Sunday, July 12, 2009

The Grass Rainbow

It's Sunday and since I don't have to go to work, the dog gets an extra-long walk. A few months back, I realized I was being followed on those early-morning walks... by a rainbow on the grass which matched my pace the way the moon sometimes seems to follow you at night.

Investigating, I discovered a few surprising things:Justify Full
  1. The Sun has to be relatively low in the sky -- it has to be early in the day.

  2. The grass has to be wet with dew, and the wetter the better, although there must be such a thing as ' too wet'. This also requires 'early' because after the Sun has been up for a while, the dew will evaporate. The droplet size also appears to affect the visibility -- the smaller, the better. This seems a reasonable assumption: the best rainbows are those seen at a distance when apparent droplet size is very small.

  3. The viewer has to be walking West, that is: away from the Sun, with the Sun at the viewer's back.

  4. The viewer has to be looking through polarized lenses. I have polaroid clip-on sunglasses, but probably any polarization will do the trick. I know this is true because when I flip up the lenses to get a better look at the colors, they vanish.
When all these things are true, you may find yourself seeing two rainbows, one on each side, emanating from about where your feet are, and arcing away at (Wikipedia says) 42 degrees, although most times I can see it only on one side, if at all. The shape of the grass rainbow is not circular as we're used to seeing with rainbows in the sky. Instead, it's parabolic or possibly hyperbolic.

They're very hard to see, even under the best conditions. I find it's easiest if I'm moving because then the rainbow itself moves against the grass and it's easier to pick out. If you go looking for a grass rainbow, I hope you find a good one.

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