Fog bow

A couple of years ago, a mate and I went out on a birding trip. We were after Bustards (we probably dipped - long story) and Red-lored Whistlers (we didn't dip).

For an overnight stop, we camped at Broken Bucket Tank. This is on the Murrayville Track, about an hour out of Nhill, getting into the Big Desert.

Getting up in the morning, we noticed this fog bow. 

A fog bow happens when light from the sun (behind us) encounters fog, which is, of course, tiny water droplets hanging in the atmosphere.

The effect is nearly - but not quite - the same as a rainbow. Like for the rainbow, light hitting the spherical water droplets gets refracted on entry to the water, reflected at the back of the droplet, and then refracted again on the way out of the droplet. (See a diagram here.) The angle the light gets reflected back at determines the apparent size of the bow.

Where it's different is that fog droplets are way smaller than raindrops. At this size, diffraction starts to become important, which means the nice beam shining in gets all fuzzy on the way out. This lack of focus means that all the colours you'd normally see in a rainbow are blurred back into each other. Mixing all the colours from the rainbow back together makes the whole bow appear white again.

Of course, the opposite of this is also true - in a Summer storm, when huge drops of water are pouring out of the sky, the diffraction is very small, so rainbows associated with these big storms are especially colourful.

Very Pink Floyd.