photo #5677566

Tim Kessler (Member 45,331) wondered why the ring of this 22-degree halo was incomplete when he spotted it in Kandiyohi County, Minnesota, US. The optical phenomenon is a large ring, centred on the Sun or Moon, created by the sunlight bending, or refracting, as it shines through ice crystals shaped as tiny hexagonal columns – little pencils of ice, sometimes clumped together. But only when the ice is clear enough, without any bubbles in it, can the tumbling crystals behave as prisms to refract the light and cause the effect. Don’t be fooled, however, by our apparent confidence in explaining this effect. In fact, no one yet knows for certain exactly how the crystals need to be shaped and oriented to cause the light effect. Clearly, the crystals to the right of Tim’s Cirrostratus fibratus cloud didn’t have quite the right shape, orientation, or clarity to make the grade for producing the halo effect. The result, as Tim put it, looked like a Pac-Man on a wild-hair day gobbling up the Sun.