Bullet Rosettes & 22° Halos
Imaged by Jakub Badelek who collected them from diamond dust. He left black paper board outside for 20 minutes at -10 Celsius.
Bullet Rosettes or cluster crystals are joined hexagonal columns. They are thought to form when several columns grow outwards from a common seed nucleus at temperatures of about -5 to -10 Celsius.
They may be the source of many showings of the most common halos in the sky, the 22° halo.
A near perfect 22° halo seen by Jonathan Shock at Santiago de Compostela, Spain.
There are distinct reds and yellow at the inner edge. The centre is dark. The halo brightness fades gently away for perhaps 10°.
This halo needs optically good ice prisms, few bubble inclusions, or indented faces. Large wobbly crystals are usually flawed. But the sun is high and smaller crystals with smaller tilts might (just) suffice.
The halo is uniformly bright, no pronounced brightening at top and bottom. A candidate for cluster crystals?
Another well defined halo, this time on the Greenland ice sheet. Imaged by Ed Stockard.
The halo beneath the horizon is mostly made by crystals resting on the surface. The path of the ground glinting crystals is a hyperbola but seen from the tip of the halo cone it looks circular.
A very diffuse and weak lunar 22° halo seen in Finland by Marko Riikonen.
We have as 22° halo forming candidates cluster crystals or - when the halo is not uniformly bright - large wobbly columns.
There is a third possibility. Small columns less than about 30 micron long are less well aligned by aerodynamic drag. If small enough they can be near random.
Smallness introduces another effect. Diffraction blurs the halo in the same way that tiny water droplets produce a broad diffuse fogbow rather than a rainbow.
A 22 degree halo competition
Unfortunately atmospheric optics are not always reliably described on the web.
Google "22 degree halo". Google features a popular and oft linked page. How many errors, misleading statements and outright scientific howlers can you find? 5, 10.. ..more?
A beautiful and rather diffuse and soft 22° halo imaged by Tom Peck.
The 22° halo
Light refracts between two facets of an ice hexagonal prism inclined at 60°. The minimum ray deviation is 22° corresponding to the halo’s inner radius. Plenty of other rays deviate more and these contribute to the halo's outer white sheath.
What crystals form the halo? The standard explanation is that the culprits are randomly oriented hexagonal columns. They could drift in high cirrus or in nearby in diamond dust. However, columns are not randomly oriented because aerodynamic drag aligns them horizontal. Short columns with their length comparable to breadth are then invoked as they will be more randomly oriented. All very well, but not enough short columns are found in crystal samples.
An alternative is that completely random orientations are not always needed. Ray tracings show that wobbly columns with large but non random tilts can generate circular halos. But the halo is not uniformly bright. Uniformity does increase somewhat at high sun. Large crystals wobble the most. Yet, they are also most likely to have flaws. They might produce the soft less well defined halos often seen. The one at right is a possibility.
Cluster crystals, bullet rosettes are fairly common. They tumble. They present icy hexagonal prism sections at all orientations. They could produce sharp, bright well defined halos. Good candidates for many 22° halos.