Brilliant halo around the Sun
Image by Roland Friend (Secunda South Afrca), using a HTC Snap S521, taken 2010.10.01 at 10:00.
Roland's beautiful image shows a remarkable halo around the Sun, widely seen over the Johannesburg area this morning.
The ring of light is known as an ice halo, and its pretty rare to see a complete ring (as I think Roland's image shows) - usually one sees only a portion of the ring.
Ice halos form only when there are cirrus clouds in the sky. Most clouds are made up of drops of water, but cirrus clouds are composed of tiny ice crystals. Each minute crystal is like a long thin pencil that has six sides. Sunlight passes through the crystal and its path is bent (refracted) so that each crystal acts like a miniature prism. With a gazillion ice crystals tumbling about in the sky, you end up seeing a complete ring.
If there isn't sufficient cloud you only see a partial halo. And if the cloud is too thick, the halo may be too faint to see. If the ice crystals are too large, then they don't create the ring effect either. And even if there are enough tiny ice crystals, they may simply not be perfectly shaped - instead of being solid with smooth sides, they may be hollow, or perhaps stuck together to form clumps. But this morning, conditions were perfect!
The ice crystals that lie inside the halo don't contribute to the ring of light - they simply block out the light instead, so that the inside of the halo appears darker than the surrounding sky - exactly as Roland's lovely image shows.
Halos can also be seen around the Moon, from time to time, such as this one I photographed in July 2009. For previous solar ice halo images, see Maciej Soltynski's photograph taken in July 2006, and an anonymous contributor's photo from 2005 November.
For more on these and related sights, browse the articles tagged as "atmospheric phenomena".
via: Hannes Pieterse, Carol Botha
Naylor, J. (2002) Out of the Blue: A 24-hour Skywatcher's Guide. Cambridge University Press.
nothing more to see. please move along.