Halo (optical phenomenon)

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 are elesewhere and even in the opposite part of the sky. They can also form around artificial lights in very cold weather when ice crystals called are diamond dust 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 jewels, 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.

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

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.

Halo over the Kluane Range viewed from the Alaska Highway

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.

A noontime Halo that was visible in Tacoma, WA, on June 19, 2007
Halo and sundog with a brilliant larger arc
Image:Solar-halo.jpg
Subsun
A sun halo seen over a stand of trees on the edge of the Alberta Legislature Grounds in Edmonton, Alberta, Canada.
22° halo around sun visible in Roscoff, France on June 15, 2008
Complex halo display at South Pole
Complex Moon halo. You could see Halo, Parry arc, upper tangent arc and Moon dogs
Circumhorizontal arc photographed in Ravenna, Michigan on May 13th, 2008
Sun Halo appeared in Medan, Indonesia September 22 2008 12:33PM
halo around sun visible in Medan, Indonesia on September 22, 2008
halo surround the sun in Medan, Indonesia September 22, 2008 with red eyes passing by.
Solar Halo, Upper Tangent Arc, Sun Dog, and Parhelic Circle : 21 June 2009 : Khonkaen, Thailand
sun Halo seen in Chennai,India on July 09,2009(forenoon).

External links

Light pillar

Circular halo

See also

External links