Sat, 04 Jun 2016
Strange northern clouds
Noctilucent cloud in 2014
The first noctilucent clouds (NLC) of the season have been reported in the past few days, and a major display was noted from norrthern Britain on 2 June. Noctilucent clouds are, as their name suggests (nocti – night, lucent – shining) clouds which glow in the night, and they only appear in the northern hemisphere's sky between the end of May and the end of July. NLC are basically icy clouds which form in the highest regions of the atmosphere, almost on the edge of space – and it's their extreme height which makes them visible. Up there they remain bathed in sunlight long after darkness has fallen on the ground, and ordinary clouds are no longer illuminated, so they shine and glow in the night sky.
Most NLC displays are modest affairs – we see a few bands and streaks of blue-white light in the northern sky, just above the horizon for a couple of hours before they fade away – but during a major display the northern sky comes alive with a beautiful splash of electric blue streamers, curls, whirls and tendrils.
If you're thinking "that sounds like the northern lights!" there's a big difference between the aurora and NLC. The aurora can move noticeably to the naked eye from moment to moment, as the beams and pillars come and go. The NLC move too, but very, very slowly, over the course of minutes instead of seconds, and are usually blue rather than green or red. Don’t confuse them either with ordinary cirrus cloud. This may look superficially similar after sunset, but appears earlier in the evening.
To see NLC, keep an eye on the sky above the northern horizon from around 11.30 pm on every clear night between now and the end of July. If you're lucky, you'll see what look like very pale streaks or puffs of blue–white in that direction as soon as darkness is falling, but you might not see anything until well after midnight. In fact, you might not see anything at all, because NLC don't appear every night, which is very frustrating! Unlike the aurora we can't predict when a display of NLC will occur.
What will you see? Well, most displays just look pretty – some bright streaks of cloud above the horizon – but a major display is a sight you will never forget. The northern sky can be painted with bright NLC for hours, looking like a special effect from a science-fiction film. If you're lucky enough to see one of those it will hypnotise and entrance you.
Unlike 2014 which was a fantastic year for NLC, with several incredible displays (such as that at the top, taken by SPA Comet Section Director Stuart Atkinson from Kendal in Cumbria), last year's season was very disappointing, with no really bright displays, so we're crossing our fingers for a better year this year!
One last thing to bear in mind – even if your sky is only partly clear it's still worth looking out for NLC because they will be visible through the gaps, with the normal clouds silhouetted against them.
Reports and photos of NLC should be submitted to the Aurora Section Director.
Alan Clitherow – 36,000 ft above Manchester on 2 June at 22.31 UT. The bright star is Capella, which is often to be seen in NLC photos as it is low in the north at this time of year
Added by: Robin Scagell