What the species is and where it came from
A halo is a natural phenomenon: an optical effect produced by light, typically from the Sun or Moon, interacting with ice crystals suspended in the atmosphere. The word comes from the Ancient Greek for threshing floor or disk. Halos range from coloured or white rings to arcs and spots. Many appear near the Sun or Moon, but others occur elsewhere in the sky. The best known are the circular halo (properly the 22° halo), light pillars and sun dogs. Some are fairly common and others extremely rare.
How it works, in terms someone would actually use
The crystals are typically suspended in cirrus or cirrostratus clouds in the upper troposphere, 5 to 10 km up. In cold weather they can also float near the ground, where the effect is called diamond dust. Their shape and orientation decide which halo you see. Light is reflected and refracted between the crystal faces, so the crystals act like prisms and mirrors, and dispersion can split the light into colours.
For the 22° halo the crystals are oriented semi-randomly. The ring has a radius of about 22°, roughly an outstretched hand at arm’s length, and the sky inside it looks darker, like a hole in the sky. Sun dogs and light pillars need crystals that sit horizontally. A pillar rises from the Sun near sunset or sunrise. Plate crystals generally make one only when the Sun is within 6 degrees of the horizon.
What it gets right
It is a useful sign in the sky. Halos often indicate that rain will fall within 24 hours, because the cirrostratus clouds that cause them can signal an approaching frontal system. Optical effects like this were part of weather lore, an empirical way of forecasting before meteorology developed. In the Cornish dialect a halo around the sun or moon is a cock’s eye, an omen of bad weather.
What it does not
A halo is made by ice, not water droplets. The glory and the rainbow are other phenomena that involve water droplets. The corona is a multicoloured disk caused by droplets and should not be mistaken for the 22° ring. A halo is also not a sure forecast, since the material says only that halos often precede rain. Rare forms are less certain still: Bottlinger’s ring, an elliptical halo that is small and hard to see in the Sun’s glare, is not well understood.
What it changed
People have written halos down for a long time. Aristotle mentioned halos and parhelia. Chinese observers recorded them for centuries, and a section of the Chin Shu in 637 gives terms for 26 solar halo phenomena. Vädersolstavlan, the Stockholm Sundog Painting, is arguably one of the oldest known depictions of a halo display, showing the sky over the city on the morning of 20 April 1535. The first European descriptions of complex displays came from Scheiner in Rome around 1630. Halos have also been reproduced artificially, and the earliest experimental studies are attributed to Bravais, who spun a glass prism in 1847.
Who it is for, and who it is not
It is for anyone who looks at the sky, since a 22° ring needs only the Sun or Moon and ice crystals in the air. It is not for anyone who wants a guarantee, and rare forms such as Bottlinger’s ring are hard to see at all.
Is it worth your time
Yes, if you glance up now and then. A ring or arc around the Sun or Moon is physics you can see unaided, and it often means rain within the day.
