The ghost of a rainbow

I love driving on sunny mornings when a thin mist hangs in the air. As the low light of the early morning sun struggles to bring some warmth to the wakening dawn, the fog can act to diffuse the light, softening out harsh shadows and instead turning into ghostly apparitions. Features that otherwise would be sitting unobstrusively on the landscape take on a moody and atmospheric quality. Trees begin to fade out in the distance, their shadows cast in front of them but with an other-wordly ephemeral existence. Now that we're in to autumn, the cooler nights hold the promise of many such mornings like this over the coming months.

Low and moody autumnal light from a couple of year back.

Today had one such morning. As I drove around south Co. Antrim, I met many patches of low fog draped casually across the landscape. But, as I was driving around enjoying the view, something fleeting seemed to catch my eye in the sky. As I turned my head towards it, it was almost as if it went off into hiding, playing some atmospheric game of hide and seek. But when I looked away, I could swear I would see a hint of it again. It seemed to be a lighter band within the fog, arcing up and away from me.

Eventually, I felt I had to pull over to take a closer look. And, when I got out of the car, there is was in all it's glory. A feature I'd seen other people's photos of, but never yet witnessed to myself. It had an immediately recognisable curve, sweeping across the sky as only a rainbow would. But - there was no rain. And indeed there wasn't much by way of colour in this bow either. In keeping with the ghostly atmosphere in the misty morning, this ghoulish white bow hung like some transient ghost rainbow above the fog bank. This was my first viewing of a fog bow.

Fog bows are formed by similar processes to rainbows, as light from behind the viewer is refracted through droplets of water. The big difference is that, as the water droplets are in the mist, they are much smaller in size that raindrops and so the light is not refracted out in the well defined way a raindrop would, separating the colours for us to see. Rather, the refracted light overlaps, the constituent colours recombining to create a white light. Sometimes, as was the case with my fog bow, the outer edge has a slight red tinge, and the inner one, slight blue. Apart from that it hangs as a ghostly dome into the blue skies above.

So, if you're fortunate enough to be driving through the countryside this autumn, and you meet a patch of mist, keep an eye out for subtle arcs of light. You too may be treated to the gloriously ghoulish experience that is the white fog bow. 

Read about my other first 'bow experience when I saw a moonbow over Lough Neagh here.

The shy rainbow