This is the first time I've seen this natural phenomenon, although it must be quite frequent: the cloudbow. That's a rainbow (a circular diffraction pattern caused by water droplets in the atmosphere) made by clouds, not by rain.

I walked out onto the lanai to enjoy the (extremely rare) evening sunshine. The sun had already set on Hau`Oli Mau, but it still shone on the ocean, unencumbered by Mauna Kea. There was a giant cloud right ahead of me, towards the wind. And from it rose a vertical rainbow.

The amazing thing about the cloudbow was that (as you can see in the larger image after the break) there was no rain. As a matter of fact, the cloudbow rose above the cloud, mostly.

From a physical perspective, it doesn't really matter how the droplets that cause a rainbow are formed. They just have to be the right size and the right density, and sunlight shone on them will reflect back to the viewer. The reason the bow is circular is that what matters is the angle between the light source (usually the sun), the droplet, and the viewer. The reason it's behind the light source is just the way the diffraction works: it's a mirroring in the back of the droplet.

Now, evidently, there was enough light and enough moisture in Ninole that evening to make the cloudbow appear. Its strength seemed to gain with time passing, which was probably mostly due to the dimming of the surrounding sky. Towards the end it was bright and clear, albeit fuzzier than a typical rainbow. I assume that's due to the much more irregular size of the water droplets.

Now, of course I wasn't thinking any of this physics stuff while I was watching this unfold. I was just surprised at the beauty of it all, feasting on the reflection of the pink clouds on the ocean, on the greens and blues around them, and the view of Mauna Kea in the background. What a fantastic way to say good-bye!