Noctilucent clouds are back

Noctilucent cloud (NLC) season is here again following a splendid display on Tuesday evening.

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.