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Crepuscular Rays and Wedge
Mon Jan 03, 2005
Listen in RealAudio 
Crepuscular Rays, and the Twilight Wedge, up next on The Weather Notebook.
Hi, I’m Bryan Yeaton, and this is The Weather Notebook.
Certainly, you have seen the glorious rays of the sun streaming through the clouds
early in the morning or as old Sol edges toward the evening horizon. You may have
even seen them beaming through the lofty, stained-glass windows of a cathedral. Well,
these magestic streaks have a solidly scientific name: crepuscular rays.
Crepuscular means "twilight," and these are sometimes called "twilight rays." Owls, for
example, are considered crepuscular creatures because they are usually spotted as
the sun is near the horizon (they actually work all night, but we usually don’t see them
then).
When light from the sun passes through a window, or a broken layer of cloud, some of
the light is scattered—that’s why these rays are visible only from the side. If the sun is
very low, you can even see these rays beaming above it. The scattering is often aided
by dust or smoke particles in the air. Crepuscular scatterings seem to fan out in wide
rays, but that’s only an illusion: light from the sun is coming in, pretty much, parallel
lines. The spreading effect is caused by the same phenomenon that makes railroad
tracks look smaller as they get farther from your sighting position.
The Twilight Wedge is actually the boundary line between light and shadow at sunset.
It is best seen in clear air, and is very sharp just after the sun has set, but becomes
more diffuse as the sun sinks farther below the horizon. Probably the best place to see
the wedge, however, is from an airplane. When you’re cruising along at 30,000 feet,
you may be right at the line of demarcation between night and day.
The Weather Notebook is a program of the Mount Washington Observatory, funded by
Subaru of America. Find us online at www.weathernotebook.org.
The Weather Notebook is generously supported by Subaru of America.
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