Halo is a ring, an arc or a spot of light around the sun, the moon or an artificial light source. Halo can be white or colored. The most common halo form what observer can see is 22° halo around sun which inner edge is usually clearly reddish. An other usually seen halo is sun pillar which is a pillar of light often seen upward from setting sun.

When and where I can see halos?
 

You can see halos more often than you might expect. In Finland one can see halos in more than 150 days in a year. In Finland and in other middle or low latitude places halos are usually in cirrus clouds which usually are at altitude 7-12 km. In the northern Scandinavia, where I live, the best season is the Spring time. In other parts of the World there is some differences which is the best season for halos. You can see halos also in middle level clouds but not so often and not so complex displays.

In Winter in Finland and specially in Antarctic and in Arctic or in other cold places halo producing ice crystals can be laying around an observer. Sometimes this situation occurs also in mountain regions like at the Alps. These are so called diamond dust displays. These displays can be very strong and very beautiful. No wonder why scientist and hard-core observers like to visit those cold places. 

Physical background for halos

Halo phenomena are produced by sun or moon light which is reflected or refracted thought airborne ice crystals. Shape of the ice crystals, orientation of the ice crystals, elevation of light source, ray path of light through ice crystal and laws of aerodynamics  are main parameters which define halo forms. 

These variable parameters make all different halo forms possible. About 60 different halo forms have been photographed today. There is some theoretical halo forms which are not seen yet and several halo candidates from the past which existence are still unclear. 

Although halo theory is old, we do not know origin of all halos yet. Some new halos have been observed lately, so... Happy hunting!

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