Picture Of The Month
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
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."