If Pigs Could Fly – A Quick Guide to Solar Halos and Other Curiosities

by Bob King on May 18, 2014

A circumscribed halo encloses the more common 22-degree halo around the sun Saturday morning (May 17. Credit: Bob King

A circumscribed halo encloses the more common 22-degree halo around the sun Saturday morning (May 17). Six-sided pencil-shaped ice crystals in cirrostratus clouds created the fine display. Credit: Bob King

Call it a porcine occultation. It took nearly a year but I finally got help from the ornamental pig in my wife’s flower garden. This weekend it became the preferred method for blocking the sun to better see and photograph a beautiful pair of solar halos. We often associate solar and lunar halos with winter because they require ice crystals for their formation, but they happen during all seasons. 

Nature keeps it simple. Light refracting through or reflecting from six-sided plate and column (pencil-shaped) ice crystals in high clouds is responsible for almost all halos and their variations.

Nature keeps it simple. Light refracting through or reflecting from six-sided plate and column (pencil-shaped) ice crystals in high clouds is responsible for almost all halos and their variations.

Lower clouds, like the puffy cumulus dotting the sky on a summer day, are composed of water droplets. A typical cumulus spans about a kilometer and contains 1.1 million pounds of water. Cirrostratus clouds are much higher (18,000 feet and up) and colder and formed instead of ice crystals. They’re often the first clouds to betray an incoming frontal system.

Cirrostratus are thin and fibrous and give the blue sky a milky look.  Most halos and related phenomena originate in countless millions of hexagonal plate and pencil-shaped ice crystals wafting about like diamond dust in these often featureless clouds.

This is the top end of a hexagonal column-shaped ice crystal. Light refracting (bending) through billions of these crystals spreads out to form a typical solar halo. Credit: Donalbein

This is the top end of a hexagonal column-shaped ice crystal. Light refracting (bending) through the 60-degree angled faces of millions of these crystals is concentrated into a ring of light 22 degrees from the sun. As light leaves the crystal, the shorter blue and purple wavelengths are refracted slightly more than red, tinting the outer edge of the halo blue and inner edge red. Credit: Donalbein with additions by the author

In winter, the sun is generally low in the sky, making it hard to miss a halo. Come summer, when the sun is much higher up, halo spotters have to be more deliberate and make a point to look up more often. The 22-degree halo is the most common; it’s the inner of the two halos in the photo above. With a radius of 22 degrees, an outstretched hand at arm’s length will comfortably fit between sun and circle.

Light refracted or bent through millions of randomly oriented pencil-shaped crystals exits at angles from 22 degrees up to 50 degrees, however most of the light is concentrated around 22 degrees, resulting in the familiar 22-degree radius halo. No light gets bent and concentrated at angles fewer than 22 degrees, which is why the sky looks darker inside the halo than outside. Finally, a small fraction of the light exits the crystals between 22 and 50 degrees creating a soft outer edge to the circle as well as a large, more diffuse disk of light as far as 50 degrees from the sun.

The sun on Dec. 6, 2013 with a 22-degree halo and two luminous canine companions or sundogs. Credit: Bob King

The sun on Dec. 6, 2013 with a 22-degree halo and two luminous canine companions or sundogs. Similar halos and ‘moondogs’ can be seen around a bright moon. Credit: Bob King

Sundogs, also called mock suns or parhelia, are brilliant and often colorful patches of light that accompany the sun on either side of a halo. Not as frequent as halos, they’re still common enough to spot half a dozen times or more a year. Depending on how extensive the cloud cover is, you might see only one sundog instead of the more typical pair. Sundogs form when light refracts through hexagonal plate-shaped ice crystals with their flat sides parallel to the ground. They appear when the sun is near the horizon and on the same horizontal plane as the ice crystals. As in halos, red light is refracted less than blue, coloring the dog’s ‘head’ red and its hind quarters blue. Mock sun is an apt term as occasionally a sundog will shine with the intensity of a second sun. They’re responsible for some of the daytime ‘UFO’ sightings. Check this one one out on YouTube.

An especially colorful sundog with a 'tail' from 2008. Credit: Bob King

An especially colorful sundog with a ‘tail’. Red light is bent less than blue as it emerges from the ice crystal, tinting the sundog’s inner edge. Blue is bent more and colors the outer half. If you look closely, all colors of the rainbow are seen. Credit: Bob King

Wobbly crystals make for taller sundogs. Like real dogs, ice crystal sundogs can grow tails. These are part of the much larger parhelic circle, a rarely-seen narrow band of light encircling the entire sky at the sun’s altitude formed when millions of both plate and column crystals reflect light from their vertical faces. Short tails extend from each mock sun in the photo above.

A couple hours after the flying pig image, the sun was beyond 50 degrees altitude. The circumscribed halo had vanished! Credit: Bob King

About 2 hours after the flying pig image, the sun climbed beyond 50 degrees altitude. The circumscribed halo vanished! Credit: Bob King

There’s almost no end to atmospheric ice antics. Many are rare like the giant 46-degree halo or the 9 and 18-degree halos formed from pyramidal ice crystals. Oftentimes halos are accompanied by arcs or modified arcs as in the flying pig image.  When the sun is low, you’ll occasionally see an arc shaped like a bird in flight tangent to the top of the halo and rarely, to its bottom. When the sun reaches an altitude of 29 degrees, these tangent arcs – both upper and lower – change shape and merge into a circumscribed halo wrapped around and overlapping the top and bottom of the main halo. At 50 degrees altitude and beyond, the circumscribed halo disappears … for a time. If the clouds persist, you can watch it return when the sun dips below 29 degrees and the two arcs separate again.

Maybe you’re not a halo watcher, but anyone who keeps an eye on the weather and studies the daytime sky in preparation for a night of skywatching can enjoy these icy appetizers.

About 

I'm a long-time amateur astronomer and member of the American Association of Variable Star Observers (AAVSO). My observing passions include everything from auroras to Z Cam stars. Every day the universe offers up something both beautiful and thought-provoking. I also write a daily astronomy blog called Astro Bob.

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