Noctilucent clouds are back

BY KEITH COOPER

ASTRONOMY NOW

Posted: 16 June, 2009

As previewed in our Weekly Guide to the Night Sky, noctilucent cloud (NLC) season is here again following a splendid display on Tuesday evening.

Noctilucent clouds seen in the evening of 16 June, imaged by Nik Szymanek.

NLCs are thin sheets of cloud 82 kilometres high that reflect sunlight when the Sun is between six and sixteen degrees below the horizon. You can tell them apart from other, more normal clouds by their often pearly-blue colour, and ribbed and interwoven appearance. Other clouds at lower altitudes will be in darkness whilst NLCs glimmer high overhead.

The best months for NLCs in the UK are June and July, and they seem to favour lower sunspot activity (which limits the solar heating on the upper atmosphere where they form) so at least the presently dull Sun offers something in exchange for its lack of activity! Even their origin is a mystery, as clouds shouldn’t ordinarily be able to form so high up.

Noctilucent clouds over Stonehenge on 16 June, imaged by Grant Privett with a 60-second exposure using a Lumix DMC-FZ7 camera.

The best time to see NLCS is just after sunset, and they make for very picturesque photographs, as these examples of last night’s display show. If you spot any NLCs, we’d be keen to hear about your observations.

Email your pictures, saved as JPG, TIF or GIF files to gallery2009<at>astronomynow.com with observation and technical details such as exposure time, telescope and camera type.

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