There’s a particular photo that most of us who do astrophotography can’t resist. At some point in our nocturnal wanderings, we will give this shot a go. It is the classic; perhaps even the cliche. But, still, we can’t resist the urge to add it to our repertoire. Is it the night time selfie - with a torch!
There we’ll stand, on some promontory, silhouetted against the starry sky, shining our torch up into the heavens above. The powerful beam of the torch reaches up into the sky, vying for attention against the vast array of stellar wonders all around, before gradually fading out high above, lost in the cosmic majesty of the universe above.
There is something of the scale of this one tiny human being and one single torch beam set against the vast, unimaginable distances of space that make this kind of image just so magical.
Enter the sun pillar!
But, as ever, nature can do one better. This time, it’s not the night sky that is the host to this light phenomenon, but rather the last few moments of sunset. And this time, it’s no mere torch that produces the column of light, but none other than our Sun itself.
For sometimes, when the conditions are right, as the sun slips down towards the horizon, painting the skies with warm washes of orange, you can discern a pillar of light, coming up vertically above the Sun, pushing far up into the twilight skies above. This is called - perhaps a bit unimaginatively - a sun pillar.
But the dull name belies a tremendous beauty. And a fascinating explanation as to how it forms. For these pillars are not the result of someone running off to the horizon with a big, powerful torch and shining it upwards.
Rather, as with most atmospheric optics, it begins with water. Or, more precisely, ice (and so are part of the family of halos that can form around the Sun and Mon).
And so it was, as I was wandering along the cliff tops at the Giant’s Causeway the other night, that I saw my latest sun pillar. The skies were mostly clear, but strands of broken cirrostratus cloud hung over the horizon. These clouds are some of the highest clouds that form, and are made up of ice crystals. As the Sun was setting, the light was passing through the cloud and being reflected by the ice crystals to produce the sun pillar. But how exactly?
The ice crystals in the cirrus clouds are hexagonal, plate-like in shape. And, if they are orientated horizontally, they basically act like millions of little mirrors. As the light from the Sun spreads out and heads towards us, some of it travels diagonally upwards above our heads. But that’s where the ice crystals come in. Their shape and orientation reflects this light back down towards us. And it’s this that causes the apparent pillar above the Sun.
Things are not always as them seem...
I say apparent pillar, because the feature itself is actually an optical illusion. The pillar of light is not like the torch light, which forms an actual pillar above the light source. Rather, its the collective reflection off millions of ice crystals that appear to lie in a vertical line above the Sun. These diagrams may help explain things (click or tap on the arrows to move through the images).
The horizontally orientated ice crystals can be jiggled around a bit by air currents, causing the reflection of light to become elongated into the full pillar of light. And this pillar can be seen even after the sun has set, as its light shines upwards from below the horizon and interacts with the ice crystals.
Nocuturnal light pillars
In fact, this same set of processes can produce other kinds of light pillars too. For us in Ireland, the ice crystals needed for this optical illusion are found typically in the high cirrus clouds. But, in places that are much colder, the ice crystals can form at ground level and can be wafted about, suspended in the air turbulence. These have the much more exciting name of ‘diamond dust’. When this happens, the light sources that are reflected include artificial lights.
This image from Dar Tanner from Alberta, Canada shows the amazing light displays that can be seen when this occurs. These are pillars are formed by reflection of street light, creating these eerie pillars of light reaching up into the night sky. The temperatures that night were below minus 32 C (not including the wind chill!), cold enough for the ice crystals to dance about in the night sky. In fact, Dar tells me that you can see the ice crystals swirling about in the light pillar, very similar to how you can see the tiny water droplets squirm around in fog at night when a light source is shining through them.
Perhaps one day I’ll be lucky enough to combine my artificial light beam with the natural light pillars and stand in front of a scene like this with my torch. I can but dream...