Rare Lowitz Arc, Ukraine Imaged in Kiev by Alexander Shynder (blog) on Monday 28th September '09. ©Alexander Shynder, shown with permission.

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The Kievan cirrus and contrail ice crystal entangled sky revealed Lowitz arcs from 14:00 to about15:00 local time. Not so many years ago halo experts even doubted their very existence. Nowadays, dedicated observers image them not exactly frequently but once every so often. Nonetheless, Lowits arcs still guard their enigmas.

The scene at left was taken at 14:03 with the sun ~25° high. The arcs are labeled in the lower (approximately) matching HaloSim ray tracing simulation. The Lowitz arc is also arrowed in the image.

The everyday halos are the 22° circular from poorly oriented crystals and an upper tangent arc from horizontal columns.

Worthy of note in any other display, there is a suncave Parry arc above the upper tangent arc.

The upper Lowitz arc crosses the tangent arc to join the Parry at its highest point.

There is little evidence in the images for Lowitz activity near the sundog here blazing in contrail ice. One or two pictures taken later on have hints of a lower Lowitz arc near the parhelion and even a middle arc. However, they are only suggestions and the convoluted sky renders identification difficult.

The simulation using traditionally oriented Lowitz plates tells us not to expect too much near the parhelion at this solar altitude.

The lack in the images of any enhanced brightness on the 22-degree halo at the 10 o'clock position produced by the strong upper Lowitz arc is less easy to explain away. We could invoke variable Lowitz crystal concentrations across the clouds. Another possibility is that the upper arc was produced not by traditionally oriented Lowitz crystals but by Lowitz columns rocking back and forth from a Parry orientation (Parry and Lowitz crystal orientations are related and the two sets of arcs are sometimes tangent to each other).

About the simulation: It used 64 grey levels and several million rays. It is for monochromatic light and hence arcs are much sharper than in the sun's white light. The circular 22° halo is distorted in the projection used to match the image. HaloSim was modified to show the Lowitz rays in red.

Lowitz arc production is horribly inefficient (one reason for their rarity) - the Parry arc needed only a sprinkling (2%) of crystals. The Lowitz needed 15X that number!