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Mystical Road Rainbows

by Mike Weaver

December 4, 2012

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Donate No matter how mundane and explicable they are, rainbows spark a sense of magic and wonder in many of us. For others, they represent evidence of conspiracy or chemical contamination. A recent article about rainbows inspired a sense of awe and wonder in my jaded, cynical old heart. What was this wondrous event? Read on!

The article, published by The Atlantic in their Cities publication, was entitled "The Weird Weather Phenomena Created by Urban Bike Paths" by John Metcalfe. Unlike the videos I linked above, the reason this article inspired me was due to the quality of the reporting rather than the rainbows themselves.

Geoff Chester noted a halo of rainbow light surrounding him as he rode along an asphalt bike trail in Arlington, VA. This halo was odd in that it followed him, keeping him in the center, and that it only appeared on the bike path, not on the surrounding ground or grass.
""The conditions were dry, so it wasn't anything to do with dew," says Chester. "And it was only visible on the asphalt, not on the grass or a concrete wall when I was looking at my shadow there.""
Rather than being overcome with his mystical experience or fearing HAARP rays or chemtrails Mr. Chester took some photos and sent them to Tony Phillips, who happens to be the NASA scientist behind the Space Weather site.

One scientist lead to another, in this case Les Crowley, who studies the effects of dust, ice, water, etc on light. The scientists arrived at two potential hypotheses for the phenomenon: glass beads on the road surface or particles of some other highly reflective material had blown onto the path. Crowley offers this comment on a potential glass bead rainbow:
""Crews marking paint lines on roads often scatter small glass beads onto the paint. The glass beads retro-reflect light and this enhances the visibility of the markings at night. The glass beads — if sufficiently spherical — also produce rainbows. The difference is that the refractive index of glass is greater than that of water and the bow is only about 21 in radius compared to the rainbow's 42. The glow around the shadow of Geoff's head is an antisolar point phenomenon... produced by refraction through the glass spheres.""
Mr Metcalfe could have left his article there, but he went that extra mile (for which I applaud him) and gave us an update on December 3rd:
"Chester says "the mystery is definitively solved! As the first comment poster suggested, the culprits are indeed glass beads. They were spread quite liberally on the trail, filling the spaces between the aggregate pebbles in the asphalt. I collected some during my ride this past weekend... perfect little spheres of glass." So that's that, then."
It is refreshing to find good science in an article which is arguably a "fluff" piece. Mr Metcalfe and Mr Chester should both be commended for applying reason and science to a phenomenon that many would have either dismissed or attributed falsely to magic or conspiracy. It filled me with a sense of awe and wonder, you might say.

by Mike Weaver

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