<p>Dieter,</p>
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<p>Wikipedia claims it can be witnessed 100 days per year - but I can't recall having seen such a halo before.</p>
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<p>You need to move to Antarctica! The same physics gives rise to the "Moon Halo", which is perhaps somewhat more familiar. Don't forget the astronomers' favourite - noctilucent clouds, which are also caused by ice, but much higher up and which remain something of a mystery.</p>
<p>Nice capture - they really aren't that common in more temperate regions.</p>