Sky watchers in Europe should be alert for volcanic sunsets. "For the past week, we've seen unusual twilight rays probably caused by high-attitude aerosols from Nabro, a volcano which erupted in Eritrea on June 13th," reports Petr Horalek from the Ondřejov Observatory of the Astronomical Institute of the Academy of Sciences of the Czech Republic. This is how the sky looked on Aug. 23rd:

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© Petr Horalek
"Around 20 minutes after sunset these significant crepuscular rays appeared like shining fingers in the western sky," he describes. "The rays were so strong, I could see them almost directly overhead; and in the south, they stretched across the horizon like great red and purple stripes."

Purple is one of the telltale colors of a volcanic sunset. Fine volcanic aerosols in the stratosphere scatter blue light which, when mixed with ordinary sunset red, produces a violet hue. Another set of photos taken last night by Martin Popek in the Czech city of Nýdek highlights the purple signature.

More examples of volcanic sunsets may be found here and here.