A solar halo is a mesmerizing atmospheric optical phenomenon that manifests as a luminous ring surrounding the Sun or Moon. This spectacular sight is caused by the interaction of light with the Earth’s atmosphere, creating a radiant circle that can sometimes display subtle colors. The appearance of a halo transforms an ordinary sky into a stunning visual display. The presence of a solar halo is entirely dependent on specific atmospheric conditions that align perfectly to create the optical effect.
How Ice Crystals Create the Halo Effect
The physical mechanism behind a solar halo begins high in the atmosphere, typically in wispy cirrus or cirrostratus clouds situated between five and ten kilometers above the surface. These clouds exist at altitudes where temperatures are significantly below freezing, meaning they are composed not of liquid water droplets but of millions of tiny ice crystals. These microscopic ice particles are structured as hexagonal prisms, possessing six rectangular side faces and two flat hexagonal end faces. The light from the Sun must interact with the geometry of these crystals to produce the halo effect.
When sunlight strikes a randomly oriented hexagonal ice crystal, the light is refracted, or bent, as it passes into and then out of the crystal, behaving much like a small prism. This refraction occurs twice: once as the light enters one crystal face and again as it exits through an alternate face. The minimum deviation angle for light passing through a hexagonal crystal is approximately 22 degrees.
This consistent geometry ensures that the light scattering is focused into a ring shape around the light source. The bending of light also causes a slight separation of colors, similar to a rainbow, with red light being refracted least and appearing on the inside edge of the halo. The remaining colors overlap, making the outer edge appear whiter or only faintly colored.
Common Types of Solar Halos
The most frequently observed manifestation of this atmospheric interaction is the 22-degree halo, which appears as a complete circle around the Sun or Moon. This particular halo is caused by light passing through ice crystals that are randomly oriented in the air.
Another common type is the sun dog, scientifically known as parhelia, which appears as bright, often colorful spots of light on either side of the Sun. These phenomena occur when the hexagonal plate-shaped ice crystals are oriented with their flat faces parallel to the ground, causing the light to refract laterally. Sun dogs are usually seen when the Sun is near the horizon and are always located at the same 22-degree distance from the Sun.
A less common but equally spectacular display is the 46-degree halo, a much larger ring that surrounds the 22-degree halo. This halo is created by light passing through the hexagonal ice crystals in a different way, specifically entering a side face and exiting an end face.
Halos Versus Other Light Phenomena
Solar halos are often confused with other atmospheric light displays, but they are distinguished by their formation mechanism and the particles involved. The most significant difference lies between a halo and a rainbow, which is created by liquid water droplets instead of ice crystals. Rainbows form when sunlight undergoes refraction upon entering a droplet, internal reflection off the back surface, and a second refraction upon exiting, typically resulting in a 42-degree arc centered opposite the Sun. A halo, conversely, involves only refraction through ice and is centered on the light source.
Coronas represent a third distinct phenomenon, appearing as a much smaller, colored disk immediately surrounding the Sun or Moon. Unlike the refraction that creates halos, coronas are caused by the diffraction of light as it passes around very small, uniformly sized water droplets or ice crystals. This process results in a series of concentric colored rings where the color order is opposite to that of a halo.