Every so often you glance up and notice a large, perfect ring of pale light encircling the sun — or, on a quiet night, the moon. It looks almost supernatural, and for most of human history people read it as an omen. There's an old weather proverb: "ring around the moon, rain by noon." The ring is real, the physics behind it is elegant, and the folk wisdom is right more often than you'd expect.
What you're seeing is a 22-degree halo, and it's made not by water droplets but by millions of tiny ice crystals floating far overhead.
A halo forms in cirrus and cirrostratus clouds — thin, wispy clouds five miles (8 km) or more up, where the air is cold enough that the cloud is made entirely of ice, not liquid water. Those ice crystals often take the shape of tiny six-sided (hexagonal) pencils and plates.
When sunlight or moonlight enters one side of a hexagonal crystal and exits another, it bends — refracts — just like light passing through a glass prism. The particular geometry of a six-sided crystal bends light by a minimum of about 22 degrees. Light is scattered in all directions, but it piles up most strongly at that 22-degree angle, so from your point of view a bright ring appears exactly that far out from the sun or moon. Anything closer than the ring looks noticeably darker, which makes the halo stand out even more.
Because the crystals bend different colors of light by slightly different amounts — the same reason a prism makes a rainbow — a bright halo often shows a faint reddish tint on the inside edge and a bluish tint on the outside. It's much subtler than a rainbow, though, because the colors overlap heavily. Most halos just look whitish. A halo and a rainbow are cousins in physics but different in almost every practical way, as the table shows.
| 22° Halo | Rainbow | |
|---|---|---|
| Made by | Ice crystals | Liquid raindrops |
| Position | Around the sun/moon | Opposite the sun |
| Shape | Full ring | Arc |
| Color | Faint, mostly white | Vivid, full spectrum |
| Best time | Any time, even at night | Sun low, rain opposite |
Surprisingly often, yes. High cirrus clouds are frequently the leading edge of an approaching warm front or storm system. As a low-pressure system moves toward you, its first sign is usually a veil of high, icy cloud spreading across the sky a day or two ahead of the actual rain. That's exactly the cloud that produces halos. So a halo doesn't cause rain, but it's a genuine early warning that the atmosphere is moistening aloft and a front may be on the way.
It's far from a guarantee — plenty of cirrus drifts by without any storm behind it. But as folk forecasting goes, "ring around the moon, rain soon" has real physics under it. If you see a halo, it's worth glancing at your hourly forecast to see whether a system is indeed approaching.
Once you're tuned in to halos, you'll start noticing their relatives. Sundogs — bright, sometimes rainbow-tinted spots sitting on the halo to the left and right of the sun — come from the same ice crystals when they're flat plates drifting horizontally; we cover them in this post on sun dogs. There's also a bigger, rarer 46-degree halo, luminous arcs called circumzenithal arcs (a sort of "upside-down rainbow"), and light pillars. All of them are the sky's optics lab at work, using nothing but frozen water and sunlight.
A halo around the sun or moon is a 22-degree ring created when light refracts through hexagonal ice crystals in high, thin cirrus clouds. Hold up your hand at arm's length to measure it, look for a faint red-inside, blue-outside tint, and remember that the icy clouds making it are often the vanguard of an approaching weather system — which is why the old "ring around the moon means rain" saying holds up more than superstition should.