On a cold and clear winter night, you might catch a glimpse of nature’s very own Christmas light show. Light pillars are optical illusions in which thin, vertical beams of light shoot into the sky above or below an artificial or natural light source. These beams are members of the “halo” family and are also called sun pillars, lunar pillars, moon pillars, and luminous pillars.
Light pillars in North Bay, Ontario. Photo: Timmy Joe Elzinga
Halos have been studied since the time of Aristotle. Many cultures have eyed these beautiful displays with suspicion and fear, believing they were signs of an approaching storm — which sometimes they are. Others used it in religious iconography as symbols of purity and sanctity. Rather underwhelmingly, they are simply a trick of the light.
Light pillars vs sundogs
Halos need two key ingredients to be formed: ice crystals and a light source. However, the way in which these two interact makes a big difference. Light pillars are often mistaken for auroras and confused with another optical phenomenon called a sundog (also called a parhelion or mock sun). Sundogs occur when light shines on ice crystals and refract light at exactly 22°, creating a ring around the sun. Light pillars form differently, by reflection rather than refraction.
Sundog on Ellesmere Island. Photo: Jerry Kobalenko
Light pillars occur mostly in northern regions but also in deserts at night when temperatures drop to near freezing. Tiny ice crystals in the air drift slowly downward until they’re below or right on the horizon. Each hexagonally shaped crystal becomes a mirror that reflects light from the sun, moon, or street lights just above. The reflection appears as an elongated pillar of light.
The bigger and thicker the ice crystals, the bigger and stronger the light pillar. The colors of the light pillars depend on the color temperature of the light source. The air must be calm because wind disrupts the image.
How a light pillar works. Photo: Ashley Brauweiler/Shutterstock
Meteorologists have linked light pillars and halos in general to the presence of cirrostratus and cirrus clouds. These are very thin, hazy-looking high-altitude clouds made entirely from ice crystals. Most halos associate with these clouds, but sometimes low or even ground-level clouds create the same conditions for pillars to form.
Diamond dust in a light pillar. Photo: w.aoki/Shutterstock
Diamond dust
Finally, diamond dust is a low-level mist that appears on frosty winter mornings. Diamond dust forms when temperatures are at least 20°C below freezing and a temperature inversion occurs. This is when colder air near the ground interacts with warmer air above it. The water vapor transforms into ice crystals. When the sun hits the crystals, it looks like the air is full of tiny sparkling diamonds.
