Definition of an Optical Halo
An optical halo refers to various atmospheric optical phenomena that manifest as luminous rings, arcs, or spots in the sky, typically appearing around the sun or moon. These captivating displays are created by the interaction of light with millions of tiny ice crystals suspended in the Earth's atmosphere, most commonly found in high-altitude cirrus clouds.
How Halos Form: The Role of Ice Crystals
Halos form when sunlight or moonlight passes through hexagonal ice crystals, which act like tiny prisms. The specific shape (e.g., plates, columns) and orientation of these crystals, combined with the angles at which light enters and exits their facets, determine the type and appearance of the halo. Light is refracted (bent) as it enters the crystal, reflected off an internal surface, and then refracted again as it exits, creating the observed optical effects.
Common Types of Optical Halos
The most frequently observed halo is the 22-degree halo, a circular ring around the sun or moon with a radius of approximately 22 degrees. Other common types include sun dogs (or parhelia), which are bright, often colorful spots that appear on either side of the sun, and circumzenithal arcs, which are bright, rainbow-like arcs high above the sun, sometimes referred to as 'upside-down rainbows.'
Significance and Observation
Observing optical halos provides insights into atmospheric conditions, particularly the presence and characteristics of ice crystals in the upper troposphere. While historically sometimes interpreted as omens, halos are purely natural phenomena that beautifully demonstrate fundamental principles of optics, showing how light interacts with matter and revealing the physics of our atmosphere.