Physics Photo of the
December 16, 2011
Almost a year ago we woke up to this colorful parhelion amidst
an impressive sunrise. The parhelion is the rainbow
pillar in the lower left of the photo. The Sun is
currently to the right of the large old pine tree, but the Sun
hasn't appeared over the edge of the mountain yet. The
clouds, however, are in the bright sunlight.
The rainbow parhelion means literally "next to the Sun"; it is
also called a "sundog". These are frequently visible
when the Sun is near the horizon when the clouds consist
of hexagonal platelike ice crystals. The flat ice
crystals slowly fall in the air in the horizontal orientation
- the most stable configuration under air resistance.
The sunlight enters a vertical side of the hexagon and exits
two sides over at an angle of 22 deg. Not only is the
sunlight bent, but it is dispersed into the rainbow of colors
like a rainbow. The hexagonal ice crystal acts exactly
as a prism. Parahelia are often seen on both sides of
the Sun due to the symmetry of the crystals as was shown on
8, 2008 Physics Photo of the Week.
On careful inspection of today's photo, we can also notice the
shadows of the clouds cast onto the sky in the center of
the photo. I would welcome any similar photos weather
phenomena from readers. Cell phone cameras have the
advantage of nearly always being on-hand to take images when
This is the final Physics Photo of the Week for 2011 due to
the winter holidays at Warren Wilson College. Look for
the next publication on January 20, 2012. Have a Merry
Christmas and a Happy New Year!
Photo of the
published weekly during the academic year on Fridays by
Wilson College Physics
Department. These photos feature interesting
the world around us. Students, faculty, and others
are invited to
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explanation. Atmospheric phenomena are especially
Please send any photos to email@example.com.
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