I Finally Saw A Monochrome Rainbow

I was out running some last-minute errands before the nor’easter hit Boston when I noticed the sky looked unusually pink. It had this strange, almost unreal glow to it. Then something caught my eye in my car’s side window.

A rainbow.

But not the kind you normally see. This one was almost entirely pink and red. A monochrome rainbow.

I’d heard of them before, and I’d read that they can look a little eerie. I’d never actually seen one, though. In person, it wasn’t eerie at all. It was beautiful. I pulled over, took a few photos, and just stood there looking at it for a while.

So how does it happen?

It comes down to the Sun being very low in the sky.

When sunlight travels through a lot of atmosphere, the shorter wavelengths, like blue and violet, get scattered more strongly. By the time the light reaches the raindrops, there’s more red and orange light left.

The raindrops then do their usual rainbow thing, bending and reflecting the light. The result can be a rainbow that looks almost completely red or pink.

There’s another cool part. The Sun can be below the horizon where you’re standing but still be high enough to illuminate raindrops farther up in the atmosphere. So you can sometimes see one just after sunset.

A pretty simple bit of physics, but a pretty spectacular thing to see!