Driving home last night, it looked like Calgary was sending messages to space, with lights shining straight up into the dark all across the city. Our neighbor Rob explained the phenomenon is known as “light pillars,” caused by “diamond dust.”

A light pillar is an atmospheric optical phenomenon in the form of a vertical band of light that appears to shoot above or below a light source. The effect is created by the reflection of light from numerous tiny ice crystals suspended in the atmosphere or clouds. A light pillar is just an optical illusion, and is not physically located above or below the light source.

Light pillars are caused by the interaction of light with ice crystals, and so belong to the family of halos. The crystals responsible for light pillars usually consist of flat, hexagonal plates, which tend to orient themselves more or less horizontally as they fall through the air. Their collective surfaces act as a giant mirror, which reflects the light source upward and/or downward into a virtual image. As the crystals are disturbed by turbulence, the angle of their surfaces deviates some degrees from the horizontal orientation, causing the reflection (i.e., the light pillar) to become elongated into a column. The larger the crystals, the more pronounced this effect becomes. In very cold weather, the ice crystals can be suspended near the ground, in which case they are referred to as diamond dust.

Diamond dust is a ground-level cloud composed of tiny ice crystals. Diamond dust generally forms under otherwise clear or nearly clear skies, so it is sometimes referred to as clear-sky precipitation. It is most commonly observed in Antarctica and the Arctic, but it can occur anywhere with a temperature well below freezing. In polar regions diamond dust may continue for several days without interruption.

When the pillars occurred here last night, it was 28 degrees Fahrenheit, -2 degrees Celcius. Cold enough to snow, but there wasn’t enough moisture for snow, just for ice crystals.

These ice crystals usually form when a temperature inversion is present at the surface and the warmer air above the ground mixes with the colder air near the surface. Since warmer air frequently contains more water vapor than colder air, this mixing will usually also transport water vapor into the air near the surface, causing the relative humidity of the near-surface air to increase. If the relative humidity increase near the surface is large enough then ice crystals may form.

That’s the science behind the phenomenon. But I prefer to think that Calgarians were chatting in space!

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