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Waterfall Bows
Fri Jun 18, 2004
Listen in RealAudio 
You don’t need a rainstorm to make a rainbow. That may sound like an early Bob Dylan
song, but it’s actually today’s topic: waterfall rainbows. Hi, I’m Bryan Yeaton for The
Weather Notebook.
We only need two things to make a bow: water drops and strong light, and we must be
standing between the drops and the light.
Rainbows appear fullest with the sun low in the sky because the center of the bow
must lie on a line from the light through our eye position. If the sun is too high, droplets
needed for the bow are actually below the horizon. But at a waterfall, spray drops rather
than raindrops refract and reflect the light to create a bow. The larger or taller the
waterfall, the more spray it produces, and thus spectacular waterfalls can generate
spectacular non-rain bows. These bows can appear any time of day depending on our
viewing location relative to the spray and the light.
Waterfall bows may even form below us. The mighty Niagara Falls provides great
examples of low-elevation bows; indeed the American side is named Rainbow Falls.
When you stand atop the gorge with the sun beaming into it, bows can appear to be
rising from the gorge below. On a sunny day, it is possible to see Niagara bows from
the American side in the morning, either side around midday, and the Canadian side
in the afternoon.
The spectacle of waterfall bows is not just limited to daylight viewing. During near-full
phases of the moon, there is enough light to produce moonbows around waterfalls at
night. Accounts from Niagara Falls indicate moonbows were once common, but today
street and commercial lighting overwhelms the delicate bow light.
Thanks to our contributing writer, meteorologist Keith Heidorn. The Weather Notebook
is generously supported by Subaru of America and the National Science Foundation.
Today's Links
Rainbows add to the beauty of the Horseshoe Falls:
http://waves.apple.com:81/people/jav/Images/Postcards/NiagaraFalls/NF10.jpg
Gorge Rainbow:
http://www.planetware.com/photos/US/NYNF4.HTM
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