Noctilucent clouds...
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"....2013 is shaping up to be a great year for NLCs. The clouds surprised researchers by appearing early this year, and many bright displays have already been recorded. Once confined to the Arctic, NLCs have been sighted in recent years as far south as Utah, Colorado, and Nebraska. They might spread even farther south in 2013.
Observing tips: Look west 30 to 60 minutes after sunset when the sun has dipped 6o to 16o below the horizon. If you see luminous blue-white tendrils spreading across the sky, you've probably spotted a noctilucent cloud."
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Tadas JanuĊĦonis photographed this display on July 18th from Vabalninkas in the Birzai district of Lithuania: