A 22-degree halo is a ring of light that forms around the Sun or Moon when its light is refracted through hexagonal ice crystals found in high-altitude cirrus clouds. These crystals act like tiny prisms, bending the light at a specific angle (22 degrees) to create the characteristic circular shape visible to an observer on the ground. ANSWER: (C) Refraction by hexagonal ice crystals
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For a halo to appear, thin cirrostratus clouds must be present. These clouds are composed of millions of tiny, randomly oriented hexagonal ice crystals. As light enters one side of the crystal and exits another, it is bent by 22 degrees. Because there are so many crystals, the light is refracted in a full circle around the light source. In folklore, the appearance of a halo is often said to predict rain, which is scientifically grounded: cirrus clouds are often the leading edge of an approaching warm front or a low-pressure system that brings precipitation.
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