Degrees of Blue Sky

July 22, 2006


Provided by: Dale Chadwick
Summary authors & editors: Dale Chadwick; Jim Foster

At first glance, the above photo taken near Wellington, Colorado shows only a featureless blue sky. Nevertheless, it illustrates a phenomenon often overlooked. The sky is not one blue color but a gradation, lighter at the horizon and darker overhead -- the more atmosphere you look through the lighter the blue. Directly overhead there are only a few miles of atmosphere (troposphere), but if you look toward the horizon, the path length of light is significantly greater. As a result, more of the shorter wavelengths (blue and violet colors) are scattered or attenuated. The deepest blues are found approximately 90 degrees away from wherever the Sun is in the sky. Photo taken on November 24, 2005.

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