Physics Photo of the Week
Smile in the clouds
bright rainbow "smile" appeared in my yard almost three years ago (Dec.
29, 2007). It is not a rainbow, but a "circumzenithal arc" and is
formed by ice crystals in the wispy cirrus clouds at high
altitudes. As this specimen shows, circumzenithal arcs are very
Like the more familiar circumscribed halo around the Sun or the Moon,
the circle around the zenith is formed by refraction of the millions of
tiny prisms of ice crystals. In this case the ice crystals are
thin hexagonal plates. The Sun was fairly low in the sky, perhaps
only about 22 degrees above the horizon. The colorful arc is
centered on the zenith; its angular radius from the zenith
is the same as the Sun's angle above the horizon.
Understanding this rare display is much simpler than the halo that
circumscribes the Sun or Moon. The diagram at right shows a
hexagonal ice crystal. The plate crystal is falling slowly in the
upper atmosphere. The friction of the air makes the plate fall in
a horizontal orientation. Any other orientation is not
stable. The light from the low Sun orientation enters the top
face, is refracted by the 90 degree prism formed by the top face and
one of the side faces. When the Sun is about 22 degrees above the
horizontal the incoming angle to the flat face of the prism and the
outgoing angle are the same. The spectrum of color is displayed
due to the prism effect of the hexagonal ice plate.
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
individual use or educational use. Any commercial use without
written permission of the photoprovider is forbidden.
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