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Feature

Sundogs in Utah

A rising sun and two sundogs were caught on February 7, 2003, near Park City, Utah. The sundogs appear in front of the scenery because they are generated by tiny "diamond dust" ice crystals in the nearby air.

Tiny crystals, huge delights

Ice crystals high in the sky or down to earth give rise to gorgeous patterns of light

By Deane Morrison

December 28, 2005

It was as if a forest of light had sprung up, its innumerable thin shoots of radiance reaching up into the dark sky. Such was the situation in many parts of Minnesota on the evening of December 3, when eerily beautiful pillars of light stood everywhere, like vertical streams of aurora borealis frozen in place. But these columns of light were no aurora borealis; rather, they were a rare atmospheric phenomenon called ice pillars, the result of reflections off ice crystals near the ground. They are cousins to such striking displays as sun pillars, sundogs, and halos, all caused by the interactions of light and microscopic ice crystals in the air--with an occasional assist from that master generator of illusions, the human brain. Ice crystals in the atmosphere cause these various displays by either reflecting or refracting light from the sun, moon, or artificial sources on the ground. Let's look first at sun pillars and ice pillars, which result from reflection. Ice crystals are essentially six-sided plates that act just like mirrors. Suppose you're looking into a big flat wall mirror at the image of a light bulb off to your right. If you could see only the bulb's reflection--not the room around you or the edges of the mirror--you would think the bulb was to your right but in front of you.

Perhaps the most spectacular manifestation of refraction is sundogs, or parhelia, which are bright spots on either side of the sun.

What happens is that light from the bulb hits the mirror at a certain angle and bounces off at the same angle, hitting your eyes. You see the image coming to you in a straight line from the mirror. Your brain extends that line and "tells" you that the bulb is behind the mirror, as far from its surface as the real bulb actually is. If you rotated the room so that the mirror was on the ceiling, the bulb would appear to be the same distance above the mirror surface as it really is below it. That's what happens when the air is full of tiny ice crystals oriented with their flat surfaces horizontal. Like so many mirrors, the crystals reflect light to your eyes and cause you to project an image above the source of the light. In the case of the ice pillars on December 3, the light sources were streetlights, store lights, and so on. The mirrors were ice crystals in a column of air midway between each light source and each observer. Image(55804)

Ice crystals midway between a person and a streetlamp reflect light into the person's eyes (solid lines). The brain projects the light rays back along the same route to points above the lamp (dotted lines). The result is a shimmering image of a pillar of light. (Diagram by John Broadhurst) When flat ice crystals are in very high, light clouds such as cirrostratus, they can reflect light from the sun. If the sun is near the horizon, sunlight reflecting off these crystals can form a vertical pillar of reddish light above the sun, much like ice pillars. Except for one thing: The ice crystals cannot be anywhere near midway between the sun and observers. In order to reflect the sun's rays into our eyes, the crystals must be tilted from horizontal. Ice crystals that are dropping through the air, rocking back and forth like falling leaves, are therefore the source of sun pillars. Sometimes, ice crystals refract light. In refraction, light rays are bent by passing through boundaries between different media, such as air and water. That's why underwater objects, seen from above the surface, are not where they appear to be. "It's why a stick 'bends' when you put it in water," says University of Minnesota physics professor John Broadhurst. Perhaps the most spectacular manifestation of refraction is sundogs, or parhelia, which are bright spots on either side of the sun. These are caused by relatively large (but still microscopic) ice crystals that are oriented horizontally with respect to the sun and the observer. They bend light coming from the sun and refract it, typically changing its angle by 22 degrees. When the light rays reach our eyes, we project them back in a straight line and see an image of the sun 22 degrees to the right or left (or both) of it. Image(55805)

Ice crystals in the upper atmosphere bend the sun's rays before they enter our eyes (solid lines). We "place" images from the light in line with the incoming rays -- that is, to either side of the sun. The result is two sundogs bracketing the sun. (Diagram by John Broadhurst) When ice crystals are smaller and randomly oriented, they refract light in every direction, and we see a halo around the sun or the moon. Halos often occur along with sundogs because the atmosphere contains a mix of crystal sizes and orientations. The above patterns are, of course, not the only ones that form. But seeing any one of them can only help make winter at this latitude a little more bearable.