That ring around the sun is actually a giant prism made of ice
In short: When a halo appears in the sky, you are witnessing light interacting with millions of tiny ice crystals suspended in the upper atmosphere. These crystals act as natural mirrors and prisms, bending and reflecting sunlight to create complex, fleeting geometric displays that have fascinated observers for centuries.
Halos are atmospheric optical phenomena caused by light from the Sun or Moon interacting with ice crystals, typically found in cirrus or cirrostratus clouds at altitudes between 5 and 10 kilometers. As light passes through these crystals, it is reflected and refracted, with dispersion often splitting the light into colors. The specific shape and orientation of the crystals determine the type of halo observed, ranging from common circular rings to rare arcs and spots.
The most recognizable form is the 22° halo, a large ring with a radius of about 22 degrees. Unlike other phenomena, the interior of this ring often appears darker than the surrounding sky, creating the illusion of a hole. While the 22° halo results from semi-randomly oriented crystals, other displays like sun dogs and light pillars require crystals to align horizontally as they drift through the air. These phenomena have historically served as weather lore, often signaling an approaching frontal system and potential rain within 24 hours.
Scientific interest in these displays dates back to antiquity, with Aristotle noting them early on. Later, researchers like Christoph Scheiner in 1630 and Johannes Hevelius in 1661 provided detailed descriptions of complex displays. Today, scientists reproduce these effects in laboratories using chemical precipitation or engineered machines that rotate crystals to mimic natural orientations. Such experiments help explain rare phenomena like Bottlinger's rings, which are elliptical and notoriously difficult to observe due to their faintness and proximity to the sun's glare.
It is important to distinguish halos from other optical events. Coronas, for instance, are multicolored disks caused by water droplets rather than ice. Similarly, while rainbows and glories also involve light interaction, they rely on water droplets. Whether viewed as an omen of bad weather or a subject of rigorous physical study, the halo remains a testament to the complex, crystalline nature of our upper atmosphere.
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Sources
- Summary by Intellect · source: Halo (optical phenomenon)
- Also: Halo | Rainbow, Sunlight Refraction & Ice Crystals | Britannica
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