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Geo-Joint: Moonbows

Posted on September 24 2015


Rainbows are one of nature’s little gifts. We used to have them here in California before it stopped raining. Sigh. Anyhow, if it’s raining and the sun comes out, there’s probably a rainbow out there somewhere, if the timing’s right. You don’t always see one because the sun has to be no more than 42 degrees above the horizon in order for the light to hit the raindrops at the proper angle. Too high, and the rays exit the raindrops at an angle that projects the colors below the horizon, where you can’t see them. Near sunrise and near sunset are the best rainbow-hunting times. Raindrops, contrary to the way they’re often portrayed, are quite round. It takes strong winds to distort them even just a little, let alone make them the “classic” teardrop shape most of us envision them being. Sunlight reflects and refracts off of and within the round drops, breaking the rays into their constituent hues and forming a rainbow. Additionally, as the light gets re-refracted inside the raindrop, all the angles get shifted, which can produce a secondary, inside-out twin – a double rainbow!

There is more to this rainbow business than you ever imagined, and once you start digging down, it gets into physics and math and optics that are way beyond my ken, so today’s subject is limited to a couple of oddball queries. First, if we can have a rainbow, how about a snowbow? Well, in the sense of sunlight hitting a snowstorm as it falls to the surface, not really. The sunlight can definitely get broken up in the crystal structure of a snowflake as it would in a prism, but the orientation of the angles of all those crystals is not uniform, and many snowflakes are complex in their form. The upshot is that the light gets scattered willy-nilly and we end up seeing white light. Simple ice crystals high in the atmosphere can form blobs of rainbow color such as those seen in what is called a sundog or parhelion. Those don’t arc across the sky, though.

The other mystery to address is this: If the sun makes a rainbow, can the moon make a moonbow? Happily, the answer to this is Yes! It takes a special set of circumstances: rain, of course, with some clearing for the rays of a low-angled moon coming from a direction opposite the rain, a dark sky, plus the added requirement of a moon in full or nearly-full phase. If you can find all these conditions at once, you may see a moonbow that exhibits a splash of colors ranging from light to dark….gray. Sadly, the amount of light a full moon can produce is insufficient to fire the color receptors in our eyes. However, though the technological magic of a long photographic exposure, Roy G. Biv can make an appearance – at least on paper or your camera screen. Since the coincidence of all the required moonbow components is frustratingly hard to find, you can experience the next closest thing in a good burst of nighttime water vapor the likes of which are pretty reliable at the base of large waterfalls. Check one out on a full moon – I suggest Yosemite National Park where I once saw a moonbow at the base of Yosemite Falls.

Meanwhile, if you want to go absolutely nuts wandering down the rabbit hole of information about atmospheric light phenomena, check out this website, and have fun!


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