Diamond Dust: Snow Without Clouds


They are long in comparison to their hexagonal cross-section. Larger column crystals fall with their long axis parallelling the ground, but at times, the falling columns may rotate like slow, miniature helicopter blades.

When the hexagonal plates and columns are less than 20 micrometres across -- so small that ten can fit, side-by-side, on the edge of a sheet of paper -- the plate crystals tumble randomly through the air as they fall, catching and reflecting the solar beam. But when the crystals are somewhat larger, they fall so that their long dimension parallels the ground, floating downward like descending flying saucers. Larger column crystals fall with their long axis parallelling the ground, but at times, the falling columns may rotate like slow, miniature helicopter blades.

As I continued walking homeward, the descending sun was only minutes away from setting. Looking westward, I saw a crimson pillar of light rising above the solar disk. This was a sun pillar, a feather of light that extends vertically above, and at times below, the sun. Sun pillars are the most common form of light pillar, though they can form in moonlight and strong artificial light as well. (For more on light pillars, see my Weather Doctor article by clicking here.) Most sun and moon pillars are seen when the source is low on the horizon (about 1 to 2 degrees above) and generally extend only 5 to 10 degrees directly above it.

Light pillars appear when the larger ice crystal plates and columns assemble in a stable "flying formation" and strong light passes through the formation from the source toward the observer. The ice crystals act like small mirrors, reflecting the light rays as they hit the crystal's upper or lower surfaces at a grazing angle.

Light pillars may form in situations when diamond dust is present in the air, but they more commonly appear when blowing snow raises ice crystals or when thin cirrus clouds containing such crystals are near the horizon between the observer and light source. (When pillars form from crystals in cirrus clouds, they may be seen in all seasons.)

Copyright 2004, Keith C. Heidorn, All Rights Reserved. (Illustrations courtesy of Spectrum Educational Enteprises, ©2004.)

The copyright of the article Diamond Dust: Snow Without Clouds in Meteorology is owned by Keith C. Heidorn. Permission to republish Diamond Dust: Snow Without Clouds in print or online must be granted by the author in writing.

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