Night isn’t the only time to see gorgeous things in the sky. Some of the most awe-invoking heavenly phenomena can only happen during the day, when the sun is shining.
If you live in a place where you (1) can see our home star and (2) have an atmosphere above your head—statistically speaking, I think that covers most of our readers—then you can see some truly lovely daytime apparitions, such as giant rings around the sun, glowing arcs and ethereal flattened rainbows. There are many different kinds of these visual delights with many different names, but in aggregate, they’re called atmospheric optical phenomena.
Almost all of them are caused by tiny ice crystals suspended in the air. Sometimes those crystals are very high up in the atmosphere, where it’s cold all the time, so you can see them in summer. But in winter the air can be cold enough to support ice at much lower altitudes, setting the stage for especially vibrant displays.
The most common of these is called a halo, or more specifically, a 22-degree halo. This appears as a ring of (usually) milky-white light centered on the sun with a 22-degree radius—about twice the width of your outstretched fist. This means such halos have a 44-degree diameter, appearing quite large in the sky.
Sometimes halos are faint, and other times they blaze so brightly that they look like solid objects. But they’re not: in a sense, they’re illusions, optical effects with no real existence on their own.
You’re really seeing the combined glints of light from countless airborne ice crystals. Although the exact mechanism behind the 22-degree halo isn’t perfectly understood, the leading hypothesis is that it’s caused by columnar hexagonal ice crystals that are shaped something like the body of a pencil.
Light bends, or refracts, when it passes from the air through the crystal, and how much it bends depends on the crystal’s shape and the angle at which the light hits it. For a hexagonal column of ice, geometric optics demands that a ray of light will bend at least 22 degrees as it passes through. (It can bend more, but most of the incoming light is refracted by this amount.)
This means that if you’re looking up from the ground, any ice crystal that is 22 degrees from the sun in any direction in the plane of the sky will bend that light right at you. So a sufficiently sized intervening cloud of crystals will project a circle around the sun with that radius; because some of the light is bent by more than 22 degrees, some crystals outside the projected halo contribute to the glow, giving it a sharp inner boundary but a fuzzy, ill-defined outer one.
Most of the time, the halo looks whitish, but it can show color, too. Different wavelengths of light—what we see as their color—bend by slightly different amounts; blue light bends more than red. This differential bending can show up as a rainbowlike distribution of colors in the halo, with red on the inside and blue on the outside. If you’re lucky, the colors can be quite striking.
Greater concentrations of ice in the air make for brighter halos, which can help our eyes see these colors. I commonly see them when there’s a thin cirrus cloud layer in the sky, and if I spot those clouds, I always stop and check for halos. I don’t know how many I’ve seen in total, but I’m sure the figure is more than 100. They can occur around the moon, too. Lunar halos are fainter, but their contrast with the dark sky can make them more obvious.
There are other phenomena that can occur along with a halo. Parhelia, or sun dogs, in the vernacular, are bright spots on each side of a halo that are set parallel with the horizon. They’re also caused by hexagonal ice crystals—ones that are flat like a floor tile rather than pencil-shaped. As gravity pulls them down through the atmosphere, air flowing around them causes the crystals to align horizontally.
That configuration means that the light passing through them is bent toward an observer who sees them edge-on; this happens when they are lined up with the sun and parallel to the horizon, creating the twin sun dogs. Again, the light is bent by about 22 degrees, so they appear on the halo. They can also be brilliantly colored, with red on the inside and blue on the outside.
Because their source crystals are flat and need to be seen edge-on, sun dogs tend to be brightest when the sun is on or near the horizon; I’ve lost count of how many times I’ve seen them in the early morning or late afternoon. They can be so bright that they look like the sun itself—which is why they’re sometimes called “mock suns.”
There are many, many other optical phenomena associated with halos, too, such as tangent arcs (gull-wing-shaped arcs at the top of halos), parhelic circles (straight white lines that extend around the horizon through parhelia, which can make some parhelia look teardrop-shaped), and circumzenithal arcs (which look like an upside-down rainbow centered on the zenith directly above your head). All of these are rare in normal circumstances but are actually quite common at ski resorts, where snowmaking machines can blast huge amounts of tiny ice crystals high into the air. Videos of them are absolutely jaw-dropping.
When I lived in Colorado, one of the best I ever saw came about when a cold front dropped the temperature overnight to a bone-chilling –25 degrees Celsius (–13 degrees Fahrenheit). When I got up and checked the weather, I immediately knew I might see a good display, so I ran to a window facing east and saw a brilliant halo, complete with sun dogs, parhelic circles and a tangent arc, shown in the photograph at the top of this article. The display changed as the sun rose, too; it got brighter, strengthening the parhelic circles. It was staggeringly beautiful.
Another cool optical phenomenon is the circumhorizon (or sometimes circumhorizontal) arc, which is like a flat rainbow near the horizon when the sun is high in the sky. Sometimes called a fire cloud—frilly, ice-filled cirrus clouds near the horizon are a common source for this—it’s stunning yet rare. I’ve only seen three fire clouds in my life. They form slightly differently than sun dogs; the light enters one of the vertical sides of a flat hexagonal ice crystal but leaves through the bottom face. This is more efficient at separating the colors, giving them their striking spectra display.
One phenomenon I’ve only faintly seen is called a light pillar: a column of light shining straight up above a bright source such as the sun. As with parhelia, they’re caused by flat plate crystals falling in the air. If such a crystal is above a bright light source, the light hitting its underside is reflected toward the observer. Any crystal directly above the source can do this, so in aggregate, many crystals will produce a vertical pillar of light. This happens with city lights, too, and sometimes the light pillars are all different colors, matching the holiday lights at night. They can stretch high into the sky, and in 2016 a display in Eura, Finland, was so bright that you could trace out the layout of the city streets in the reflected streetlights!
I have stood flabbergasted by sights such as these and many more, and it’s no surprise at all that throughout history they were sometimes interpreted as omens or something supernatural in the sky. I understand how they form, and it’s still a magical experience. If I’m around people when these fantastic phenomena grace our sky, I always point them out. It’s always a surprise to me how few people know halos exist, even though they’re fairly common—hence my constant exhortation: look up.
Not enough people do, and they are missing out on some of nature’s most incredible displays. The more you look up, the more wonder you’ll experience.
