July 03, 2008
On the evening of June 5, 2008, we had a powerful squall line blast through our part of east-central Kansas. The atmosphere the following day was extremely clean, and while wearing my polarized, prescription sunglasses, I noticed the marked difference in color between the Cumulus humilis clouds a mile or two away (2 or 3 km), and the mature thunderstorm 140 miles (225 km) distant over Joplin, Missouri. I then imaged the scene using a polarizing filter to reproduce what I saw with my sunglasses. While I knew that polarizing lenses and filters increase color saturation and reduce blue light, I did not know the precise mechanisms at work. Atmospheric optics expert Les Cowley kindly provided the following explanation: Two effects produced this cloud color. Firstly, sunlight scattered by the cloud towards our eyes is scattered again by air molecules. Shorter wavelength blues and greens are scattered out of the direct line of sight more than red, causing the cloud's light to be reddened. The reddening of the sun at sunset is the same effect. The second effect is that the air also scatters blue light towards us; this is called “airlight”. It is responsible for the blue sky and partly for the blue color of distant mountains. Airlight is polarized and so its intensity depends on the setting of a camera polarizing filter. The two effects acting together produced the pink coloration of the cloud.