Light rays bending in a sky that is unusually heavy with moisture create strange and unsual light displays. Human beings have reported seeing such displays for hundreds of years.
Sometimes rings of light can be seen in the sky. The most common types of celestial rings are halos and coronas.
Halos, which are mostly seen in the winter, are created by the refraction (bending) of sunlight or moonlight by ice crystals floating high in the upper atmosphere.
Coronas may appear in an overcast summer sky. They are created when many tiny water droplets of uniform size, which are suspended in a thin cloud, are distorted by light waves.
Coronas may occur either singly around the Sun or Moon, or appear in sets.
Halos and coronas are usually almost colorless.
However, some of them can be very colorful, especially those that form around the Sun.
Smaller halos may possess vivid red interiors, changing to bluish white on their edges. The largest halos, if not pure white, show these same colors reversed, with the blue being on the inside.
Colored coronas are also blue on the inside.
The aureole, a type of poorly developed corona, ranges in color from bluish white on the inside to brown at its outer rim.
Rings around the Sun are usually less evident than those around the Moon, because the Sun's brilliance obscures them.
A haloed sky is overcast with high-altitude clouds made up of tiny ice crystals. Coronas appear in a thin lower cloud cover of suspended water droplets.
Coronas vary greatly in size, but are usually much smaller than halos.
There are variations on simple halos, some of which are extremely spectacular. The pseudohelion, or false sun, for example, is visible only from aloft. It appears as a reflection of the Sun on the upper surface of a horizontal bank of clouds.
At daybreak or dusk, the low-lying Sun will occasionally project a person's shadow against a distant cloudbank. This enormously enlarged shadow forms what is known as a Brocken specter. This name comes from a mountain in Germany where the phenomenon has frequently been witnessed.
An individual looking at his shadow on a cloud may see a series of from one to five colored rings around the shadow's head, called a glory or Ulloa's halo, after the 18th century Spanish astronomer Antonio de Ulloa.
These circles are formed, much like coronas, by the diffraction of light rays in the cloud.
The same effect can often be seen by passengers in an airplane flying between the Sun and a cloud. The plane's shadow is usually surrounded by at least one colored ring.
Bouguer's halo, named for the 18th century French mathematician and astronomer Pierre Bouguer, is a large ring of light hanging in the sky opposite the Sun. It is colorless, and often called a white rainbow, because, like a rainbow, it appears in the sky opposite the Sun.
This halo is also caused by light reflected from ice crystals suspended in the atmosphere.
On still nights, strange vertical shafts of light often rise above the horizon. These light pillars may be caused by artificial lighting. When flat snow crystals float in the air parallel to the Earth's surface, lights from the ground are reflected back from the surfaces of the snowflakes, causing shafts of light to appear in the sky. The beams may be either white or colored, depending on the color of the light source on the Earth's surface.
Sometimes when the Sun is on the horizon a mock sun will appear just above it, occasionally with a sun pillar projecting from the mock sun high into the dry.
First recorded by the German astronomer Hevelius on April 10, 1682, this phenomenon has since been attributed to the reflection and refraction of the Sun's light by airborne ice crystals.
When the Sun is slightly above the horizon, mock suns may appear both above and below it, with sun pillars extending both upward and downward.
The refracting angle of the ice crystals in the air determines the radius of the halo that appears around the Sun or Moon.
Most commonly seen is the 22 degree halo, whose arc extends 22 degrees on either side of a line between the observer and the center of the Sun.
A 46 degree halo, also known as the Great Ring, is seen less often and has rarely been measured.
Another halo, the Halo of Hevelius, has a 90 degree radius. It is very rarely seen.
Unusual atmospheric conditions can create extremely complex halo patterns.
Both 22 degree and 46 degree halos may form around the Sun at the same time, each shimmering with color.
In contact with these halos, bright arcs of light may appear as if drawn tangent to the halos with a compass.
Passing through the true Sun, parallel to the horizon, there is often a white, luminous band - the parhelic circle, along which brilliant secondary images of the Sun are formed.
Technically known as parhelia, these images are more commonly they known as sundogs or mock suns.
The most brilliant mock suns or sundogs occur on the parhelic circle at or just outside the 22 degree halo. Their distance from the halo increases as the Sun rises higher in the sky.
Moondogs or mock moons, technically known as paraselenae, have also been observed.
Although sundogs and moondogs are occasionally seen toward sunset in winter, they occur more often on a chilly dawn.
They are commonplace in the Arctic and Antarctic.
As the Sun rises, the sundogs move out of their halos and assume a cometlike appearance. By noon, the sundogs have usually disappeared, leaving the true Sun to complete its course through the sky alone.
Sundogs, moondogs and sun pillars are created by the refraction and reflection of light rays from atmospheric ice crystals that have a particular orientation with respect to the Earth's surface.
The parhelic circle and ordinary sundogs are produced when the Sun's rays strike the prismatic faces of hexagonal ice crystals floating in a vertical position in still air.
The white parhelic light is reflected from the crystals, while the sundogs are created by refracted light.
Sundogs that lie above and below the real Sun, vertical pillars through the Sun and tangential arcs are caused by refractions from those ice crystals that are horizontally oriented.
The brightness and distinctness of the sundogs depend upon the number and arrangement of the ice crystals in the air.
Sundogs are often reddish in color on the side toward the Sun and may appear elongated where they intersect the parhelic circle.
In rare cases, a 22 degree sundog may also be surrounded by its own halo, a portion of which passes through or near the mock image itself.
Solar halos are most common during the spring in the Northern Hemisphere, particularly in March.
In the Southern Hemisphere, solar halos are most common in late autumn.
Lunar halos are most often seen in the Northern Hemisphere during January.
Some solar halos have persisted as long as ten hours, but on the average, both solar and lunar halos remain visible in the sky for less than two hours.
The appearance of clouds that produce coronas is often a sign that it will rain. The saying, "the moon with a circle brings water in her beak" reflects this, as does the Native American proverb: "When the Sun is in his house [corona], it will rain soon."
The high cloud cover that causes halos often precedes bad winter weather.
The cross that the Emperor Constantine reportedly saw in the sky around 312, after which he converted to Christianity, may have been a complex halo display.
A parhelic circle, with pillars above and below the Sun, would form a cross that appeared to be on fire.