Triple Cloud Shadow
November 10, 2006
This intriguing photo featuring multiple cloud shadows was captured above Huntsville, Alabama, in June of this year. Repeated shadows of clouds such as these are formed when a shadow is cast upon several layers of thin dust or haze. The smaller, darker, closer-in shadow likely lies on a layer that's only slightly hazy and is visible in contrast because the air surrounding the clouds is somewhat more hazy and so scatters sunlight more strongly -- as though the air itself was brighter than it actually is. The larger, dimmer shadow is cast on a more distant haze or dust layer. There's even a faint third shadow further out.
Technically, this is referred to as differential scattering cross section, and it's a strong function of scattering angle. You're more apt to see such shadowing only when look toward the Sun, usually within about 10 degrees of it.
This photo is as I saw it through the lens and has not been superimposed or altered in any way. It was taken with a Canon A95 Powershot.