When it’s -25 and the wind’s blowing, we tend to keep our faces turned firmly to the ground, with occasional glances up to make sure we’re not about to walk into traffic. But if, during the recent cold spell, you were brave (or foolish) enough to raise your head, you may have been treated with the sight of two bright spots on either side of the sun: sundogs.
It doesn’t have to be cold for sundogs (also called “mock suns” or “parhelia”) to form, but it helps. That’s because they’re caused by the refraction of light through ice crystals in the atmosphere.
Light refracts, or bends, as it passes from one medium (for example, air) to another that’s more dense (for example, ice), because it moves more slowly in the denser medium. (This is what makes a pencil look bent if you stick it into a glass of water.) Refraction is one of four basic mechanisms that account for all of the various optical effects produced by the sky, from the mere fact of its being blue to the spectacular summer rainbow.
The others are reflection (the bouncing of light off of a surface), scattering, in which light is thrown off by particles in the atmosphere in all directions, and diffraction, in which light is bent as it passes around the edge of an object (for example, a cloud droplet) in the air.
Scattering makes the sky blue and the clouds white. Oxygen and nitrogen molecules in the air are more effective at scattering the short blue wavelengths of the light striking them than the longer reddish wavelengths, while the large cloud droplets scatter all wavelengths more or less equally. Diffraction accounts for the silver lining sometimes seen around the edge of clouds. Reflection gives us pillars, those beams of light that seem to shine straight up from every streetlight and pair of headlights on a cold night: you’re seeing the reflection of the light off the bottom of countless flat little ice crystals. And, as noted, refraction gives us sundogs.
Sundogs are formed by the refraction of light through hexagonal plate-like ice crystals whose flat faces are oriented horizontally and whose diameter is larger than 30 micrometers.
As sunlight passes through the crystals, it’s bent 22 degrees. As a result, it appears to us that light is originating from two spots on either side of the sun as well as from the sun itself.
A complete halo around the sun (or the moon) is caused by the same thing; the only difference is in the ice crystals. Halos usually form when the crystals, instead of taking the shape of little plates, form tiny columns, instead. If these columns are oriented horizontally a “tangent arc,” may appear, a particularly bright spot directly above the sun, similar to a sundog but in a different place.
On rarer occasions, a much larger halo may appear. This happens when most of the ice crystals are oriented with one end toward you. This means the light reaching your eyes is refracted twice, once at it enters the long, flat side of the column that’s facing up, and again as it exits through the end of the column. The total refraction is then 46 degrees instead of 22, and so the resulting halo is more than twice as big as usual.
Even if enough crystals are oriented the right way to produce a 46-degree halo, there are usually plenty more that aren’t, so 46-degree halos are usually seen at the same time as a 22-degree halo, producing a double halo.
Most sundogs and halos are white, but occasionally, some will show some color. That appears because white light is actually made up of many different wavelengths, or colors, of light. Each wavelength is slowed, and thus bent, at a slightly different angle than the next, which has the effect of separating the colors and making them individually visible. That’s how rainbows are formed, but water droplets, which cause rainbows, are better refractors than ice crystals, and so the colors of a rainbow are more distinct than those of a sundog.
So there you have it. Next time it’s really cold and ice crystals fill the air, bravely look up from the frozen ground and into the frozen sky at the sundogs. And when your friends ask you how you froze your face, tell them you were admiring a fine pair of parhelia.
I’m sure they’ll be suitably impressed.
