Antarctic Halos pictured at Halley 5, Brunt Ice Shelf, Coats Land by Jon Oldroyd (site). Image ©Jon Oldroyd, shown with permission.

Halley 5 is a permanently manned station of the British Antarctic Survey. It researches atmospheric sciences, geology and glaciology and contributes to understanding of climate change. It was here that dramatic ozone depletion by CFCs, the ‘ozone hole’, was first detected in 1985.

The halo display has the classic Polar appearance of an inner 22° halo flanked by bright sundogs and a large outer halo, both crossed by a sweeping parhelic circle.

Is a large outer halo a 46° degree circular halo or supralateral and infralateral arcs?

Usually it is a supralateral and infralateral arc from rays passing between side and end faces of horizontal column crystals. A further possibility is a transitional halo between a supralateral and 46° halo produced by columns with large tilts. A pure 46° halo from 'randomly' oriented crystals is rarer.

Here, unsharp image masking suggests the presence of a 46° halo plus supralateral and infralateral arcs.

There is a weak Parry arc and it is tempting to ascribe the localised bright patches on the outer halo to Parry supralateral and infralateral arcs (Tape arcs) but they could equally be zones of greater ice crystal density.

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