HALO (Icebow)

A halo (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 (3–6 miles) in the upper troposphere. 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 -- see link to a previous post about sun dogs link). 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.

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