A sun halo is a bright ring of light that encircles the sun, formed when sunlight refracts through millions of ice crystals in thin, high cirrus clouds. The most common one sits at a fixed radius of about 22 degrees from the sun, which is why it is often called a 22-degree halo. Halos are harmless, they appear whenever the right high clouds drift overhead, and because that cirrus often arrives ahead of a weather system, a ring around the sun has long been read as a sign that rain or a storm may be on the way. Over Central Oregon's clear high desert, they are a common and striking sight.
What Is a Sun Halo?
A sun halo is a complete circle of light centered on the sun, usually appearing as a pale white-to-faintly-colored ring against a milky, high-cloud sky. The classic version, the 22-degree halo, sits about a hand-span and a half from the sun at arm's length, and the sky inside the ring often looks slightly darker than the sky just outside it. When colors are visible, the inner edge tends toward red and the outer toward blue, the same prism effect that tints a rainbow or a sun dog.
The same kind of ring can form around the moon on a cold, clear night, where it is called a moon halo or lunar halo and works by exactly the same physics, just with moonlight instead of sunlight. Sun halos can be faint and easy to overlook or bold and unmistakable, depending on how thick and uniform the cirrus is. Once you know to scan the sky around a sun partly veiled by high cloud, you will spot them far more often than you would expect.
What Causes a Halo Around the Sun?
A halo forms when sunlight enters and exits the hexagonal ice crystals that make up thin cirrus clouds, bending the light by a minimum of about 22 degrees in the process. Each tiny crystal acts like a six-sided prism, and because there are countless crystals oriented in every direction, their combined refraction concentrates light into a ring at that 22-degree radius all the way around the sun. The geometry of the ice crystal is what fixes the ring at its characteristic size.
The difference between a halo and a sun dog comes down to crystal orientation. When the crystals are randomly oriented, as they are in most cirrus, light bends evenly around the whole circle and you get a full halo. When flat plate crystals lie horizontally, they concentrate the light into bright spots at the sides of the sun, producing sun dogs instead. The two often appear together, with sun dogs sitting on the left and right of a faint halo, all built from the same cloud of ice crystals in different orientations.
A 22-degree halo is the most common form, but the same ice crystals can produce a whole family of rarer arcs when the geometry is right, including a larger, fainter 46-degree halo and bright, colorful arcs that curve above the sun. A sun halo is also different from a corona, the small set of colored rings that hug close to the sun or moon, which is made by diffraction through tiny water droplets rather than refraction through ice. The size is the giveaway: a halo sits well out from the sun at a fixed 22 degrees, while a corona presses right up against it.
Does a Halo Around the Sun Mean Rain Is Coming?
A sun halo often does signal that rain or a storm is on the way, because the high cirrus clouds that create it frequently arrive ahead of an approaching weather system. Cirrus is usually the leading edge of a front, streaming out far in advance of the actual precipitation, so a halo can appear a day or more before the change reaches the ground. The old saying, ring around the sun, rain will come, has a genuine basis in this sequence of clouds.
That said, a halo is a hint, not a guarantee. Cirrus can drift through without any storm following, and halos appear plenty of times on days that stay dry. They are best understood as a sign that high, moist air is moving in, which sometimes, but not always, precedes a system. Watching whether the cirrus thickens and lowers over the following hours tells you more than the halo alone, and in Central Oregon a halo followed by lowering clouds is a reasonable cue that a Pacific system may be approaching, the kind discussed in atmospheric rivers and Central Oregon snow.
Is a Sun Halo Dangerous?
A sun halo is completely harmless. It is simply light bending through ice crystals, with no weather hazard of its own. The only real caution is the same one that applies to looking anywhere near the sun: never stare directly at it, halo or not, because the sun's brightness can damage your eyes. The safe way to view a halo is to block the sun itself behind a hand, a pole, or the edge of a building so you can study the ring without looking into the glare.
With the sun blocked, a halo is one of the easiest atmospheric optics to appreciate, often spanning a huge arc of sky and occasionally accompanied by sun dogs or other arcs for a full display. It poses no danger to people, aircraft, or anything else; it is purely an optical phenomenon, a trick of light and ice high overhead.
Where Do You See Sun Halos in Central Oregon?
Sun halos are common over Central Oregon's clear high desert whenever thin cirrus drifts in, which happens often ahead of Pacific systems and during high, moist flow aloft. The wide, open skies around Bend, Redmond, and Sisters make it easy to take in the full ring, and the region's generally clear conditions mean the cirrus that produces halos stands out cleanly rather than being lost in a gray overcast. They can appear in any season but are especially noticeable in the cold months.
They belong to the same family of ice-crystal optics as the region's sun dogs and light pillars, and seeing one is a good prompt to scan the rest of the sky for those companions. A halo is the high-desert sky quietly announcing that a layer of icy cirrus has moved in overhead, and sometimes that a change in the weather is not far behind.