A halo is a common atmospheric sight, appearing as a large, often brilliant, ring of light encircling the Sun or the Moon. This striking optical phenomenon is a predictable result of physics and geometry played out in the upper atmosphere. The appearance of this perfect luminous circle is governed entirely by the way light behaves when it encounters frozen water high above the ground. The explanation for this specific, unchanging ring involves the precise shape of ice crystals and a principle of light bending known as minimum deviation.
The Necessary Ingredients
Creating a 22-degree halo requires the presence of two distinct components: a powerful light source and a particular atmospheric medium. The light source is always either the Sun during the day or the Moon at night, providing the parallel rays of light needed to initiate the effect.
The medium responsible for refracting the light is a vast collection of high-altitude hexagonal ice crystals. These crystals are typically suspended in thin cirrus or cirrostratus clouds, which float at elevations of 20,000 feet or higher. The specific six-sided, columnar structure of these frozen particles is what makes the halo possible. Each tiny crystal functions as a miniature optical prism, ready to bend incoming light at a fixed angle.
How Light Interacts with Ice Crystals
The formation of the halo begins when a ray of light enters one face of a hexagonal ice crystal and exits through an alternate face. Since the hexagonal shape creates a 60-degree angle between adjacent faces, the light passes through what is essentially a 60-degree prism. The light bends, or refracts, twice—once upon entry and again upon exit—from the denser ice material.
This double refraction causes the light to deviate from its original path at a wide range of angles. However, a large concentration of light rays exits the crystal at a specific, smallest possible angle, known as the angle of minimum deviation. For the hexagonal ice prism, the angle of minimum deviation is consistently near 22 degrees, a value determined by the 60-degree apex angle and the refractive index of ice, which is approximately 1.31.
Why Halos Are Always 22 Degrees
The 22-degree radius of the halo is a direct consequence of this minimum angle of deviation. Light rays that exit the ice crystal at the minimum deviation angle are the ones that are most efficiently directed toward the observer’s eye. Rays that are deflected at smaller angles cannot reach the observer, which is why the sky inside the halo ring appears noticeably darker than the surrounding sky.
The full circular shape of the halo is achieved because the millions of ice crystals are randomly oriented in the sky. Any light ray that passes through it and is refracted at the minimum angle of deviation will land on a cone of light with an apex angle of 22 degrees relative to the observer and the Sun. The observer sees a complete ring because this cone of light is continuously formed by different crystals all around the sky. Therefore, the halo’s radius is not a physical distance but an angular measurement—the light reaching the observer has been deflected by 22 degrees from the direct path of the Sun or Moon.