Pillar and Arcs Above Amarillo, Texas
February 25, 2011
The photo above showing a Sun pillar and a pair of dazzling arcs (sundogs) was taken the morning after a winter storm passed through Amarillo, Texas. Temperatures were in the low single digits Fahrenheit (about -15 C). Because arcs are observed rather than halos, it's known that the ice crystals responsible for their formation are more or less oriented in particular ways. Rays of sunlight passing through oriented hexagonal crystals are responsible for the commonly observed sundogs -- the bright arcs on either side of the Sun. Light enters one of the crystals’ side faces and exits through an alternate side face, refracted about 22 degrees. Therefore, sundogs or parhelia are found 22 degrees on either side of the Sun and at the same solar altitude.
The bright burst of light extending vertically from the Sun is a pillar. Sun pillars are reflection phenomena. Thus, they're rather colorless; whereas arcs, formed by light refracting through ice crystals, have at least some coloration. Pillars form when sunlight is reflected from the bottom, or occasionally, the top surface of aligned plate-shaped ice crystals. Photo taken on February 6, 2011.
Photo details: Camera Maker: Apple; Camera Model: iPhone 3GS; Focal Length: 3.9mm; Aperture: f/2.8; Exposure Time: 0.0008 s (1/1327); ISO equiv: 64; Metering Mode: Average; Exposure: program (Auto); White Balance: Auto; Flash Fired: No (No Flash available); Orientation: Normal; Color Space: sRGB.