Lowitz Arc at Yosemite

Matthew Walker of New Zealand captured this magnificent rare halo display over the High Sierra near May Lake at Yosemite, California - November 4, 2010. A sharp and clear upper Lowitz arc curves upwards from the 22° halo, crosses a tangent arc and touches an upper suncave Parry arc.
  ©Matthew Walker, shown with permission.

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Lowitz arcs were first recorded at St Petersburg, Russia in 1790.   From then on their very existence was argued about because they were rarely observed with any clarity and they were not finally photographed until about two decades ago.  

Lowitz arcs can be simulated using ice crystals pivoting around a near horizontal axis passing through opposite prism edges. The orientation is improbably and is very inefficient in halo production. The latter might account for the paucity of observations.

The classical model assumes plate crystals and all rotational orientations are allowed. But there is little reason to be so restrictive. Some displays are a little better explained by positing restricted rotation and column crystals rather than plates.

The Yosemite display could have been produced by classically oriented Lowitz plates as shown in the HaloSim ray tracing at right.

Another possibility is that the upper arc was produced by column crystals rocking back and forth about a Lowitz axis from a horizontal Parry orientation position.