A halo (ἅλως; also known as a nimbus, icebow or Gloriole) is an optical phenomenon that appears near or around the Sun or Moon, and sometimes near other strong light sources such as street lights. There are many types of optical halos, but they are mostly caused by ice crystals in cold cirrus clouds located high (5–10 km, or 3–6 miles) in the upper troposphere. The particular shape and orientation of the crystals is responsible for the type of halo observed. Light is reflected and refracted by the ice crystals and may split up into colors because of dispersion, similarly to the rainbow.
Sometimes in very cold weather optical halos are formed by crystals close to ground level, called diamond dust. The crystals behave like jewels, refracting and reflecting sunlight between their faces, sending shafts of light in particular directions.
A light pillar, or sun pillar, appears as a vertical pillar or column of light rising from the sun near sunset or sunrise, though it can appear below the sun, particularly if the observer is at a high elevation or altitude. Hexagonal plate- and column-shaped ice crystals cause the phenomenon. Plate crystals generally cause pillars only when the sun is within 6 degrees of the horizon, or below it; column crystals can cause a pillar when the sun is as high as 20 degrees above the horizon. The crystals tend to orient themselves near-horizontally as they fall or float through the air, and the width and visibility of a sun pillar depends on crystal alignment.
Light pillars can also form around the moon, and around street lights or other bright lights. Pillars forming from ground-based light sources may appear much taller than those associated with the sun or moon. Since the observer is closer to the light source, crystal orientation matters less in the formation of these pillars.
An icebow is phenomenon similar to a rainbow except that it is formed by the refraction of sunlight through cloud suspended ice crystals as opposed to raindrops or other liquid water suspended in the air. Generally the appearance is as arc sections as opposed to a full circle. Brighter sections usually occur above, below, and lateral to the center (where the sun is visible). These bright areas are referred to as "sun dogs," "parhelia" (plural), or mock suns because of their bright appearance and possible confusion with the actual location of the sun. Those icebows that are caused by very small ice crystals are one colour, because diffraction blurs the colours together. A 22 degree icebow has red on the inside and blue on the outside.
A diffraction disc or Airy disc has similar appearance, but is a disk, rather than a ring, and has a red border on the inside. Its size depends on the size of the ice or water particles that cause it. These are also known as coronas, but are not to be confused with the thin streaming luminous gas that makes up the sun's own corona.