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Upper Lowitz Arcs

A display at Dun-sur-Grandry, France imaged by Hans Stocker.

Always search hard for an upper Lowitz arc when - as here - there is a strong Parry above an upper tangent arc.

The Lowitz is tangent to the 22° halo at 10 and 2 o' clock. It then curves away upwards, crosses the upper tangent arc and becomes tangent to the suncave Parry.

This is the region to search for most Lowitz arcs. A factor why it went unrecorded for some 200 years after Lowitz saw it in 1790 was that early searches concentrated on the region between 22° parhelia and the 22° halo.

Scroll down for an enhanced view of the latter region in this display. There are maybe traces of an arc there - but it is hard to see.

The dearth of 'parhelion Lowitz' sightings is likely because many Lowitz arcs arise from 'Parry-Lowitz' columns' that rock about an axis perpendicular to the crystal long axis. This is the traditional Lowitz orientation but with a restricted range of angles.

Images ©Hans Stocker, shown with permission

A colour subtraction process clearly shows the coloured Lowitz arc.

The Lowitz is strongest at the top of the 22° halo. It is weak or even non-existent at the traditional search area around the sundog.