Refraction, or a combination of refraction and reflection, of light by ice crystals in the atmosphere may cause a halo to appear. The most common form is Occasionally a faint, white circle with a radius of 90° appears around the Sun. This is called a Hevelian halo. It is probably caused by refraction and internal reflection of the Sun's light by bipyramidal ice crystals. A halo formed by refraction is usually faintly colored like a rainbow, with red nearest the celestial body, and blue farthest from it.

A brilliant rainbow-colored arc of about a quarter of a circle with its center at the zenith, and the bottom of the arc about 46° above the Sun, is called a circumzenithal arc. Red is on the outside of the arc, nearest the Sun. It is produced by the refraction and dispersion of the Sun's light striking the top of prismatic ice crystals in the atmosphere. It usually lasts for only about 5 minutes, but may be so brilliant as to be mistaken for an unusually bright rainbow. A similar arc formed 46° below the Sun, with red on the upper side, is called a circumhorizontal arc. Any arc tangent to a heliocentric halo (one surrounding the Sun) is called a tangent arc. As the Sun increases in elevation, such arcs tangent to the halo of 22° gradually bend their ends toward each other. If they meet, the elongated curve enclosing the circular halo is called a circumscribed halo. The inner edge is red.

A halo consisting of a faint, white circle through the Sun and parallel to the horizon is called a parhelic circle. A similar one through the Moon is called a paraselenic circle. They are produced by reflection of Sunlight or Moonlight from vertical faces of ice crystals.

A parhelion (plural: parhelia) is a form of halo consisting of an image of the Sun at the same altitude and some distance from it, usually 22°, but occasionally 46°.A similar phenomenon occurring at an angular distance of 120° (sometimes 90° or 140°) from the Sun is called a paranthelion. One at an angular distance of 180°, a rare occurrence, is called an anthelion, although this term is also used to refer to a luminous, colored ring or glory sometimes seen around the shadow of one's head on a cloud or fog bank. A parhelion is popularly called a mock Sun or Sun dog. Similar phenomena in relation to the Moon are called paraselene (popularly a mock Moon or Moon dog), parantiselene, and antiselene. The term parhelion should not be confused with perihelion, the orbital point nearest the Sun when the Sun is the center of attraction.

A Sun pillar is a glittering shaft of white or reddish light occasionally seen extending above and below the Sun, usually when the Sun is near the horizon. A phenomenon similar to a Sun pillar, but observed in connection with the Moon, is called aMoon pillar. A rare form of halo in which horizontal and vertical shafts of light intersect at the Sun is called a Sun cross. It is probably due to the simultaneous occurrence of a Sun pillar and a parhelic circle.

• Zone Time In Celestial Navigation - 2009-04-21 - 01:09:16
• Some common local winds are listed by local name - 2009-04-21 - 12:35:43
• Wind Patterns - Modifications of the General Circulation - 2009-04-21 - 05:46:03
• Maneuvering to Avoid the Storm Center - 2009-04-21 - 05:03:03
• Weather Routing - Benefits - 2009-04-19 - 07:35:50
• Weather Routing - Communications - 2009-04-19 - 07:25:17
• Weather Routing - Southern Hemisphere Routing - 2009-04-19 - 07:21:29
• Weather Routing - Types of Recommendations and Advisories - 2009-04-19 - 07:16:12
• Special Weather and Environmental Considerations - 2009-04-19 - 07:06:59
• Weather Routing - Synoptic Weather Considerations - 2009-04-19 - 06:15:22

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