Cloudbows are also called white rainbows. They are similar to rainbows in that the sun reflecting off of water droplets in the atmosphere causes the effect. Cloudbows appear white because the water droplets are very tiny. The larger the water droplet, the more colors will be visible in the bow. Fogbow droplets are larger than cloudbow droplets, so fogbows may have very weak colors. Rainbow droplets are bigger still, thus reflecting "all the colors of the rainbow." Cloudbows and fogbows are usually seen in foggy conditions.
I have seen only one--the one depicted in these photos. On 25 August 1992, we were returning home to Arizona on the last day of a four-week vacation. It was early morning, around 7 a.m., and we were driving through New Mexico in the high country between Magdelena and Datil. A heavy fog was just beginning to break up in the morning sunlight. I looked up through the car's windshield and there it was. We pulled to the side of the road and I snapped several photos.