arc between the UTA and Parry without producing enhanced brightness on the 22-degree halo in the 2-3 o'clock position. The latter is not seen in the image. The HaloSim Lowitz arc was instead made by ' Parry type' column crystals rocking up to 40� from horizontal in the Lowitz mode. Speculations like these are interesting to pursue but....

In this display several rather beautiful cloud streaks masquerade as halos. It is dangerous to speculate over much from one image. Likely the 'Lowitz arc' was one of the streaks but that does not detract from the marvel of the halos.


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