They are called fire rainbows, but the name doesn’t do justice to the display. No fire burns, it’s only light, bent and scattered across high, icy clouds. Over Hawaii these rare sights sometimes appear like brushstrokes of flame. Visitors who glance upward at just the right time can count themselves fortunate to see one.

A fire rainbow—scientifically known as a circumhorizontal arc, is caused when sunlight strikes thin, wispy cirrus clouds made of hexagonal ice crystals miles above the earth. When the sun climbs high in the sky, its rays enter the plate-like crystals at a specific angle—about 58 degrees—and refract, or bend, into bands of color that fan out horizontally. Unlike a traditional rainbow, which forms in raindrops opposite the sun, a fire rainbow sits high and parallel to the horizon, shimmering in the same direction as the clouds hosting it.

Hawaii’s tropical climate makes it a fitting stage for these sky-borne apparitions. Cirrus clouds drift frequently above the islands, trailing across the upper atmosphere. But even here, fire rainbows are fleeting. The angle of the sun must be just right, and that happens only when it climbs higher than 58 degrees above the horizon—most commonly during the summer months, between May and August. Add the right altitude of ice crystals, the correct orientation, and the absence of denser clouds, and you have the makings of a spectacle that might last only a few minutes before fading.

Fire rainbows appear in many parts of the world, but only within certain latitudes—roughly between 55 degrees north and 55 degrees south—where the sun can reach the necessary height. That makes the Hawaiian Islands one of the few places where the phenomenon is even possible. In contrast, fire rainbows are nearly impossible near the poles, where the sun never climbs high enough. Even in Hawaii, though, the conditions are exacting: the right kind of cirrus cloud must form at the right time of day, with ice crystals perfectly aligned to catch and split the light. For that reason, the displays appear only a handful of times each year, most often around midday when the summer sun blazes nearly overhead.

When they do appear, they paint the Hawaiian sky in colors that seem too vivid to belong to daylight—electric blues, fiery oranges, molten pinks. Often, they appear just before noon, when the heat shimmers off the ocean and the wind carries a salt haze inland. Tourists on beaches sometimes mistake them for halos or auroras, but those are different phenomena. Fire rainbows are purely an interplay of geometry and light during a brief alignment of sun and ice.

In Hawaiian tradition, skies have long been seen as messengers, taken as omens, or guidance. Though fire rainbow is a modern term, it’s easy to imagine ancient Hawaiians interpreting such a vision as a sign. Modern onlookers feel a similar hush when they see one. Cameras come out, but often the moment passes before the shutter clicks.

And that’s part of their magic. Fire rainbows, like many of Hawaii’s wonders, live in the balance between science and spirit—where sunlight meets ice, where an ordinary sky can catch fire.