Sunday, July 23, 2006

Fire and Ice

Viewers are kind enough to pass along photos to us here in the StormTracker 46 Weather Center all the time. Many times the photos are of things they shot around here, sometimes they are pictures of a recent vacation, and sometimes the photos are of rare weather events that they find while surfing around the internet.

The latter was the case today as a viewer forwarded the above picture she found on the internet. The above photo was taken along the Idaho/Washington state border and it of an optical phenomenon called "circumhorizontal arc."

Circumhorizontal arcs form when sunlight passes through cirrus clouds. The sight occurs only when the sun is at a very high angle and when the hexagonal ice crystals that make up cirrus clouds must be shaped like thick plates with their faces parallel to the ground.

What happens is light enters through a vertical side face of such an ice crystal and leaves from the bottom face, it refracts, or bends, in the same way that light passes through a prism. If a cirrus clouds crystals are aligned just right, the whole cloud lights up in a spectrum of colors.

This particular arc spanned several hundred square miles of sky and lasted for about an hour on June 3rd when it formed.

A circumhorizontal arc is similar in how it forms to the more common sundog (pictured to the left) which we see can see quite frequently here in Georgia.

Always make sure to keep an eye to the sky. You never know what you will see!


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