Rainbows

 

Question:
On a recent flight aboard a regional jet, I saw what appeared to be a rainbow that was following us.  It disappeared, but then another one appeared and this time it was a complete circle!  Can you tell me what created this perfect circle of a rainbow?  C. Stokes, Huntsville, Ala.

Answer:   Let’s talk first about the type of rainbow you might see when earthbound.  The sun must be shining behind you while in front of you must be water drops from rain or even a garden hose.  The water drops act as individual prisms that refract (bend) the light passing through them, creating—you guessed it—all the colors of the rainbow.

Since each drop reflects only one color, there have to be a lot of drops to create a rainbow.  The “bow” of the rainbow is formed because the light exits each drop at an angle of 40 to 42 degrees from its angle of entry.  The reason we don’t see a full circle is because the lower half is blocked by the earth.

However, standing up in a tower or observing a rainbow from an airplane, the lower half of the rainbow is not blocked and you can indeed see a full circle that appears to travel with you as long as you have the sun behind you and waters drops in the direction you’re looking.

It's quite possible what you saw was actually a "glory," which is not that unusual to spot from a plane. A glory is produced when light reflects off a cloud of water droplets of uniform size. If it was a glory, there was most likely a visible image of your airplane right in the center.  While I've witnessed some glories with centers so faint I would not have seen the airplane in the middle had I not known to look for it, sometimes the airplane image in the center is spectacularly sharp. You can see some interesting photos along with additional information about glories here:   
http://en.wikipedia.org/wiki/Glory_(rainbow)

You can create a full rainbow yourself with a garden hose if you spray a fine mist below eye level with the sun behind you.  You’ll lose just a bit of the full circle with your own shadow.

By the way, double rainbows are formed when there is a secondary reflection from the raindrops with a wider radius.  The colors are reversed.  You can see part of both halves of a double rainbow taken near my home in rural Colorado here in the “At Home” album of my Photo Gallery:
http://www.fromthecockpit.com/Gallery/thumbnails.php?album=8

A few decades ago, when I had just earned my Instrument Rating (which allows flight referring to cockpit instruments as opposed to outside visual reference), I was flying a Cessna 172 solo from San Diego to Long Beach.  There were towering cumulus and other clouds all along my route of flight, which took place just before sunset. 

Once, I broke out between clouds and happened to look to my right.  For a very few seconds, I saw what I have referred to ever since as an incredible “valley of rainbows.”  Dozens of rainbows, some partial, some “normal” arcs and some full circles, appeared before me before I went into the next cloud.  For the rest of the flight I kept trying to see the phenomenon a second time but that was my one and only experience, in over thirty years of flying, with such a dazzling display of color.