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Most people are unaware of it, but several times each month, colourful pillars, arcs, rings and patches of light make their appearance in the sky. These displays are called halos. They are an optical phenomenon produced when light from the Sun or the Moon reflects and refracts off tiny ice-crystals in thin high-level clouds. Certain halos bear a strong resemblance to rainbows which, despite the fact that everyone is familiar with them, occur less frequently (contrary to halo phenomena, rainbows are caused by sunlight reflecting not off ice-crystals, but from inside spherical water droplets). The tiny six-sided crystals that cause halo displays usually form at high altitudes in cold cirriform clouds (i.e. clouds from the cirrus family, notably cirrostratus, cirrus plumes and dispersed contrails).

One of the most common halo types is the 22° halo. A whitish ring around the Sun with a radius of 22 degrees, often with a reddish tinge at the inner edge. Another often observed halo phenomenon are the sun dogs, also known as mock suns or parhelia. These are white or rainbow coloured patches of light, 22 degrees to the left or right of the Sun. Next to 22° halos and sun dogs, there exists a wide variety of less common halo types, some of which are extremely rare and occur almost exclusively in cold climates, where ice clouds can also form close to the ground (many of the most stunning halo images come from Finland and Alaska).

Halos seldom give rise to UFO reports, and if they do, it is usually because an overzealous ufologist interpreted a witness's description in a rather personal and biased manner.