Photographs of vivid rainbows are a dime a dozen, but a spectacle rarely captured is the appearance of light pillars. Whereas rainbows are produced when sunlight is refracted, or bent, as it passes through water droplets in the atmosphere, light pillars are produced when light is reflected, or bounced, off flat ice crystals suspended in the air. Here, however, the resemblance to rainbows ends. These luminous vertical shafts, known to scientists as light pillars, are a considerably less familiar phenomenon, materializing only under particular conditions and constrained by the temperature of the air.

A precise combination of optical and atmospheric conditions must coincide for a light pillar to take shape. To begin with, the air must be cold enough for tiny, plate-shaped ice crystals to form and linger near the ground. Furthermore, these flat crystals must settle almost horizontally, tilting only slightly as they drift downward, so that their broad faces act as a multitude of tiny mirrors. Finally, there must be a source of light beneath or behind them, whether the sun sitting low on the horizon or a bright lamp on the ground. When these requirements are met, the crystals reflect the light upward and an observer perceives a slender column rising vertically from the source.