With all the understandable excitement in Northern Ireland about all things aurora over the past few months, it's worth remembering that the aurora is not the only stunning night time light show to which we may be treated. Enter noctilucent clouds, the aurora's lesser know distant cousin. As we come toward the time of the year when these enigmatic clouds make an appearance again, here's a bit of a story about them and one display I saw at Portrush last June.
There is something strangely compelling about the paradox. That weird combination of aspects that clearly don't belong together but yet are held so in some ongoing, seemingly unresolved tension. One of the best examples of this was surely penned by Charles Dickens, who opened his novel with these most famous of words:
Such paradoxes can exist naturally too, of course, and last weekend I was privileged to be able to spend an hour and a half in the wee small hours of the night basking in one such display: the clouds that shine at night. Hanging over West Bay at Portush was the most glorious display of noctilucent clouds that I have ever seen, all on show in the most splendid of locations with the bay in the foreground, the lights of the sleeping town in the middle, and the still glowing embers of those most tenacious of June sunsets still tracing the line of the horizon hours after the sun had finally set. But it was the night-glowing clouds themselves that stole the show, their enigmatic glow drawing me in, capturing me in their compelling spell.
But how to explain this strange paradox? How is it that noctilucent clouds can glow bright all throughout the night?
These clouds are the highest that can form in our atmosphere, suspending some 50 miles high, in the the last few miles of what can properly be called our atmosphere and before space proper begins. At such elevations, the sun that has set below our horizon still shines obliquely up over the horizon, lighting these whispy tendrils from below.
The mystery still remains, however. For 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.
But we just don't know for certain. And that seems entirely appropriate, in a sense. Although the scientist in me wants to discover the explanations, the artist in me is actually quite happy that these enigmatic clouds that glow in the dark retain something of their air of mystery. After all, I like a good paradox as much as the next man.