Ring around the moon: what the halo means and if rain is coming

A ring around the moon is almost always a 22-degree halo: moonlight bent by millions of tiny six-sided ice crystals in thin, high cloud. The ring does nothing to the weather itself, but the ice cloud that makes it often runs ahead of a warm front, so the old saying that it means rain is right more often than most weather lore. It is a clue, though, not a guarantee.

You usually notice it on an ordinary evening. The Moon sits behind a thin, milky veil, and around it hangs a huge pale ring, so wide that your outstretched hand barely spans the gap between Moon and ring. The sky inside the ring looks darker than the sky outside, and the inner edge has a faint reddish tinge. This article covers how the halo forms, when the rain lore holds up, how to tell a halo from a corona or moon dogs, how to measure one with your hand, and which full moons of 2027 give you the best chance.

What causes a ring around the moon?

The ring forms in cirrus and cirrostratus, the highest common clouds, which the UK Met Office places at 20,000 to 40,000 feet (about 6 to 12 kilometers). At that height it is well below freezing even on a summer night, and these clouds are made entirely of ice. Many of the crystals are tiny hexagonal columns and plates, like microscopic pencil stubs.

Moonlight that enters one side face of such a crystal and leaves through another face, set at 60 degrees to the first, is bent the way a glass prism bends light. The amount of bending varies, but it is never less than about 22 degrees, and light piles up close to that minimum. Across millions of crystals tumbling at every angle, the result is a bright circle with a radius of 22 degrees around the Moon. That makes the ring 44 degrees across, compared with a Moon only half a degree wide.

That geometry explains the rest of what you see. No light is bent by less than 22 degrees, so the sky just inside the ring is darker. Red light is bent slightly less than blue, so the inner edge is reddish and the outer edge fades toward blue. Around the Moon, though, colors are hard to make out, because moonlight is too faint for the color-sensing cells in your eyes to work well. A moon halo mostly looks white, for reasons explained in how bright is moonlight.

The Sun makes exactly the same halo, and plenty of people have walked under one without looking up. At night the Moon is the only light bright enough to produce a clear ring, which is why moon halos are easiest to see in the nights around full moon. Folk names for it include moon ring, storm ring and winter halo.

Does a ring around the moon mean rain?

Often, yes. The saying comes in many versions, among them “When halo rings the Moon or Sun, rain’s approaching on the run,” and unlike a lot of weather lore it rests on real meteorology.

Cirrus and cirrostratus commonly form at the leading edge of a warm front, where warmer, moister air slides up over colder air. The highest, most forward part of that sloping warm air is ice cloud, and it arrives well before the rain. The classic sequence runs like this:

1. Wisps of cirrus appear high in the sky.
2. They spread into a thin, milky veil of cirrostratus, and the halo appears.
3. The veil thickens and lowers into gray altostratus, the halo fades, and the Moon turns into a blurry smudge.
4. Low, dark nimbostratus moves in, and steady rain or snow begins.

Once that veil has thickened, rain usually begins 12 to 24 hours later. The Met Office puts it just as plainly: if a smooth, featureless sheet of cirrostratus covers the sky, an incoming warm front is likely to bring persistent rain within a day.

So when is the lore wrong? Cirrus does not always come from a front. It can be the leftover anvil of a distant thunderstorm, or a band stretched out by the jet stream, and random, isolated cirrus has no particular meaning. Plenty of halos fade away with the weather unchanged. Two checks make your own forecast much better:

• Watch the trend. If the veil thickens over the next hour or two and the ring melts into a dull glow, a front is probably on its way. If the sky stays thin and the ring stays crisp, it may come to nothing.
• Watch the direction. Between about 30 and 60 degrees latitude, in both hemispheres, the prevailing westerlies steer weather systems from west to east. A veil spreading in from the west is a stronger sign than one drifting from another direction.

A ring around the moon does not predict the weather. It reveals the ice cloud that often runs a day ahead of the rain.

Halo, corona or moon dogs: what are you seeing?

Not every glow around the Moon is a halo. Two other sights get mixed up with it all the time.

The corona

A corona is a much smaller glow hugging the Moon: a bluish-white disk that fades to reddish-brown at its edge, sometimes with faint pastel rings around it. It forms by diffraction, as light bends around tiny water droplets in thin cloud such as altocumulus, rather than by refraction through ice. Smaller droplets make a larger corona, and the rings are sharpest when the droplets are all the same size. Notice the color order: red on the outside, the reverse of a halo.

Moon dogs

Moon dogs, or paraselenae, are bright spots of light on one or both sides of the Moon, level with it and about 22 degrees away, often sitting right on the halo. They come from flat, plate-shaped hexagonal crystals that drift down with their broad faces horizontal. They are the night version of sun dogs, and rarer, because the Moon must be at least as bright as a quarter moon for them to show. Even then they usually look pale or white.

A quick way to tell them apart:

• Large, pale ring, reddish inside, with a darker sky within: a 22-degree halo, made by ice.
• Small glow close around the Moon, red on the outside: a corona, made by water droplets.
• Bright patches beside the Moon at halo distance: moon dogs, made by flat ice plates.

How do you measure 22 degrees with your hand?

You need no equipment. Stretch your arm out fully and spread your fingers as wide as they will go. The radius of a 22-degree halo is roughly the span of your outstretched hand at arm’s length. Put the tip of your thumb on the Moon: if the ring runs close to the tip of your little finger, you are looking at a halo. If the ring sits snugly around the Moon, well inside your hand, it is a corona.

Photographing a halo is harder than it looks, because the Moon is so much brighter than the ring. Use the widest lens you have, since the ring is 44 degrees across. A phone’s ultra-wide camera fits it with room for a tree or rooftop. Tap on the sky beside the ring rather than on the Moon, lower the exposure until the ring appears, and let the Moon burn out. Rest the phone on something solid for night mode. More tips are in how to photograph the Moon with your phone.

When can you see a ring around the moon in 2027?

You cannot schedule cirrus, but you can schedule a bright moon. A halo is easiest to see in the few nights either side of full moon, and winter often brings the best mix in the mid-latitudes: long nights, a moon riding high and a steady stream of fronts. Because winter falls in opposite months north and south of the equator, the best halo nights do too.

In the northern hemisphere:

• Wolf Moon, January 22 (12:17 UTC): the closest and brightest full moon of the year, the supermoon of 2027, high in the winter sky.
• Snow Moon, February 20 (February 21 in India and Australia): a bright late-winter moon at 363,296 km.
• Beaver Moon, November 13 or 14 (November 13 in the US, November 14 in the UK, India and Australia): the start of the long nights.
• Cold Moon, December 13 (December 14 in Australia): the highest full moon of the year in the northern sky, so a halo around it stands well clear of rooftops and trees.

In Australia, New Zealand and South Africa, look to the winter moons instead: the Strawberry Moon of June 19 (June 18 in the US) and the Buck Moon of July 18 (July 19 in Australia and New Zealand). The Buck Moon is the farthest full moon of 2027, yet down south it still rides high through a long night. Local times are on the Australia full moon page.

Every date and UTC time is in the 2027 full moon calendar, and you can add the full moons to your own calendar so a reminder arrives in time to look up. And if you do spot a ring on one of those nights, look out of the window again the next morning.