Rare Elliptical Halo

A superb specimen imaged Saturday 12th February 2011 by Olli Leivo at Lahti, Southern Finland.
  ©Olli Leivo, shown with permission.

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Elliptical halos are a comparative rarity but Finland had widespread displays on 12th February.    This beauty was imaged at 14:02 with the sun 12° high.     It is 10.4° across and measures 9.9° to its upper tip.  

Ellipticals are small halos just a few degrees across.  Unlike most other halos they are not of fixed size and their size also depends on solar elevation.   

Their mode of formation is still problematic.

Reflections from very flat pyramidal crystals drifting as horizontal plates have been invoked.   The problem is that the required face inclinations (here 85° plus from the crystal ‘c’ axis)  are very unphysical.    All crystal face angles are effectively ‘quantised’ because they must correspond to planes through the underlying atomic lattice.   These flat crystal angles do not.

Another possibility is formation by dendritic snow-type crystals.