Sundog mystery explained

A faint halo, sun pillar and an upper tangent arc (top) appeared for a brief time around 8 a.m. today around the sun. Photo: Bob King

Hexagons. Bees build them out of wax. A snowflake’s beauty is based on their floor plan. Every time you pick up a pencil you hold one. This morning, they were at it again, when hexagonal ice crystals created a halo and sun pillar display around the sun.

Three different phenomena were happening, but all were created by sunlight refracted through or reflected off of 6-sided, columnar ice crystals shaped like very tiny pencils. Halos are fairly common and occur when a light ray is bent or refracted as it passes through one side of a crystal and emerges at an angle out the other side. Millions of crystals all doing the same thing concentrate the sun’s light into a circle with a radius of 22 degrees (44 degree diameter).

Light reflecting from the undersides of hexagonal, plate-like crystals gives rise to sun pillars. Credit: Keith C. Heidorn

The same crystals form the upper tangent arc, but only the ones that are horizontal or parallel to the ground. Sunlight glinting off the undersides of hexagonal, plate-shaped crystals falling parallel to the ground create the vertical streak of light or sun pillar.

By the time I got back to the house from walking the dog, the clouds had changed and all of it went away. You never know when nature, always unmindful of human scheduling, will unveil something out of the ordinary.


Watch the sundog (at right) disappear in a flurry of shock waves as the rocket rises. At left, keep an eye for a streak of light to form and fade.

Halo phenomena and in particular sundogs were in the news again late this week when atmospheric optics expert Les Cowley cracked the mystery on why the rocket that launched the Solar Dynamics Observatory (SDO) “destroyed” a sundog on its way to orbit.

Each of these sundogs comes with its own tail. Photo: Bob King

Sundogs are brilliant, round patches of light on either side of a solar or lunar halo, and they form by light refracted by those plate-shaped hexagonal ice crystals. When the rocket was launched on February 11, 2010 and rose through a layer of cirrus clouds, shock waves disturbed the ice crystals causing a multi-colored sundog to completely disappear. At the same time, a streak of cloudy light appeared alongside the rocket.

Cowley and colleague Robert Greenler at first thought that randomly-distributed crystals might be responsible for the streak, but that didn’t jive with their computer models. Then they realized that the crystals were organized by the shock waves into a “dancing army of spinning tops” that in unison produced a small halo around the rocket itself.

A simulation of a hypothetical halo around the SDO rocket last February. Credit: Science@NASA

The streak you see in the video is only part of a hypothetical halo around the rocket. “This could be the start of a new research field—halo dynamics,” said Cowley. To learn more about this interesting new phenomenon, please check out this NASA release.

One last tidbit: Algol, the “Demon Star” in Perseus we visited in a blog earlier this week, will be up to its old tricks again this evening. It’s at minimum light at 6:07 p.m. Central time, which means if you go out when it first gets dark, it will shine at its faintest. Take a look again around 8 or 9 o’clock and the star will have returned to normal brightness. Very easy to see!

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About astrobob

My name is Bob King and I work at the Duluth News Tribune in Duluth, Minn. as a photographer and photo editor. I'm also an amateur astronomer and have been keen on the sky since age 11. My modest credentials include membership in the American Association of Variable Star Observers (AAVSO) where I'm a regular contributor, International Meteorite Collectors Assn. and Arrowhead Astronomical Society. I also teach community education astronomy classes at our local planetarium.

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