Unusual Lamp Fogbows captured by Marko Krusel (atmospheric optics site) at Tuula, Estonia on July 16, '09. ©Marko Krusel, shown with permission.

".. at 02:00 I went out to hunt solar fogbows. I left in dark and thought why not to take the lamp with me, it's still dark so I can entertain myself in the meantime. The fog looked three-dimensional, every single drop was visible in the light. Little did I know that the fogbow would turn out to be so unusual.. It was very very cool."

The camera is pointing away from the lamp, the bright glow at the fogbow centre is direct backscatter from the fog droplets.

Let's start with the familiar, the bright inner primary fogbow. It is faintly coloured red at its diffuse outer edge. Only the very broad fogbows made by exceedingly small water droplets less than 10-micron diameter are devoid of colour.

Inside the primary are at least two supernumeraries analogous to those seen inside rainbows. In contrast to the rainbow, fogbow supernumeraries are more strongly coloured than the primary. Their hues are reversed with red on the inside. An IRIS Mie calculation gives an approximate drop size of 20 micron dia. - approximate because the calculation is for parallel light whereas lamp fogbows, like lamp ice halos, have divergent light.

Now for the less familiar: Outside the primary is a second broad bow. This is a secondary fogbow - again analogous to the secondary rainbow and arising similarly from two reflections inside the fog droplets.

Even less familiar: Outside the secondary are at least two further rings - faintly red on their outside and greenish inside. These are rare supernumerary fringes to the secondary bow. Rainbows show them sometimes (naturally or better using a hosepipe) but this capture in a fogbow is very unusual and might even be a first. Marko could see the first ring by eye.

Marko used a 15s exposure with an 8mm lens (12.8 mm 35mm camera equivalent, 108� horizontal FOV) on a Canon 350D.


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