Have you ever seen a large ghostly disc around the Moon on a cool, calm, hazy night? If so, you have likely seen what is called an “Ice Halo” or “22° Halo.” Not only can the Moon display these ghostly rings of light, but the Sun does so in the day time too.
22° halos are visible all over the world and throughout the year; look for them whenever the sky is wispy or hazy with thin cirrus clouds – even in the hottest countries.
So what are they and why do they appear?
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Ice halos or 22° radius Halos are in fact an optical illusion caused by 3 to 5 mile high, cold and very tenuous cirrostratus cloud, containing millions of tiny ice crystals.
The tiny ice crystals in the atmosphere create halos by refracting and reflecting light from the Moon. The halo is always the same diameter regardless of its position in the sky, though sometimes only parts of the circle are visible.
The much smaller coloured rings directly around the Moon or Sun are a corona produced by water droplets rather than ice crystals. They often form a rainbow effect or Moonbow.
Some people even believe they herald the onset of wet weather, but this has yet to be proved.