AS I WRITE this we're almost two weeks into the 2017 Noctilucent Cloud season. By now we would have expected to see one, maybe even two curtain-raising displays of these eerily beautiful clouds. However, the season has got off to an unusually slow and boring start: there has yet to be a single noteworthy display. This is very disappointing, and troubling too; we're all crossing our fingers it's not a bad omen for the rest of the summer.

But it might be. Really impressive NLC displays used to happen a dozen or so times a year a decade ago, but they have been getting rarer in the past few years. The past two years have been really poor for noctilucent cloud watchers, with less than half a dozen good displays during the whole of 2015 and 2016. Some experts think it's another sign of global warming, and suggest that just as the seas and lower atmosphere are heating up, the upper atmosphere is heating up too, preventing NLC from forming.

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Whatever is going on, please keep an eye on the sky after midnight from now until the end of July. Sooner or later a big display of NLC will paint the northern sky with stunning streamers, curls and swirls of electric blue. You'll see them best from somewhere with a low and unobscured northern horizon, and if you see a modest display don't just give up in it and go indoors; all the most incredible displays I have seen over the years started off as barely a few wisps of silvery white above the horizon before blossoming into something stunning.

Stuart Atkinson