|Rare pyramidal halos surround the sun as it sets over the Shetland Islands of Scotland. Imaged Thursday 19th March 2009 by Chris Brown. ©Chris Brown, shown with permission.
The ordinary hexagonal prisms that generate most halos have ice facets inclined at only three angles - 60, 90 and 120° - to one another.
In contrast, pyramidal ice crystals with up to 20 sides have their facets inclined at many more angles - 28,52,56,60,62,64,80 and 90 degrees.
As they tumble, light passing between these variously inclined faces produces a series of circular halos often called 'odd radius halos'.
The Shetland display had most of them.
Outwards from the setting sun is a distinct 9° halo. There are fragments of 18 and possibly 20 degree halos. Beyond that is a broad band in the position of the 22,23 and 24° circles. These last three are characteristically difficult to see as separate halos.