Copy of The clouds that glow in the dark


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:

“It was the best of times, it was the worst of times, it was the age of wisdom, it was the age of foolishness, it was the epoch of belief, it was the epoch of incredulity, it was the season of Light, it was the season of Darkness, it was the spring of hope, it was the winter of despair, we had everything before us, we had nothing before us, we were all going direct to Heaven, we were all going direct the other way. ”

There are natural paradoxes too, of course. And probably none quite so beautiful as the enigmatic night sky visitors of May to August - the noctilucent clouds.

In this eBook, I want to introduce you to the wonders just waiting to be discovered over the next few months, revealing something of the wonder of the science behind them, and the beauty of the displays they put on.

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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 formation of noctilucent clouds

Noctilucent clouds form in the mesosphere, at the furthest edge of our atmosphere, around 80-100 km high. Too high for weather balloons, too low for satellites, this is a strange and mysterious place. The perfect place for paradoxes to dwell…


Source: NASA

 Clouds need particles for the water vapour to condense around (condensation nuclei, as they're called in the business). Scientists believe that these tiny specs of dust that act as seeds for the clouds have been deposited there by meteors as they burn up on entry to our atmosphere. Bits of cosmic rock that have been travelling around the solar system for billions of years, burning out in one last glorious fiery display, have one final trick up their sleeves, so it would seem.

Then, in June and July, when the mesosphere is at its coolest, water condenses around these nuclei to create the ice crystal clouds (when the temperatures are warmer, the extremely low pressure there does not allow the ice crystals to form). The clouds that form create the most wonderful of patterns: tendrils here, herringbone ripples there - quite unlike any other cloud you will see in the sky.

 Why do noctilucent clouds glow in the dark?

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But they are also incredibly thin and ephemeral. So translucent are they that they are invisible during the day, washed out by the overwhelming brightness of the sun (it is estimated that they only reflect 1000th of the light they receive). We can only see them at night, when the skies are dark. How then do they shine?

By the sun - shining from below them! The sun, long since set below the horizon, is now shining upwards from the other side of the world. And these clouds are so high in the sky that the sun lights them from below. It's as if the sun itself is so impressed by this last hurrah of ancient meteor dust that it just has to shine its spotlight on this wonderful display as if to say: "Here - look at this!"

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The structure and forms of noctilucent clouds


If you see these clouds, one of the first things you will notice is the often intricate patterns them form. Often described as a herringbone pattern, they form wispy tendrils, weaving throughout the night sky.

 But, in fact, if you shoot a timelapse video of the NLCs, you will see just how dynamic they are. It’s not even so much that the clouds themselves move - it’s more that ripples seem to pass through them, almost as if waves were crossing over them.

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 And that’s exactly what is going on. These features are produced by the effect that gravity waves are having on the clouds. The most common example of gravity waves are the waves formed on the sea by the wind. They form when the fluid is disturbed from a position of equilibrium. The wave initially moves in one direction, and gravity acts to pull it back towards its starting position - but it ‘overshoots’ and begins the wave-like undulations we see on the ocean.

 Such gravity waves can also form in the atmosphere. For example, when air is displaced upwards over a mountain, the column of air above the mountain is displaced upwards, knocking the air out of its equilibrium state and producing the oscillating gravity waves. As these waves move up into the upper atmosphere, up to where the NLCs are found, they can cause oscillations there which produce wave-like structures in the noctilucent clouds.

 In fact, these gravity waves are the same thing that is responsible for producing the patterns and undulations in air glow (a night glow display that can be seen during the darker months - see my previous eBook, ‘Not so dark after all’ for more information).


But you don’t need to go to the highest reaches of the atmosphere to see them in action. Lower down, in the troposphere, when you get a band of cloud with another band of air above it moving at different speeds, the upper band can disturb the lower band, settling up the wave undulations known as Kelvin-Helmholtz clouds, themselves a beautiful sight to behold.


Photographing noctilucent clouds

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One of the great charms of watching NLCs is that they are a good example of WYSYG (what you see you get). They look to the eye exactly how they look in these pictures. That, combined with the fact that at this time of year it never gets fully dark, makes it easier to photograph that, say, a faint aurora on a moonless night in the middle of winter.

 As far as camera settings go, here are a few guidelines to start you off.

  • Lens: usually, I would recommend ultra wide lenses for astro and night photography, to be able to take in as much of the night sky as possible. But, unless the NLC display is a very big one, it’s likely to be concentrated in one part of the night sky, close to the horizon. So, you could experiment with a longer lens to take in a bit more detail from the clouds.

  • Tripod

  • Aperture: open your aperture up close to its widest setting.

  • Shutter speed - this will depend partly on your focal length. An ultra wide lens will go up to 30 seconds; a longer focal length will have a shorter time. When photographing stars, we use the 500 rule to determine the maximum shutter speed for a focal length. In the case of NLCs, there are likely to be few stars out, so you have a bit more flexibility.

  • ISO: set this to get a good exposure following on from setting your aperture and shutter speed. A strong NLC display can be very bright - so expose for the highlights and make sure you don’t blow their exposure when taking your shots.

  • Set your camera with an intervelometer and shoot some timelapse footage. NLCs are very dynamic indeed, and a short timelapse video can help you capture something of that.

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Alistair HamillComment