Circumzenithal Arc Above London

June 05, 2004


Provided by: Chris Sargent
Summary authors & editors: Chris Sargent; Jim Foster

The above photo of a lovely circumzenithal arc was captured in late April over London, England. Although mostly low level, cumulus, clouds are seen on the picture, because ice crystals are responsible for halo phenomena such as circumzenithal arcs, higher level, cirrus, clouds are present as well. Circumzenithal arcs form when sunlight passes through the uppermost basal faces of similarly oriented hexagonal plate crystals and then exits through vertical side faces. Because sunlight is very nearly parallel, there is little overlap between the colors that result, and thus the hues are very pure, purer than those of a rainbow.

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