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Name: Angela
Status: Student
Age: 16
Location: N/A
Country: N/A
Date: N/A 


Question:
What are sundogs and how are they formed?
What are halos and how are they formed?
I am doing a project on weather optics and they are in the list to classify and describe their formations.



Replies:
The single best source for this information is: Robert Greenler _Rainbows, Halos, and Glories_, Cambridge University Press, 1980 (reprinted 1994). ISBN 0-521-38865-1

David Falk also speaks about some of these in his book _Seeing The Light_

Nathan A. Unterman


Angela, halos form as a result of the reflection and refraction of sunlight by hexagonal ice crystals in the shapes of plates and columns. Reflection occurs when light bounces off of the surface of the ice crystal and refraction occurs when sunlight is bent to a different direction or angle as it passes through the crystal and comes out a different side. Halos can occur at either 22 or 46 degree angles from the sun. A 22 degree halo occurs when the refracted light passes through the sides of ice columns and a 46 degree halo occurs when the light is refracted through one side and the end of an ice column. 22 degree halos are the more common because the small range of size of ice columns that is needed to produce a 46 degree halo. Halos of 9, 17, 32, and 90 degrees are also possible, but extremely rare. Hexagonal ice crystals also contribute to 22 degree halos and when very large in size, produce bright and often colored light patches to either side of the halo; these are commonly called sun dogs or mock suns, and are technically refered to as parhelia. Other arcs of light can also be produced by ice columns; these are known as tangent arcs, because they touch the halo at one or more points. Tangent arcs can take a wide variety of shapes and orientations around the halo.

Dr. Cook



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