You wait ages, and then three come along at once. Since spotting our first sundog on 10 September, we've now seen another three. And the most recent display was accompanied by a circumzenithal arc, which looks like a small rainbow almost directly overhead. Like sundogs, these are not particularly rare ... but who looks vertically up?
Note that these phenomena have little to do with rainbows, which of course are always opposite the sun. They're not connected with rain, but are caused by ice crystals very high up in the atmosphere (hence the connection with cirrus clouds, also very high). If the air is stable and conditions are right, the crystals form with identical sizes and alignments; light is then refracted and reflected within the crystals to give various halo effects.
Fig 2: Sundog [Leeds UK, 30 Sep 2002 9:40:37am; Fuji FinePix 4800Z digicam]
The geometry of the crystals governs the size and nature of the halos. People have investigated what halos might form on planets with different atmospheres -- crystalline ammonia on Jupiter and carbon dioxide crystals on Mars, for example. The Atmospheric Optics web site has a simulation program, using ray-tracing techniques to show some possible displays.
Below is a detail of the sundog, zoomed in three times.
Fig 3: Sundog detail [Leeds UK, 30 Sep 2002 9:40:43am; Fuji FinePix 4800Z digicam]
For further information, explanations and photographs, visit the splendid Atmospheric Optics web site. See also the extraordinary book Color and Light in Nature by Lynch and Livingstone (second edition; Cambridge University Press 2001, £19.95).
Original image: Leeds UK, 2002:09:30 08:49:57 * 1280x960, 324 Kbytes
Camera: FUJIFILM FinePix2200 * Ref Dscf0541.jpg
1/157 sec * ISO100 * f11.00
Focal length 5.80mm * Bias 0
Program: Normal * Metering: Pattern * Flash: Not fired
Processing (Photoshop): Montage of three shots; hue/saturation adjustment
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