Physics Photo of the Week
on January 23, 2011 was preceded by this majestic pillar - the vertical
column of scattered sunlight that is in-line with the Sun. The
Sun lies below the horizon.
Sun pillars are very simple to explain. They are formed by flat
plates of ice crystals that form the thin cirrus clouds. When the
flat plates fall in the air, they fall to maximize the air resistance,
which is horizontal. The millions of tiny flat-plate crystals
reflects light from the Sun. Each tiny crystal could be thought
of as a tiny mirror that reflects an image of the Sun, but the plates
too small and so numerous that the images blend into a smear.
The small hexagonal plates are also responsible for parhelia - sun dogs
- seen at about 22 degrees on either side of the Sun. Later,
after the Sun had risen, a parahelion is visible (future PPOW).
Cirrus clouds almost always consist of ice crystals. Whether the
crystals form needles (PPOW for January 28, 2011), hexagonal plates, or
snow stars depends on the temperature and humidity. Often cirrus
contain both plates and needles as evidenced by the presence of both
parhelia and halo (PPOW
3, 2010). Usually cirrus clouds show neither a
halo nor parhelia. Such cirrus clouds must mostly consist of snow
Photo of the
published weekly during the academic year on Fridays by the Warren
Wilson College Physics
Department. These photos feature interesting phenomena in
the world around us. Students, faculty, and others are invited to
submit digital (or film) photographs for publication and
explanation. Atmospheric phenomena are especially welcome.
Please send any photos to firstname.lastname@example.org.
All photos and discussions are copyright by Donald
Collins or by the person credited for the photo and/or
discussion. These photos and discussions may be used for private
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written permission of the photoprovider is forbidden.
the Physics Photo
the Week Archive.
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