In message , Padraig
writes
and yet this strange sort of rainbow

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You get that pretty spectrum effect in thin clouds reasonably often at
high latitudes. It's caused by tiny ice crystals all pointing the same
way. They're one of the rewards of cycling, people in cars often can't
see them.

Detailed science and some world-class pics he

http://www.atoptics.co.uk/highsky/psc1.htm

--
Sue ]