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Halos are due to randomly oriented ice crystals less that 20 microns in diameter that are abundant in the atmosphere.

http://www.physics.isu.edu/weather/kmdbbd/notesc4.pdf

Halos - appear with ice clouds. Crystals have to be small 15 or 20 microns. The 'rainbows' thrown by lead crystals are more similar to sundogs then rainbows and are more. Sun dogs are generated when light interacts with hexagonal plate like ice crystals that are approximately 30 microns in size and fall horizontally.

http://cimss.ssec.wisc.edu/wxwise/class/optics.html

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.

http://www.weatherpictures.nl/halo.html

Ice Fog (Also called ice-crystal fog, frozen fog, frost fog, frost flakes, air hoar, rime fog, pogonip.) A type of fog, composed of suspended particles of ice; partly ice crystals 20 to 100 micron in diameter, but chiefly (especially when dense) ice particles about 12–20 micron in diameter, formed by direct freezing of supercooled water droplets with little growth directly from the vapor. It occurs at very low temperatures, and usually in clear, calm w

http://www.weather.gov/glossary/index.php?letter=i

it’s orientation is crucial as well. When ice crystals are very small (<100 microns or so) they are oriented more or less in any old direction in space,

http://www.met.reading.ac.uk/Data/CurrentWeather/wcd/blog/the-parry-arc/

A ring of light 22 degrees from the moon (or sun) is the most common type of halo observed from earth and is formed by millions of hexagonal ice crystals with diameters less than 20.5 micrometers within the clouds.

Read more: Halo Moon — Infoplease.com http://www.infoplease.com/askeds/halo-moon.html#ixzz1Dn7YuOoE

http://www.infoplease.com/askeds/halo-moon.html

Electron microscopy of etched and nascent surfaces of single ice crystals shows a characteristic microstructure of elongated, hexagonal prisms. The small hexagonal units vary greatly in size with an expected width of the order of 3 microns and a length of 6 microns.

http://ieeexplore.ieee.org/Xplore/login.jsp?url=http%3A%2F%2Fieeexplore.ieee.org%2Fiel5%2F4915369%2F5121058%2F05121063.pdf%3Farnumber%3D5121063&authDecision=-203

Ice crystals found in the cirrus-family of clouds that form halos are usually hexagonally shaped columns, rather similar to the shape of a common lead pencil but much smaller (around 15-25 micrometres in diameter).