A sun halo, which appears as a large, often colorful ring around the sun, is a common atmospheric optical phenomenon. However, the presence of a sun halo does not negate the severe danger of looking directly at the sun. The primary concern is the intense, focused solar radiation that can cause serious and permanent eye damage regardless of any atmospheric display.
The Atmospheric Science Behind Sun Halos
Sun halos, also known as 22-degree halos, form when sunlight interacts with tiny ice crystals suspended high in the atmosphere. These crystals are typically found in thin, wispy cirrus or cirrostratus clouds, which float in the upper troposphere. The ice crystals possess a hexagonal structure, acting like miniature prisms as light passes through them. Light enters one face of the crystal, is refracted, and then exits another, bending twice and deflecting the light by a minimum angle of approximately 22 degrees from its original path. Since millions of these crystals are randomly oriented, the refracted light creates a complete circle with a radius of 22 degrees around the solar disk, often making the sky appear noticeably darker inside the halo.
Direct Hazards of Viewing the Sun
The danger in observing a sun halo lies in the human tendency to look toward the center of the ring, which is the sun itself. Staring directly at the sun, even for a short time, can result in a condition called solar retinopathy. The eye’s lens focuses the sun’s intense light and ultraviolet (UV) radiation onto the retina, causing photochemical and thermal damage to the photoreceptor cells. Since the retina lacks pain receptors, the injury is not felt as it occurs, and damage can happen in a matter of seconds. Symptoms like blurred vision, central blind spots, or distorted vision may not appear until hours later, and the damage can be permanent.
To observe a sun halo safely, it is necessary to completely shield the direct view of the sun. This can be accomplished by positioning a building, a tree, a telephone pole, or even a hand to block the sun’s bright disc. By covering the sun, the halo, which appears 22 degrees away, can be safely viewed using peripheral vision. Never use regular sunglasses, as they do not offer sufficient protection against the sun’s harmful rays.
Do Sun Halos Predict Dangerous Weather?
Sun halos have long been associated with changing weather, and this folklore has a basis in atmospheric science. The cirrostratus clouds that host the ice crystals responsible for the halo often form ahead of an approaching large-scale weather system, such as warm fronts. These systems typically carry significant moisture and can precede rain or snow within the next 24 hours. While the halo itself is not dangerous, its presence indicates that atmospheric conditions are shifting due to an approaching weather pattern, though halos are not a guaranteed forecast.