Fogbows – sometimes called white rainbows, cloudbows or ghost rainbows – are made much as rainbows are, from the same configuration of sunlight and moisture. Rainbows happen when the air is filled with raindrops, and you always see a rainbow in the direction opposite the sun. Fogbows are much the same, always opposite the sun, but fogbows are caused by the small droplets inside a fog or cloud rather than larger raindrops.
Look for fogbows in a thin fog, when the sun is bright. You might see one when the sun breaks through a fog. Or watch for fogbows over the ocean.
Because the water droplets in fog are so small, fogbows have only weak colors or are colorless.
Les Cowley of the great website Atmospheric Optics says:
Look away from the sun and at an angle of 35-40° from your shadow which marks the direction of the antisolar point. Some fogbows have very low contrast so look for small brightenings in the misty background. Once caught, they are unmistakable.
The sun must be less than 30 – 40° high unless you are on a hill or high up on a ship where the mist and fogbow can be viewed from above.
Fogbows are huge, almost as large as a rainbow and much much broader.
Bottom line: Fogbows are rainbow’s cousins, made in much the same process, but with the small water droplets inside a fog instead of larger raindrops. Because the water droplets in fog are so small, fogbows have only weak colors or are colorless.