Saturday, April 4, 2009

Halo

This afternoon, I was sitting in the back yard, soaking up some sun. The sky was somewhat hazy. Momentarily glancing at the sky in the direction of the Sun (with my eyes shielded against it), I noticed a faint halo around the Sun. Here is the image, taken with a Canon Rebel Xt, 18-200mm Sigma zoom lens at 18mm, ISO 100, f22, 1/1000th sec exposure.

According to Wikipedia, a halo is an optical phenomenon that appears near or around the Sun or Moon, and sometimes near other strong light sources such as street lights. There are many types of optical halos, but they are mostly caused by ice crystals in cold cirrus clouds located high (5–10 km, or 3–6 miles) in the upper troposphere. The particular shape and orientation of the crystals is responsible for the type of halo observed. Light is reflected and refracted by the ice crystals and may split up into colors because of dispersion, similarly to the rainbow.

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