A sun halo, often called a 22-degree halo, appears as a bright, circular ring of light that encircles the Sun. This optical phenomenon creates a striking spectacle in the sky, appearing as a luminous band with a radius of approximately 22 degrees around the Sun. The ring can be white, but it sometimes shows subtle colors, with reds and blues visible on its inner edge. This visual effect is a result of light interacting with specific atmospheric conditions.
How Sun Halos Form
Sun halos are optical phenomena that occur when sunlight interacts with ice crystals suspended in the Earth’s atmosphere. These ice crystals are found in thin, high-altitude clouds, such as cirrus or cirrostratus clouds, which exist in the upper troposphere, between 5 to 10 kilometers (3.1 to 6.2 miles) above the Earth’s surface. In very cold conditions, these crystals can also be present closer to the ground, a phenomenon known as “diamond dust.”
The formation process involves both refraction and reflection of light by these ice crystals. When sunlight enters an ice crystal, it bends, or refracts, and then bends again upon exiting, acting much like a prism. This double refraction bends the light by a specific angle, 22 degrees from its original path, creating the circular halo effect. The hexagonal shape and orientation of the ice crystals are important for determining the type of halo observed.
How Often Sun Halos Appear
Sun halos, particularly the common 22-degree halo, are not as rare as many might perceive them to be. They are observed more frequently than rainbows, with some regions in Europe and parts of the United States reporting sightings on average twice a week. The presence of high-altitude ice crystals, even on seemingly clear days, contributes to their common occurrence.
Public perception of their rarity often stems from people not looking up or recognizing the phenomenon when it occurs. While these halos can appear anywhere in the world and throughout the year, their frequency can vary with factors such as latitude, weather patterns, and local atmospheric conditions. Colder climates tend to have a higher abundance of the ice crystals necessary for halo formation, making sightings more common. The appearance of a sun halo can sometimes signal approaching weather systems, as the cirrus clouds that form them often precede storm fronts.
Common Types of Halos
Beyond the overarching term “sun halo,” various types of halo phenomena exist, each with distinct visual characteristics. The 22-degree halo is the most frequently observed and appears as a large ring with a radius of approximately 22 degrees around the Sun or Moon. This halo often shows a reddish tint on its inner edge.
Another common type is sun dogs, also known as parhelia or mock suns. These appear as bright, sometimes colorful, spots of light located on one or both sides of the Sun, often at the same altitude. Sun dogs are formed by sunlight refracting off plate-shaped ice crystals that are horizontally oriented in the atmosphere. They frequently accompany the 22-degree halo.