Fire the crepuscular ray, Mr Sulu!

The non-sci-fi story behind a stunning North Coast meteorological display

The crepuscular display just after sunset

The display that decorated the sky in the hour before sunset

It seems to be an iconic staple of sci-fi movies. The crowds gather below, gazing at the sky. Above, an unusual cloud formation appears and moves across the sky, hinting of some extra-terrestrial craft of unimaginable proportions hidden inside. The music quickens; the tensions rise, all the way up to the inevitable reveal - we are, it turns out, not alone.

There was an atmospheric visitation at Portrush last night. Not of the inter-galactic variety this time. But rather we were treated to a wonderfully long crepuscular display, one of the best I've seen in quite some time. These distinctive rays form as the shafts of light scatter out from behind a puffy cumulus cloud or shine down through the gaps in a broken strato-cumulus layer of cloud. 

They have inspired awe ever since people first laid eyes on them, I'm sure. In the Renaissance period, religious artists used these crepuscular rays to signify the essence of the divine. And it's not hard to see why, as they emanate down from behind the clouds, a glorious sight and a reminder of the Divine eyes that are looking down upon us.

They are formed as the light passes through tiny droplets of water and dust in the sky, causing the light to be refracted into the distinctive columnar shapes, similar to the illuminated shards that form when light shines into a dusty room. The regions where the cloud is blocking the light appear as the darker bands between the rays themselves.

The rays appear to converge on the sun, of course. But if you were to trace the light rays back to their source, you would actually discover that this would place the sun a mere few thousand feet above the surface of our planet. Given how cold our August has been, this clearly cannot be the case. So what's going on?

It's actually the effect of perspective, similar to that you see if you look down a railway line. Although you know the lines are parallel, they appear to converge. Similarly with the crepuscular rays. They seem to rush to gather together just behind the clouds; in reality, this is just a trick of perspective, and the lines are more or less parallel.

As impressive as the first display was, little did I know that an even more impressive show was less than an hour away.

The stunning sunset crepuscular display

As the sun slipped further down behind the clouds, the light started shining upwards and was split into that distinctive upward crepuscular pattern. To the right, another amazing meteorlogical feature was found. It may have appeared that the oranges were the bottoms of the clouds being lit. But the very soft edges and wavy vertical bands that you can see in them reveal that this is something else. This is a feature known as virga and it occurs when precipitation falls out from the bottom of a cloud but, before it has a chance to reach the ground, it evaporates away. So what was being lit by the crepuscular rays was the rain precipitating from below the clouds, tossed around by the wind currents aloft, before it vanished into thin air right before my very eyes. 

To finish it all off, add in a sweet reflection from the sands of West Bay, mimicking the full on upwards and downwards crepuscular displays that can emanate out of the most impressive cumulus clouds.

Truly a stunning sight, and such an unusual one that it was captured by a variety of photographers who happened to be out and about at the time. A friend was at the Giant's Causeway at the time and he remarked that the photographers there, who only saw the virga and not the rays, were calling it the Mother Ship. 

But it wasn't a rerun of Independence Day. Neither, despite my imagination, was it anything to do with Star Trek. But it was another wonderful meteorological display that stirred the hearts and imaginations of those of us fortunate enough to witness it.