Ask the meteorologist: What causes an arctic white rainbow?

I recently saw a picture of an arctic white rainbow taken at the North Pole. What causes an arctic white rainbow? — Barrett Allen

Answer: What you may have seen in what is called a "fogbow."

It is is similar to a rainbow except that the water droplets involved are much, much smaller than the raindrops that form a rainbow.

In these much smaller droplets, diffraction effects join with the refraction of the path of light passing through the droplets to "smear" the colors so that most of them overlap, instead of being separated as they are by larger drops.

When most of the colors of the visible spectrum overlap, they mix together to produce white light.

If you look closely at the photo in question, you'll notice that toward the inner and outer edges, there is in fact a bit of color.

As with rainbows, the outer edge of the main fogbow has a reddish tint, while the inner edge shows a bluish cast.

You can learn much more about these bows, and see lots of good photographic examples from the Atmospheric Optics blog.