Halo: Information from Answers.com

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Sun Halo captured behind Semen Padang Main Office building, Padang, Indonesia, October 2rd, 2009 at 11:09 am. Captured by Hermawan Ardiyanto.

Halo around the sun at the South Pole featuring a parhelic circle, a 22° halo with two sundogs, and an upper tangent arc topped by a faint Parry arc.

A halo (ἅλως; also known as a nimbus, icebow or Gloriole) is an optical phenomenon produced by ice crystals creating colored or white arcs and spots in the sky. Many are near the sun or moon but others are elsewhere and even in the opposite part of the sky. They can also form around artificial lights in very cold weather when ice crystals called diamond dust are floating in the nearby air.

There are many types of ice halos. They are produced by the ice crystals in cirrus clouds 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. The crystals behave like prisms and mirrors, refracting and reflecting sunlight between their faces, sending shafts of light in particular directions.

Atmospheric phenomena such as halos were used as part of weather lore as an empirical means of weather forecasting before meteorology was developed.

Other common optical phenomena involving water droplets rather than ice crystals include the glory and the rainbow.

1 Light pillar
2 Circular halo
3 Gallery
4 See also
5 External links

Light pillar

Sun pillar in San Francisco.

Main article: 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 closer to the light source, crystal orientation matters less in the formation of these pillars.

Circular halo

22° halo around the Moon

Further information: 22° halo

A circular halo, often referred to as just a halo, is formed by the refraction of sunlight through cloud suspended ice crystals as opposed to raindrops or other liquid water suspended in the air. Sometimes only arc sections are visible as opposed to a full circle. Brighter sections can occur above, below, and lateral to the center (where the sun is visible). The lateral 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. The ones above and below the sun are the upper and lower tangent arcs, respectively. A 22 degree halo 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.

Gallery

A halo visible at noon in Tacoma, WA, June 19, 2007	Halo and sundog with a brilliant parhelic circle by NOAA, January 1979	Halo over the Kluane Range viewed from the Alaska Highway	Halo and subsun on December 14, 2005	22° halo around sun visible in Roscoff, France, June 15, 2008
A sun halo seen over a stand of trees on the edge of the Alberta Legislature Grounds in Edmonton, Alberta, Canada.	Complex halo display at the South Pole	Complex moon halo with halo, parry arc, upper tangent arc, and Moon dogs	Circumhorizontal arc photographed in Ravenna, Michigan on May 13, 2008	Halo around sun visible in Medan, Indonesia on September 22, 2008
Complex halo display around the sun visible in San Francisco on November 19, 2006	Halo surrounding the sun in Medan, Indonesia on September 22, 2008 with red eye passing by.	Solar halo, upper tangent arc, sun dog, and parhelic circle in Khonkaen, Thailand, June 21, 2009.	Sun Halo seen in Chennai, India on July 9, 2009	Sun halo viewed from Medan, Indonesia on September 22, 2008 at 12:33 pm.