A ghostly ring around the moon on a cold winter's night, snow glittering on the ground below: Moon halos materialize when conditions are just right, as moonlight streams through icy high clouds.
But although moon rings depend on ice crystals in the atmosphere, halos aren't just a winter phenomenon. All year long, even in hottest July, the highest clouds are made of drifting bits of ice, rather than liquid droplets of water. When moonlight (or sunlight) passes at certain angles through the crystals, we may see a halo around the moon (or, in daytime, the sun).
Unlike the round droplets that form puffy cumulus clouds, higher cirrus-type clouds are made of six-sided ice crystals. The crystals are frozen into shapes ranging from flat plates to columns. Many resemble the six-sided glass prisms that hang from a chandelier.
Pale halos are created when moonlight enters and exits through the sides of hexagonal ice columns, at least 15,000 feet up in the air. As moonlight pierces one side of a tiny ice prism, it bends (refracts). Shooting through the center of the crystal and out the other side, the moonbeam bends again as it emerges into open air.
The two bends add up to a 22-degree angle off the moonlight's original path. If enough randomly oriented crystals are scattered across the sky, the angled moonlight streams from all directions, forming a cone of light. And we see a halo around the moon, with a 22-degree radius.
Rarely, the halo may extend further across the sky, and we may be lucky enough to see a 46-degree ring. These huge-but-faint halos occasionally appear when moonlight passes first through one side and then through the base of cirrus ice crystals, exiting at a larger angle.
The inside edge of a moon halo is sharpest, while the outer edge is indistinct fading into the sky. Since moonlight is weaker than direct sunlight, moon rings show less of the rainbow effect seen in much brighter sun halos. But look closely, and you may see a pale rainbow spectrum, extending out from the moon.
Just as with sunlight passing through raindrops, different wavelengths (colors) hidden in white light are bent more or less as they pass through ice crystals. So instead of traveling together, making white, they go their separate ways. Result: In a moon halo the innermost edge of the halo is red, while the outermost edge is violet.