|Rare Elliptical Halo imaged by veteran halo observer Michael Ellestad (Mike's Atmospheric Photography) during its two hour long appearance in altocumulus cloud over Ohio March 23, '08.
Image ©Michael Ellestad, shown with permission.
Elliptical halos are rare and something of a mystery.
They can be simulated by ray tracing by invoking pyramidal crystals with very large apex angles such that they are plate-like and quite unlike ordinary pyramidals.
All very well, but such crystals are crystallographically extremely unlikely. Crystal facets take much more simple low energy directions across the arrays of the molecular lattice.
Elliptical halos can also have two or three rings. The three ray paths shown below pass between faces with different interfacial angles and yield three rings not unlike those of observed halos. However, there the similarity ends for simulations never seem to get it quite right. More is going on!