A circumhorizon arc appears as a rainbow-like patch of color in the 
afternoon sky above Greer, SC, on April 25, 2005.  Image taken by Neil 
Dixon, National Weather Service.

A relatively rare optical phenomena, known as a circumhorizon arc, was 
observed in the sky above Greer, South Carolina, on April 25, 2005.  
The circumhorizon arc appears as a vivid, rainbow-like patch of color in 
cirrus clouds as sunlight refracts through the ice crystals in the cloud.  
Unlike rainbows which appear in the sky as an arch after it rains and the 
sun is at a low angle, a circumhorizon arc appears in more of a gentle 
curve during fair weather when the sun is at a very high angle.  The 
visible portion of the arc is actually part of a much larger halo at 
relatively low angles parallel with the horizon that would be visible 
if the proper type of clouds were present across the entire sky.  The 
circumhorizon arc is often mistaken for a sundog, which is much smaller 
and always appears at a position that is 22 degrees to the right or 
left of the sun.

The very high sun angle required for circumhorizon arcs is one of the 
reasons why they are so rare.  In fact, the sun must be at least 
58 degrees above the horizon in order for the phenomena to be observed.
At the latitude of Upstate South Carolina (roughly 35 degrees North), 
the sun only rises above 58 degrees over the horizon between 1 April 
and 15 September, and only during the middle part of the day.  Even on 
the summer solstice, the sun is above 58 degrees for only about 4 hours 
and 30 minutes. The farther north one is located, the rarer the phenomena 
is because the sun spends less and less time above 58 degrees as latitude 
increases.

Not only that, but precisely the right type of cloud formation must be 
present in the right part of the sky for the proper sunlight refraction
to take place.  The clouds must be composed of plate-like ice crystals 
that are oriented such that sunlight enters the nearly vertical edge of 
the crystal, refracts through it at an angle of about 46 degrees (as in 
a prism), and exits the relatively flat bottom face.  Only certain types
of cirrus clouds have the right composition of plate-like ice crystals.

Circumhorizontal arcs have been witnessed in many other locations, 
including this particularly vivid one seen over Spokane, Washington.  
For more information, look up "atmospheric optics" with your favorite 
Internet search engine. 

If you have interesting images of weather or other atmospheric phenomena,
send them to the webmaster at GSP at the e-mail link below and perhaps 
they will appear in a future edition of the "Picture of the Month."

Patrick Moore
Neil Dixon