NASA spaceweather Update: BROKEN HALO: It is well known that ice crystals in high clouds can catch the light of the sun, bending its rays to produce beautiful circular halos in the sky. Last month, Alan Clark of Calgary, Alberta, saw such a halo, but it was strangely broken: I saw this unusual halo on May 17th, says Clark. It appears that a sharp transition between clouds of significantly different ice crystal types crossed in front of the Sun, [producing jagged edges around the circle]. To investigate this possibility, Clark simulated the display using the HaloSim program written by atmospheric optics experts Les Cowley and Michael Shroeder. The results are shown in the upper right, above. I used different crystal types in the upper and lower parts of this halo, explains Clark. In the simulation, one cloud consisted of 30% of horizontal hexagonal columnar crystals and 70% hexagonal flat-plate crystals with a wide dispersion of angles of their faces to the horizontal. The other cloud contained hexagonal columnar crystals with their axes distributed randomly. The computer-generated halo was a good match to what Clark saw. I agree entirely with Alans interpretation, notes Les Cowley. It is a very unusual observation indeed. The upper halo is a fragment of a circumscribed halo generated by the horizontal column crystals. The lower halo is a fragment of the familiar 22-degree halo from randomly oriented crystals. spaceweather/
Posted on: Thu, 05 Jun 2014 02:07:02 +0000
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