Pink Cloud at Noon imaged through a polarizing filter by Doug Zubenel in Kansas.
"We had a big squall line blast through here night before last, and yesterday the atmosphere was very, very clean. With the sun nearly overhead, my polarized sunglasses hightened the atmospheric reddening seen in the distant thunderstorm compard to the Cumulus humilis clouds only a mile or two away. Without the polarization, the difference was not as pronounced." Image ©Doug Zubenel, shown with permission.
Why pink? Thunder clouds are very dense and dense clouds have no intrinsic colour. Light rays are multiply scattered by their water droplets and so the wavelength dependence of angular scattering that, in very thin clouds, gives rise to iridescence or the colours of coronae is averaged out. Thick cloud colours result only from those of the light incident upon them be it the reds of sunset, the blues of the summer sky or the greens and earths reflected from the ground.
Two effects produced this cloud's pinks. 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 preferentially scatters blue light towards us, this is called 'airlight'. It is responsible for the blue sky and partly for the blue colour of distant mountains. Airlight is polarized and so its intensity depends on the setting of a camera polarising filter. The two effects acting together produced the pink.