Belgium Halo Display
June 05, 2011
The photo above showing a memorable halo display was captured from the parking lot of the local supermarket in Mouscron, Belgium on May 6, 2011. With a sky streaked with cirrus clouds, I noticed that for much of the day halos were visible. The halos appeared and disappeared depending upon the position of these ice crystal clouds with regard to the Sun. As the Sun climbed higher, I observed not only the 22 degree halo but also a colorful sundog or parhelion visible at the lower left, along with a bright circumscribed halo in the center and a pallid parhelic circle extending through the sundog and the solar disk.
Circumscribed halos have an oblong shape and form in randomly oriented, columnar shaped, hexagonal ice crystals. Sunlight enters through one of the crystal's side faces and exits, via refraction, through an alternate side face. Circumscribed halos can only be seen when the Sun is higher than approximately 30 degrees above the horizon. They're tangent to the 22 degree halo, directly above and below the Sun, where they're at their brightest. Their shape changes as the solar altitude increases. Sundogs are caused by refraction of sunlight through hexagonal ice crystals that are more or less similarly oriented. The parhelic circle, however, is due to reflection rather than refraction -- sunlight reflects off the front faces of ice crystals having a preferred orientation. Be sure to protect your eyes when looking at halo phenomena.
Photo details: Camera Maker: PENTAX; Camera Model: Optio E70L; Focal Length: 5.7mm (35mm equivalent: 32mm); Aperture: f/5.5; Exposure Time: 0.0005 s (1/2000); ISO equiv: 100; Exposure Bias: none; Metering Mode: Matrix; Exposure: program (Auto); White Balance: Manual; Light Source: Daylight; Flash Fired: No (enforced); Orientation: Normal; Color Space: sRGB.