|Not so ordinary 22° halo
A sighting by Jenny & Colin Summerell near Takaka on the northern coast of South Island, New Zealand.
©Jenny & Colin Summerell, shown with permission
|The 22° circular halo is oft quoted to be the commonest of the ice crystal halos. Depending on location, sundogs might better claim that title but fragments or complete versions of both can be seen on average more than once a week over most of Europe and the less arid parts of North America.
The 22° halo might be an almost everyday sight but it is by no means ordinary. In one sense it is one of the least ordinary of the halos because we are unsure of what crystals produce it.
The halo, with a bright and pastel coloured inner rim of 22° radius, is certainly produced by refraction of sun rays through the side faces of hexagonal ice prisms with near random orientations. That much we know.
However, ordinary hexagonal prism crystals are strongly oriented by aerodynamic drag forces as they drift within clouds or at low level as diamond dust. Plates have their large hexagonal faces horizontal to within a degree or so. Columns are similarly well oriented with their long axes nearly horizontal.
To overcome these awkward facts, it has been suggested that the 22° halo is created by short equi-dimensional crystals that would tumble and take all orientations. Maybe some are – However, crystal samples taken during halo displays do not have many of the necessary equi-dimensional crystals.
What are often found, especially in ‘poor’ displays showing mainly a 22° halo, are clusters of column or bullet crystals. These would tumble. Refraction through their hexagonal arms would produce the 22° halo.
Maybe – But we are not certain. The commonplace can have less than commonplace cause.
Cluster crystals, assemblies of hexagonal columns, would tumble as they drift downwards relative to in-cloud air currents.
|Note the soft pastel colours. Typical of the 22° halo where there is strong colour overlap. The near circular high sun circumscribed halo often mistaken for a 22° has sharper and more saturated colours|
The halo is produced by refraction between ice crystal faces inclined 60°. To produce the circle, the face pairs must be near randomly oriented to horizontal