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Diamond Dust
01/08/2003
Listen in RealAudio 
Some folks believe it can be too cold to snow. But on many frigid winter days, light snow may
fall without a single cloud in the sky. Hi, I'm Bryan Yeaton for The Weather
Notebook.
Our Canadian correspondent, meteorologist Keith Heidorn, wrote to us about such a day.
It came during a February Arctic outbreak, with the temperature barely exceeding 0 degrees F
in southern Ontario. As he walked home in the late afternoon, the soon-to-be-setting sun
illuminated the horizon in brilliant light. Looking through his parka hood, he noticed that,
although the sky was clear above and the winds calm, a soft flutter of snow encircled him. The
snow, so fine that individual crystals were distinct on his dark sleeve, floated around gently
drifting slowly earthward. In the brilliant sunlight, the snow crystals glittered like
diamonds against the darkening eastern sky. The brilliance of the small, illuminated crystals
gives this snowfall its name: diamond dust.
At temperatures below 0 F, ice crystals may form as irregular hexagonal plates or unbranched
ice needles or columns directly from water vapour in the air, through a process called
deposition. When their size is less than 20 micrometers across, these crystals tumble randomly
through the air as they fall. But when larger, they fall so that their long dimension
parallels the ground, floating downward like descending flying saucers.
As the sun sank lower in the sky, only minutes away from setting, a crimson pillar of light
rose from the top of the solar disk. This was a sun pillar, a column of light that extended
vertically above the sun. These formed when the solar light passing through air filled with
diamond dust was reflected off the crystals.
Thanks to Keith Heidorn for sharing this experience. The Weather Notebook comes to you from
the Mount Washington Observatory, with funding by Subaru of America, and the National Science
Foundation.
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