What is a sundog? Are they just another rainbow?

Sundogs are bright spots of light seen on both sides of the Sun. They are also called halos or parhelia. The best time to see sundogs is in the late afternoon, when the Sun is close to the horizon and can be blocked by trees or buildings. Winter seems to be the best season, because the atmosphere tends to be dry and cold, but sundogs are visible throughout the year and around the world when conditions are right. The two bright spots are 22-23 degrees from the Sun, about the distance covered by your outstretched hand held at arms' length. A line though the sundogs and Sun is parallel to the horizon.

Sundogs are a type of ice crystal halo. Scientists had to understand the size, shape, and motion of ice crystals in the Earth's atmosphere to understand how halos are produced.

Sundogs are caused by sunlight passing through flat, 6-sided ice crystals high in the troposphere. The crystals fall through the atmosphere like leaves, flat side more or less down and slowly rocking back and forth. Because all of the crystals face down, light from the Sun is refracted into definite patterns. If the crystals tumbled as they fell then sundogs would not be produced.

Other halos are possible. Each has a name that describes its location in the display around the Sun and possibly the name of the person who first recorded that halo. A circle of light 22 degrees from the Sun (passing through the sundogs) is fairly common. It is caused by column crystals. They are 6-sided, but have grown long enough to fall in a random pattern. This 22-degree halo can also be seen around a bright moon and may be an indicator of possible snow.

The crystals must be small (between 0.05 and 1 mm) in size to produce halos. They are found at many altitudes but have to be in clouds 5-10 km above the ground to form halos. If the crystals grow in cold, dry air, they grow slowly and give the best displays. Spectacular pictures from the South Pole (shown in the book by Tape) are a result of the slow growth of crystals in the polar atmosphere.

Sundogs are not a type of rainbow. Sundogs and halos are seen when looking toward the Sun. Rainbows are caused by sunlight reflecting from raindrops. To see a rainbow you must stand with the Sun behind you. Both sundogs and rainbows use the Sun to study the water cycle in our atmosphere.

When looking for halos please remember: Never look directly at the Sun without adequate protection for your eyes!

For more information on sundogs see:

http://www.sundog.clara.co.uk/atoptics/phenom.htm (look under Ice Halos)
W. Tape, Atmospheric Halos, Antarctic Research Series, Vol. 64 (American Geophysical Union), 1994.

This week's question comes from Dr. W. Dean Pesnell. Dr. Pesnell is an Astrophysicist at Goddard in the Laboratory for Solar and Space Physics (LSSP).