Things that glow in the night
Now that the 2016 noctilucent cloud season has kicked off for me, I thought I'd publish a short blog in praise of these wonderful night glowing clouds.
The month of June means one thing for this avid low light photographer. It means staying up each night until at least 23.00, and periodically peering out the window towards the northern horizon. At this time of year, the glow of sunset hangs on tenaciously to the north. But it’s just above that, in the deep, rich cobalt blues of twilight, where I cast my gaze. I am looking for something in particular: hints of ghostly sky tendrils, glowing a silvery blue, wispy drapery shining brightly in the ever darkening sky. And what is this phenomenon I’m seeking? Noctilucent clouds.
I feel very defensive towards noctilucent clouds. At times, they seem to play the role of the aurora's frumpish sister. It's not uncommon for folks on social media in the middle of summer to yearn for the end of NLC season, the return to dark skies, and the chance of that spectacular view of the northern lights.
Don't get me wrong. I love a good aurora show as much as the next person. I just happen to love the short season of noctilucent displays.
Maybe it's something to do with the fact that during the never-ending summer twilight, when you're out chasing the NLCs, you can get away with wearing a t-shirt and light jacket, being as mild as it is.
Maybe it's something to do with the mesmerizing light displays they can put on, as their wave-like structures undulate and ripple across the twilight sky.
Maybe it's something to do with the fact that they are largely unappreciated, and that being into them is kind of like being into that cool band just before everyone else realises how cool they are and they go stratospheric - or should that be mesospheric (*meteorologists' pun alert)?!?
Maybe it's something to do with the mind-blowing nature of the phenomenon itself. These clouds are seeded by the dust of meteors, billions of years old, a kind of atmospheric last hurrah as they go out with a glorious visual bang. They are the highest clouds that form, found at about 85 km high, so thin and ephemeral that they can only be seen in the semi-darkness of twilight, when they are lit from below by the sun that has already set.
The conditions for the formation of these clouds are still not known for certain. Where does the water vapour come from? The mesosphere is exceedingly dry, a hundred million times drier than the Saraha desert. Theories suggest sources such as water vapour escaping up through gaps in the tropopause, before being wafted aloft by atmospheric gravity waves. The ice crystals from which they are formed are a tiny 1/10,000mm in diameter; but at the extremely low pressures of the mesosphere, ice will only form when temperatures are below -123°C. Paradoxically, the temperatures here are only this low during the summer months. Ice will also only form if it has nuclei around which to crystalise. In the lower atmosphere, this are often particles of dust from below. But this high, in the mesosphere, where does the dust come from? Some suggest that the dust may be the last remains of thousands of tiny meteroids, space rocks that burnt up quickly as they entered our upper atmosphere, scattering their last dusty residue across the skies of the mesosphere.
I love the paradoxical, mysterious nature of these clouds. It seems entirely fitting that clouds that glow at night should have an air of enigma about them.
So, I encourage you to join me in casting your eyes northwards as we come towards the midnight hour over the next few weeks. Unlike the aurora, noctilucent clouds are WYSIWYG - they look to the naked eye exactly as they do in photos. So, whether you’re a photographer or not, there is much to enjoy in this most wonderful of summer night time displays.