"Rainbows" from the plane

In my many endeavours to get good photographs from the plane, I once noticed some faint rainbow like colours in the sky below. I wasn't imagining it. On careful observation, I noticed it was more pronounced with clouds, with the overall mist providing a background. I took some photos to later analyze on my computer. Here is what I got. The phenomenon was real indeed. The left are the unprocessed images. The right are the same images with colour greatly enhanced (using Irfanview). Your eyes automatically do these things and can spot the slightest of colour.



While it puzzled me, I left it in the background to ask a physicist friend sometime. It was such a small effect that I would need to show the photo to ask the question. I forgot about it until I undertook a flight a year later. This time, the effect was very clear. With the sun behind me, I could see clear colour bands on clouds, near the shadow of the plane (Open in new tab to see better).



It made sense. I was seeing a rainbow. A rainbow is formed when light undergoes total internal reflection off a water droplet. Clouds also have water droplets. Sunlight reflected off water droplets made the rainbow. Here are some explanatory images from wikipedia .



Simple, isn't it? Well, not so. A rainbow is formed at 42 degrees. So, if you drew a line from your location (your eyes) to your shadow (with conventional rainbows, if you don't see the sun, you might have to imagine where your shadow will fall), and another one from your location to the rainbow (anywhere on it), it will make an angle of 40-42 degrees. A second rainbow (if present) forms at an angle of 50-52 degrees. However, the "rainbows" I was seeing formed at an angle 5-10 degrees and they had multiple ones close by.

With wikipedia, I found I was seeing a phenomenon called "optical glory". The name derives from the Brocken, the tallest peak of the Harz mountain range in Germany. Because the peak is above the cloud level, and the area is frequently misty, the condition of a shadow cast onto a cloud layer is relatively favored and often seen with the coloured halo around the observer's head.

Glory cannot be explained by simple ray diagrams. It is said to be due to some form of interference and back-scattering. Mie scattering, the theory which includes all kinds of scattering off particles of a certain size, predicts that 10-20 micron sized droplets (note - rain drops are 100 times larger) would produce such coloured rings. The angle of the glory tells about the size of the droplet. There are also a host of similar optical phenomenon which produce colour in the sky, depending on the kind of cloud and the position of the sun (Optical Phenomenon).

Everything explained. We're good, right? Well, not quite. Having studied optics, I would have liked an explanation which gave me a reasonable picture in mind (like the rainbow ray diagram) as opposed to "crunch the numbers and you'll get it". Mie scattering theory is one of the messiest theories, which offers very little easy insight. So, I will have to give up my privileged insider status, remain a layman and just repeat what I'm told, just as I do for questions on cosmology and String Theory. I hope the theory of optical glory does not change very soon.

Meanwhile, whenever you get a chance, take a look at the sky. And choose a window seat when flying. You never know what you'll see!