Morning Fogbow

Morning Fogbow

Imaged in New Brunswick, Canada by Richard Blacquire.

"The morning was cool and calm and a fairly dense fog lay in the Kennebecasis River Valley. By 8:30 the sun was beginning to burn through it. I drove to a vantage point so the sun was behind me and I was looking down onto an old field above the Hampton marsh. Sure enough, a fogbow had formed. Not a complete bow - the fog was not sufficiently high to show the top of the arc. Even so, I took many pictures.

Then, down the road to another elevation looking down on a farm field, a better bow this time. The fog was higher and the top of the bow was visible. Many more pictures."

This picture is from two images stitched together by PTgui and with very little further adjustment.

©Richard Blacquire, shown with permission

This fogbow shows a hint of red on its outer rim and perhaps some blue on the inner one. There is a characteristic dark band inside representing the gap between the main fogbow and its first supernumerary fringe. Notice Richard's shadow at the bow's centre, the antisolar point.

The bow was made by quite small water droplets and the lack of clear supernumeraries shows that the fog was old and had evolved into droplets with a wide distribution of sizes.

Why a fogbow?

Sunlight making a fogbow follows the same recipe as that for a rainbow. Light enters a water sphere, reflects once from the opposite side and leaves in several directions almost back towards the sun. The difference is that large (1-5mm diameter) raindrops give rainbows while small fog droplets (0.01 - 0.2mm diameter) produce fogbows.

The smaller droplets are only 10-100 times the size of wavelengths of visible light and wave effects are therefore significant. At each surface the light waves are scattered in many directions and these outgoing scattered waves overlap and interfere - in other parlance they are diffracted. The result is a broad and diffuse circle of light rather than the sharper rainbow. However, rainbows themselves are not innocent of diffraction effects - witness their supernumerary fringes.