Halo Above Sao Paulo

February 08, 2007


Provided by: Fabiano Diniz
Summary authors & editors: Fabiano Diniz, Jim Foster

During a flight from Sao Paulo to Brasilia, the capital of Brazil, I observed this dazzling 22 degree halo when the aircraft entered what seemed to be a cirrus-stratus type of cloud. The interesting thing was that the halo extended both above and below the horizon -- the altitude of the Sun was approximately 0 degrees (sunrise). Ice crystals that are for the most part randomly oriented are responsible for the formation of all halos -- the 22 degree halo is one of the most common. A bright sundog (parhelia) can also be seen -- positioned right on the horizon. Notice that what looks like a tail on this sundog, pointing away from the Sun, is actually a portion of the parahelic circle. Sundogs are caused by hexagonal crystals having a preferential orientation, like leaves falling to the surface. The parahelic circle is a colorless horizontal line that can extend all the way around the sky -- it passes through the Sun and parhelia. It's caused by reflection of sunlight from vertically oriented ice crystal faces. Photo taken on November 2, 2006.

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