The 22-Degree Halo: What Ice Crystals Do to Sunlight and Moonlight

Look up on the right kind of afternoon and you will see it: a clean white ring sitting about two fist-widths away from the sun, sometimes faintly coloured red on the inside edge and blue on the outside. The same ring appears around the moon on clear winter nights. It is one of the most common optical phenomena in the sky, and most people who notice it have no idea what they are looking at.

Cirrus clouds sit between six and ten kilometres above the ground, well above the weather you feel. At that altitude, even on a warm day at the surface, the air is cold enough, typically below minus 20 degrees Celsius, that water freezes directly into crystals rather than forming liquid droplets. The crystals that produce the 22-degree halo are hexagonal columns: six-sided prisms, roughly the shape of a pencil but microscopic, tumbling slowly as they drift.

The number 22 is not a label someone chose. It is the minimum angle at which light can exit one of those hexagonal prisms after entering through a side face and leaving through another side face 60 degrees away. When light tries to pass through at a shallower angle, the geometry of the crystal will not allow it, the light undergoes total internal reflection instead. So the minimum deflection is 22 degrees, which means light piles up there. The sky at exactly 22 degrees from the sun or moon becomes brighter than the sky around it. That brightness is the halo.

The region inside the ring, between the sun and the 22-degree circle, is noticeably darker than the sky outside it. This is not an illusion. Because 22 degrees is the minimum deflection angle for that crystal geometry, no light from the ice layer arrives inside that radius. The crystals simply cannot bend light to land there. The darkness is an absence with a geometric cause, the same way a shadow has a sharp edge when the light source is small and the object blocking it is precise.

The colour separation, red closer to the sun, blue at the outer edge, happens because different wavelengths of light refract at slightly different angles through ice, just as they do through a glass prism. Red bends least, so it piles up at the smallest angle. Blue bends more, so it sits further out. The separation is subtle compared to a rainbow, because the refractive index difference between wavelengths in ice is small, but on a clear day with a bright sun and a well-developed cirrus sheet, it is visible.

The lunar halo follows identical physics. Moonlight is reflected sunlight, and it refracts through ice crystals at the same angles. The ring appears at 22 degrees from the moon's disc, and on nights when the moon is bright enough, near full, the colour separation is just visible, red on the inside, blue on the outside, though far fainter than the solar version.

Because the human eye shifts to scotopic vision in low light, losing colour sensitivity, lunar halos tend to look white or pale silver rather than the warmer tones of the solar halo. The geometry is identical. The experience is quieter.

Indian folklore in several regions holds that a ring around the moon predicts rain within a day or two. The meteorological logic behind this is real: cirrus clouds often precede a warm front, and a warm front brings rain. The halo does not cause the rain. It is produced by the same cloud system that is moving toward you. The folk observation is a genuine pattern, even if the causal story is wrong.

Hexagonal ice columns do not only make the 22-degree ring. When the crystals are oriented, drifting with their long axes roughly horizontal rather than tumbling randomly, they produce different phenomena. Flat hexagonal plates oriented horizontally create sun dogs: two bright spots sitting at the same altitude as the sun, one on each side, at roughly 22 degrees of azimuth. A circumzenithal arc, which looks like an upside-down rainbow near the zenith, comes from the same plates with light entering through the flat top face and exiting through a side face.

The 22-degree halo specifically requires randomly oriented crystals. If the cirrus layer is turbulent enough that no preferred orientation develops, the halo is the result. If the air is calm and the crystals settle into alignment, the halo weakens and the oriented phenomena strengthen. On some days you see both: a partial halo with sun dogs sitting on it.

A 46-degree halo, larger and rarer, appears when light enters through the end face of a hexagonal column and exits through a side face, a different pair of surfaces, a different geometry, a larger minimum deflection angle. It sits roughly twice as far from the sun as the 22-degree ring and is pale enough that most people miss it even when it is there.