Steve's Atmospheric Phenomena
Sundogs and Ice Crystal Effects
Hexagonal ice crystals produce a wide variety of optical effects through reflection and refraction. Antarctica seems to
be the premier place to observe these phenomena, but sometimes even here in southern Arizona, we are treated to a display. When thin
sections of hexagonal crystals fall to earth, they orient themselves like leaves falling from a tree. Sunlight refracting
through these crystals produces strong light spots 22-degrees on either side of the sun (called Sundogs or Parhelia). Randomly
oriented long pencil-shaped hexagonal crystals produce a beautiful 22-degree halo around the sun containing the sundogs. If you are
even luckier, there will be flat hexagonal crystals distributed uniformly throughout the entire sky. They act as little mirrors
that reflect sunlight and produce an arc that is parallel to the horizon intersecting the sundogs and 22-deg halo (called the parhelic
circle). Add any number of long and short hexagonal crystals spinning and oriented in preferred directions and a large variety
of ice crystal phenomena can be observed.
One sundog amid a high cloud layer. . This is figure 5.31 in Ackerman 2003.
The color in the sundog is caused by the wavelength dispersion in the refraction process.
Here are a couple of particularly bright sundogs illustrating that the effect can be very efficient.
A 22-deg solar halo and 2 weak sundogs. At the top of the halo is the upper tangent arc formed by pencil shaped ice crystals
rotating and oriented like helicopter blades.
This is about as good as it gets. The 22-deg halo, both sundogs, the parhelic arc, and the upper tangent arc are clearly shown.
Notice the difference that elevation makes. The image here shows a concave-down upper tangent arc and sundogs displaced from
the 22-deg halo (contrast this with the image directly above where the sun is almost on the horizon). The line intersecting the sun,
halo, and sundogs is the parhelic circle. Despite the perspective of this image, the parhelic arc travels parallel to the horizon.
Even though I didn't get the perspective quite right in this multiple-image stitch, it clearly shows that the parhelic arc ran
continuously above the entire horizon. Too bad I didn't yet have the fisheye converter... The images were taken completely around
the horizon, so the center of the black area is the zenith.
Two internal reflections in a thin hexagonal crystal produce 120-deg parhelia (also called paranthelia). The blue arrows point to
the faint sundogs. Follow the parhelic circle out to the yellow arrows which seem to be the paranthelia. The split in
the stitched images is part of a building I used to shield the sun.
Random long hex crystals crystals can form moon halos too. I have never seen a moondog though!
Scanned from a very old 35mm slide, here's a large sundog with a complicated structure. Taken from downtown Tucson.
Here's a nice upper tangent arc with the sun near setting on 10-31-07. The left sundog is made from crystals falling out of a cloud
and then getting sheared by the wind (below).
Earlier, I saw my first Parry arc. This one was suncave over the sunvex upper tangent arc (UTA). Like the UTA, the Parry arc is created
by long hex crystals spinning like helicopter blades, but the Parry crystals are constrained to have one of the hex sides oriented
down. This added constraint means that Parry Arcs are only observed 1% of the time a 22-deg halo is present while the UTA is
observed about 30% of the time.