Our Oct. 21 Ask Tom column attempted to explain why some thunderstorm clouds have a greenish tint, and we explained the phenomenon in terms of scattering of the varying wavelengths of light contained within sunlight, when in actuality it results from absorption of the varying wavelengths.
Dr. Roger Johnston of Argonne National Laboratory noticed our error and sent this letter, which we reproduce here with his kind permission:
I greatly enjoy your mini weather column, “Ask Tom”. Your column today (Oct. 21) in the Chicago Tribune on why severe thunderstorms turn the sky green, however, was all wet.
The sky looks green with severe storms mostly because water is intrinsically (slightly) blue (see http://www.dartmouth.edu/~etrnsfer/water.htm), not because of differential scattering of different wavelengths.
We don’t ordinarily notice the blue color of water because we don’t usually look through long enough pathlengths of water. When clouds are very dense, however, there is a lot of multiple scattering of light.
This effectively increases the pathlength of the light, allowing the water in the cloud an extra chance to absorb red light. This is what mostly makes the sky appear green with dense thunderstorm clouds.
(There is a little bit of reflection of the blue scattered sky light off of clouds, too, but that is a very minor effect.)
The reason that thick, dense clouds make the sky look green is the same reason that glaciers look blue and mountain lakes look blue-green: multiple scattering allows extra pathlength so that water can absorb red light more. Yes, scattering plays a role, but the true explanation involves differential ABSORPTION of light, not scattering!
(Glaciers look blue, not green because there is a huge amount of multiple scattering inside the glacier from dirt, bubbles, and ice variations so even more red gets absorbed than for clouds. Mountain lakes look blue-green because the suspended particles from mountain runoffs allow multiple scattering inside the water, and thus more red absorption than clouds, but less than glaciers. The thicker the glacier, the bluer it looks.)
Lots of people over the years have gotten confused on this point (including lots of scientists such as the brilliant optical physicist Raman), so you are in good company with your confusion about scattering vs. absorption as the true color mechanism.
Roger Johnston, PhD
Argonne National Laboratory