 Halo around the sun at the South Pole (NOAA) Halos are optical phenomena that appear near or around the Sun or Moon, and sometimes near other strong light sources such as street lights. There are many types of 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 they are also 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.
Atmospheric phenomena such as halos were used as an empirical means of weather forecasting before meteorology was developed.
Halos can also have unusual shapes, for example a cross. Emperor Constantine I of the Roman Empire is said to have seen such a halo in 313 near Trier. This sign is supposed to have prompted him to become a Christian.
 Halo with sun dogs visible to the left and right (NOAA) Sun dogs, also known as a parhelia (single parhelion), appear as near-horizontal colored spots or bars on both sides of the sun, at nearly a 22 degree angle. Sun dogs are uncommon and typically appear only when a low sun shines through loose cirrus clouds, e.g. in a milky-white winter afternoon sky. The orientation of the ice crystals involved in this process is important. The crystals are hexagonal cylinders, and they have to be oriented vertically.
When the sun dog phenomenon is seen around the Moon rather than the Sun, it is called a mock moon, moon dog, or by the proper name paraselene.
 Sun pillar near Harpers Ferry, West Virginia (NOAA) A sun pillar appears most often 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 generally closer to the light source, crystal orientation matters less in the formation of these pillars.
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