On many flights above a solid deck of clouds, a small set of colored rings appears around the airplane's shadow, crowning it like a target with your transport at the bull's-eye. It is not a rainbow, not a halo, and not anything painted on the cloud. It is called a glory, and unlike nearly every other optical display in the sky, it is anchored to you: it forms around the point exactly opposite the sun, which is precisely where your shadow — or your whole airplane's — happens to fall.
That geometry is the first clue to what you are seeing. Rainbows are centered on the same anti-sun point, which is why you can never drive to the end of a rainbow — it moves as you do, because it belongs to the line joining the sun, you, and the droplets. A glory obeys the same rule in miniature, compressed from a rainbow's wide arc to a tight ring of a few degrees.
Find the point opposite the sun
Picture the sun behind you and imagine a straight line running from the sun through your head and onward to the clouds below. Where that line lands, your shadow sits, and around that shadow the glory rings arrange themselves. Everyone on the plane sees essentially the same glory, because the plane's shadow marks the shared anti-sun point and the angular size of the rings is tiny compared with the distance to the cloud. Stand on a misty ridge with the sun at your back and the same rings can gather around the shadow of your own head, though from an airplane window the silhouette of the aircraft makes the geometry unusually easy to read.
Seeing one is mostly a matter of getting the geometry right. You need the sun behind you rather than across the aisle, a reasonably uniform deck of cloud or mist below, and a view of where your transport's shadow falls. Glories stay close to the shadow and refuse to wander: unlike the halo family that rings the sun itself through ice crystals — sun dogs and their twenty-two-degree kin — a glory appears nowhere except around the anti-sun point, so the airplane's shadow is effectively the address where the light is delivered. If the aircraft banks, the whole display slides across the cloud deck with it, which is the surest proof that you are looking at shadow-pointed geometry rather than a feature of the cloud.
Tiny droplets, backward light
The machinery inside the rings is stranger than the one inside a rainbow. A rainbow's light passes through raindrops, bending as it enters, reflecting once off the back, and leaving bent again at a characteristic angle near 42 degrees. A glory works with light that goes back where it came from: sunlight strikes the cloud's tiny droplets and a portion is returned nearly toward the sun, toward you. Something so small — a droplet a few thousandths of a millimeter across — should not be able to reverse light so neatly, and the full explanation took physicists decades to agree on, settling on wave optics: the light waves bend around the droplets' surfaces, interfere with one another, and the surviving wavelengths fan out in rings.
Droplet size runs the show, as the atmospheric-optics reference literature emphasizes: smaller droplets spread the rings wider, and a uniform cloud makes them crisp. The same sensitivity explains the glory's pale cousin. When the droplets are extremely small, the colors blur into a ghostly white arc — a fogbow — and a fogbow is a rainbow that lost its colors for exactly this reason. Raindrops, cloud droplets, and fog droplets are all water; only their size, and what that size does to wave optics, separates the three displays.
The rings themselves are the fingerprint of interference. Light waves returning from droplets of very similar size stay in step only at certain angles, and the favored angles differ by wavelength, so instead of a smooth glow you get nested colored crowns that sharpen when the droplets are uniform and smear when they are not. This is also why the display prefers clouds to rain: a raindrop is enormous compared with the wavelength of light and sends its brightness off in the refracted directions of a rainbow, while a cloud's tiny, evenly matched droplets keep the backscattered rings tight enough to survive the trip to your window.
One glory per observer
Because the rings belong to the anti-sun point, the display is fundamentally one viewer at a time — the shadow is shared, but each person's eye sits on its own line to the sun, a few inches or feet apart. In practice, from a window seat, you are close enough to the geometry that the plane's shadow serves as everyone's marker. The next time the seatbelt sign dims and the cloud deck glides past, look for the shadow first; the colored rings will find it on their own.
You can practice the color-reading part anywhere. The rings present spectral hues in a compressed, reversed order compared with a rainbow's familiar band, and sorting neighboring hues by eye — deciding what grades into what — is exactly the skill exercised by Hue Sorting. It is a modest skill until the sky hands you a glory, and suddenly the difference between one ring and the next is the difference between understanding a pretty halo and reading the geometry of light.