Fogbow, Fog Droplets and Pumpkin Spider
October 30, 2012
The photo at top showing a ghostly, 360 degree fogbow was taken at Twin Peaks in San Francisco, California. As their name implies, fogbows form in fog not in rain or thundershowers. As with the case of rainbows, however, fogbows are only observed at the antisolar point – note the shadow (spectre) of the photographer (me) in front of the fogbow. Because fog droplets are so much smaller than raindrops, sunlight is largely diffracted. With rainbows, most of the light entering a raindrop is refracted and then reflected. The low contrast colors of the fogbow result because colors overlap, due to wave interference, before emerging from the droplets. Colors are much more pure when the drop size is larger.
The photo at left was taken about the same time and from the same location as the fogbow picture. Just in time for Halloween, an aptly named Pumpkin Spider’s (Araneus diadematus) web captures beads of water -- the very smallest ones illustrate the size of the fog droplets. Photos taken in late October 2011.
Photo details: Fogbow - Camera Maker: Canon; Camera Model: Canon EOS DIGITAL REBEL XTi; Exposure Time: 0.0040 s (1/250); ISO equiv: 100; Exposure Bias: none; Metering Mode: Matrix; Exposure: Manual; Exposure Mode: Manual; White Balance: Auto; Flash Fired: No (enforced); Orientation: Normal; Color Space: sRGB; Software: Adobe Photoshop CS3 Windows. Spider web - same except Focal Length: 70.0mm; Aperture: f/8.0; Exposure Time: 0.0063 s (1/160); ISO equiv: 400.