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Anti-crepuscular rays are beams of sunlight that appear to converge on a point opposite the sun. They are similar to crepuscular rays, but are seen opposite the sun in the sky. Anti-crepuscular rays are most frequently visible near sunrise or sunset. This photo of anti-crepuscular rays was taken at sunset in Boulder, Colorado. Crepuscular rays are usually much brighter than anti-crepuscular rays.
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Image Courtesy of Carlye Calvin

Atmospheric Optics

Have you ever seen clouds in the sky that looked different than "normal" clouds? Or have you wondered why rainbows form? Sometimes there are phenomena in the sky that are affected by light and make clouds and the atmosphere look very colorful or unique. Atmospheric optics shows us how light behaves as it passes through the atmosphere. From rainbows to the northern lights, these optical features are dynamic and allow us to learn about atmospheric conditions. Some of these phenomena can be seen very often, and some are once in a lifetime sights.

In some instances, dust, small particles, and moisture droplets scatter light to make the sun's rays visible while clouds and mountain shadows are dark by comparison, creating crepuscular rays or anti-crepuscular rays. In other cases, air and very small particles can scatter colors selectively to make skies blue or sunsets appear to be on fire. Misty clouds and fog contain tiny water droplets that produce strange optical effects that are mostly ringed and brightly colored, including iridescent clouds and glory. Tiny ice crystals in the atmosphere can create halos by refracting and reflecting light.

There are many beautiful examples like these of light and color at work in the atmosphere. Visit the Photo Album of Atmospheric Optics and the Atmospheric Optics Image Gallery to see images of many types of these phenomena, as well as information on how they form.

Last modified February 10, 2009 by Becca Hatheway.

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The Fall 2009 issue of The Earth Scientist, which includes articles on student research into building design for earthquakes and a classroom lab on the composition of the Earth’s ancient atmosphere, is available in our online store.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part by the National Science Foundation and NASA, our Founding Partners (the American Geophysical Union and American Geosciences Institute) as well as through Institutional, Contributing, and Affiliate Partners, individual memberships and generous donors. Thank you for your support! NASA AGU AGI NSF