Albino rainbow?

Despite all the rain we’ve had lately, I haven’t seen many rainbows kicking around. While rainbows can be seen at any time of year, they are more common in the summer because of the convective nature of the rain. Afternoon pop-up showers allow for some sun to filter between the clouds and the sun’s rays produce the rainbow. But do we really need the rain? Not always… One afternoon last week Bob Brownrigg noticed something interesting over the Halifax harbour: “The fog rolled in and produced this halo effect and disappeared as fast as it had appeared.”

It certainly is lovely Bob and not all that common. What Bob saw was not quite a halo but a fogbow! Back to rainbows for a moment, a rainbow’s appearance is caused by dispersion of sunlight as it goes through raindrops. The light is first refracted as it enters the surface of the raindrop, reflected off the back of the drop, and again refracted as it leaves the drop. The light, reflected back over a wide range of angles, produces a rainbow.

Fogbows are similar but instead of raindrops, we’re dealing with very small droplets of moisture suspended in the air. Because the droplets are so tiny, you won’t get the same separation when the light enters and exits so you don’t usually see any colour. That’s why a fogbow is often called a white rainbow. Having said that, if you look closely you might see very weak colours with a red outer edge and bluish inner edge. Thanks for asking Bob!

By the way, rainbows and fogbows are like vampires – they have no reflection and cast no shadow.