Physics Photo of the Week
March
13, 2009
Sun Pillar
Sometimes
before dawn or after sunset, the reflection from special clouds forms a
pillar of light called a Sun Pillar. This was photographed on
Wednesday, March 4, 2009 before sunrise.
Besides the pillar, notice the gold touch on the base of many of the
cloud strata. the Sun is below the horizon, but at the elevation
of the cloud layers, the Sun is above the horizon. Because the
sunlight has traveled through a much longer path through the Earth's
atmosphere, the short wavelengths have been scattered out. Only
the long red wavelenghts remain, so the base of the clouds, as well as
the sun pillar are magnificently reddened.
The sun pillar itself is caused by ice crystals in the clouds.
Atmospheric ice crystals can take many forms, but the forms responsible
for ice pillars are thin hexagonal plates. The plates fall
through the atmosphere from where they form in the cloud. When
falling, they tend to lie horizontal and thus act as tiny mirrors to
the sunlight that is reaching them. Millions of tiny horizontal
mirrors
(hexagonal ice crystals) reflect the light toward the observer -
similar to the elongation of the Moon's reflection from ripples on a
lake.
A sun pillar was featured on Physics Photo of the Week on September
16, 2005. The 2005 sun pillar was the most recent sun pillar
before this week's. This indicates that sun pillars are a
relatively rare occurance.
See PPOW for January 26, 2007 for close-up photos
of ice crystals showing hexagonal structures.
Due to Spring Break at Warren Wilson College, there will be no Physics
Photo of the Week next week. The next Physics Photo of the Week
will be published on March 27, 2009.
Physics
Photo of the
Week is
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 dcollins@warren-wilson.edu.
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
individual use or educational use. Any commercial use without
written permission of the photoprovider is forbidden.
Click
here to see the Physics Photo
of
the Week Archive.
Observers are invited to submit
digital photos to:
