Halo (optical phenomenon)
From Wikipedia, the free encyclopedia
- For other uses, see Halo (disambiguation).
A halo (Greek: ἅλως; also known as a nimbus or Gloriole) is a ring of light that surrounds an object.
Halos, also known as icebows, are also 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 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.
Atmospheric phenomena such as halos were used as an empirical means of weather forecasting before meteorology was developed.
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[edit] Sun dogs
Sun dogs, also known as parhelia (single parhelion), appear as colored spots or bars on both sides of the sun and at the same altitude. Their distance from the sun is close to 22° when it is low but they increase their distance as the altitude of the sun increases. Sun dogs are almost as common as the circular 22° halo and Europe and middle latitudes can be seen 2-3 times each week on average although obvious and very bright sundogs occur less frequently. They are produced when sunlight passes through cirrus clouds or, in very cold weather, when ice crystals are in the nearby air. Sundog forming ice crystals are hexagonal plates which drift downwards with their large hexagonal faces nearly horizontal.
When the sun dog phenomenon is seen around the Moon rather than the Sun, it is called a moon dog, or by the Latin name paraselene.
[edit] Light pillar
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 generally closer to the light source, crystal orientation matters less in the formation of these pillars.
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