Weather Pictures and Storm Chasing
Halo: a ring of light surrounding the sun or moon. Most halos appear as bright white rings but in some instances, the dispersion of light as it passes through ice crystals found in upper level cirrus clouds can cause a halo to have color.
Halos form when light from the sun or moon is refracted by ice crystals associated with thin, high-level clouds (like cirrostratus clouds). A 22 degree halo is a ring of light 22 degrees from the sun (or moon) and is the most common type of halo observed and is formed by hexagonal ice crystals with diameters less than 20.5 micrometers. A 46 degree halo is a ring of light observed 46 degrees from the sun or moon. Although they are less common than 22 degree halos, the process by which they form is similar. What determines if a 46 degree halo or a 22 degree halo develops is the path of the light as it passes through hexagonal ice crystals. A 22 degree halo results from "in one side, out another side"; a 46 degree halo from "in one side, out the bottom".
Sundog(s): also known as mock sun or "parhelia", are a pair of brightly colored spots, one on either side of the sun. Sundogs form as sunlight is refracted by hexagonal plate-like ice crystals with diameters larger than 30 micrometers and their flat faces horizontally oriented. Sundogs are visible when the sun is near the horizon and on the same horizontal plane as the observer and the ice crystals. As sunlight passes through the ice crystals, it is bent by 22 degrees before reaching our eyes, much like what happens with 22 degree halos. This bending of light results in the formation of a sundog.
Parhelic Circle: look further out from the sun, beyond the sundogs. There is often a white band at sun's altitude, so white that it is sometimes easy to mistake it for cloud.
Tangent arc is a patch of bright light that is occasionally observed along a halo. This occurs when sunlight is refracted by falling hexagonal "pencil-shaped" ice crystals whose long axes are oriented horizontally.
Circumzenithal arc is the most beautiful of all the halos. The first sighting is always a surprise, an ethereal rainbow fled from its watery origins and wrapped improbably about the zenith. It is often described as an "upside down rainbow" by first timers.
Circumhorizontal arc Look for a circumhorizontal arc near to noon near to the summer solstice when the sun is very high in the sky (higher than 58°). It lies well below the sun - twice as far from it (two hand spans) as the 22º halo. The arc is a very large halo and is close to, and parallel to the horizon. Usually only fragments are visible where there happen to be cirrus clouds.
Sun pillar: a vertical shaft of light extending upward or downward from the sun. Typically seen during sunrise or sunset, sun pillars form when sunlight reflects off the surfaces of falling ice crystals associated with thin, high-level clouds (like cirrostratus clouds). When the sun is low on the horizon, an area of brightness appears in the sky above (or below) the sun as sunlight is reflected off the surfaces of these tipped ice crystals.
Subsun: the most common subhorizon halo. It is a direct reflection of the sun from millions of cloud crystals acting together as a giant mirror. Plate crystals are the usual source. Rays can reflect externally off the uppermost horizontal face or internally from the lowermost.
My most impressing displays
June 24 2008, Utrecht NL:
May 15 2008, Agios Gordis, Corfu GR:
June 25 2005, Soest NL:
April 29 2004, near Emmen NL:
November 3 2002, over southern Belgium:
More information about halos (external links)
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