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Sun Pillars
Wed Feb 19, 2003
Listen in RealAudio 
Hi, I'm Bryan Yeaton for The Weather Notebook. When looking toward the setting sun, not
directly, of course, we sometimes notice a pillar of light appear as a bright feather
extending vertically above it. This is a sun pillar, formed by sunlight reflecting off
airborne ice crystals.
Most sun pillars are seen when the sun is low, near the horizon. They usually extend 5 to 10
degrees above the solar disk but can appear below it as well. Because the solar rays are
reflected, they show the colour of the incident light. When the sun is higher in the sky,
pillars are white. But when the sunlight is predominantly yellow, orange, or red, so too is
the resulting pillar.
Ice crystals shaped like plates or columns provide excellent reflecting surfaces. Both can be
found in ice clouds, ice fogs, plus falling and blowing snow.
When ice plates and columns assemble in a stable "flying formation," they can act like small
mirrors to light passing among them. Pillars appear when the light rays reflect off the
crystal's surfaces toward the viewer.
The pillar's shape depends upon the type and orientation of the crystals, their altitude and
distance from the observer. When crystals float near the surface, reflections may be seen from
the upper surface of some crystals and the lower surface of others, producing pillars which
extend both above and below the sun.
Other light pillars may appear from other strong sources of light such as the moon or even
street lights. They can occur in all seasons but are most frequent during winter.
Thanks to our contributing writer, meteorologist Keith Heidorn. The Weather Notebook is a
production of the Mount Washington Observatory. Support is provided by Subaru of America, and
the National Science Foundation. Special funding comes from Davis Instruments of Hayward, CA.
On the Web at www.davisnet.com.
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