Why Halos Appear: The 22° Ring and the Physics of Ice Crystals

The 22° halo is arguably the most common atmospheric optical phenomenon that most people have never consciously noticed. On perhaps one day in three across the temperate latitudes, if you shield the Sun with your thumb and let your eyes adjust, you will see a soft, luminous ring at roughly the distance of an outstretched hand, faintly reddish on its inner edge. The physics behind it is beautiful, its identification is trivial once you know what to look for, and — perhaps most usefully for amateur observers — its appearance is a genuinely reliable short-range weather signal.

The halo is produced by hexagonal ice crystals suspended in high cirrus cloud, typically between six and ten kilometres above the surface. When sunlight enters one side face of a hexagonal prism and exits another face two sides away, it is bent through a minimum angle of about 21.84 degrees. Crystals oriented at every possible angle scatter light in every direction, but the geometry of a hexagonal prism enforces a sharp cutoff: almost no light is bent through less than 22 degrees. The result is a bright ring at exactly that angle around the Sun, dark on the inside and gradually fading on the outside.

Because the refraction angle depends slightly on wavelength, red light is bent least and violet most. The inner edge of the halo is therefore faintly red, and the outer edge fades through white — never through a full spectrum, because the geometry does not focus other colours as sharply. This is one of the easiest ways to distinguish a real 22° halo from a corona (which is caused by diffraction around water droplets and shows pastel colour bands) or from an aircraft contrail glint (which is highly localised).

Halos require ice crystals of a specific quality. If the crystals are too small, they diffract instead of refract and no halo forms; if they are too large, they tumble as they fall and their orientations become non-random, producing sundogs, upper tangent arcs, and other more specialised phenomena rather than a clean ring. The Goldilocks conditions — a thin, uniform sheet of small hexagonal columns and plates in cirrostratus — are surprisingly common in the twenty-four hours ahead of a warm front, which is why old sailor lore ties halos to approaching rain.

That lore is empirically defensible. Warm fronts push warm, moist air up and over colder surface air along a very shallow slope, and the first visible sign of the front at a given location is a thickening veil of cirrostratus arriving from the direction of the approaching low. A 22° halo is essentially a diagnostic that cirrostratus of the right ice-crystal composition is overhead. Studies of surface observations against subsequent precipitation show that in the mid-latitudes, a halo raises the probability of rain within twenty-four hours from a climatological baseline of around thirty percent to roughly sixty percent — not a certainty, but a meaningful shift.

For the amateur observer, the practical skills are three. First, always block the Sun with a thumb, a lamppost, or the edge of a building before looking for a halo; direct staring at the Sun is dangerous and also destroys your ability to see the faint ring. Second, measure the angle. A fist held at arm's length subtends about ten degrees; two fists plus a little is roughly twenty-two. Anything much larger — around forty-six degrees — is a rarer, fainter halo produced by refraction through the end faces of hexagonal columns rather than side faces. Third, note the sky context. A halo with no cirrus thickening around it is a curiosity; a halo embedded in a slowly thickening, uniform cirrostratus sheet that arrived from the west is a signal.

There is a family of related phenomena worth watching for whenever a halo appears. Sundogs are bright, sometimes strongly coloured spots at exactly 22° to the left and right of the Sun, produced by plate-oriented crystals; they are common in winter and at high latitudes. The upper tangent arc kisses the top of the halo and looks like a bright cap; the parhelic circle is a faint white horizontal line running through the Sun at its own altitude. Seeing multiple members of this family at once, sometimes called a halo display, is a strong indicator that the upper atmosphere is filled with very well-oriented crystals — a moderately rare and photogenic event.

Halos are also visible around the Moon on nights when a thin cirrostratus sheet advances ahead of a warm front. Lunar halos are dimmer and less colourful than solar halos because moonlight is weaker, but they are otherwise identical in geometry and, because they announce themselves at night, they often surprise observers who thought the sky was completely clear. A lunar halo has the same forecast value as a solar one and is easier to photograph because the Moon can be blocked without over-exposing the image.

The habit of scanning for halos costs nothing and rewards well. Once you have seen a clear 22° halo you will never mistake one again. It is one of the small daily pleasures of amateur atmospheric observation, and it is the cheapest weather-forecast improvement most people can add to their day.