For Kathy: A Rainbow of Explanations

Kathy said: Yesterday a friend and I were admiring some beautiful clouds on a perfectly sunny and lovely day. We noticed that one very large, oblong cloud was surrounded by not one, but two rainbows. It hadn't rained a drop all day. How is this phenomenon possible without rain?

Such a good question and so many cool things to say and show you. Most people know that rainbows are the refraction of light through water droplets (like rain) in the sky with the water droplet acting like a prism. The full spectrum being seen is a result of seeing light reflected in many water droplets as an individual watching could only see the light refracted at a particular wavelength through a particular droplet. Wikipedia (damn, I love that site) has an excellent write-up of the physics/optics of it as well as some slick pictures.

Here's a key factor: droplets don't have to be falling on you for them to be seen. In fact, if you're standing in the sun and looking on rain in the distance, you stand a much better chance of seeing one. Rainbows can be caused by seaspray (or other water spray), dew, and other mists, basically any water droplets in the atmosphere. Some other cool things about rainbows I gleaned looking into it was that the light below the primary rainbow is lighter than just above it. Look at the picture below and you'll see what I'm talking about.

By Eric Rolph taken at Wrangell-St. Elias National Park, Alaska

You can see the lovely arc of the primary and, above it, the dim vision of the secondary. Notice it's not raining. The Wikipedia link shows rainbows on seaspray and in the spray of a geyser

However, there are other cool types of rainbows even than these like moonbows, rainbows from the light of the moon. How cool is that. Here's a picture.

Photo of a Lunar Rainbow taken from the Zambia side of Victoria Falls. The constellation Orion is visible behind the top of the moonbow. Taken by Calvin Bradshaw.

And there are more cool phenomena, including apparent rainbows in the ice crystals of cirrus clouds like this little lovely. It's called a circumhorizontal arch and there are some cool pictures here, too. And a real stunner taken over Idaho here.

Circumhorizontal arc, photographed in Idaho, June 3, 2006, by Gavin Anderson

So, hope that helped. It's cool stuff.