Pastel Clouds
These pictures from my backyard show soft pink and green pastel hues at the perimeters of clouds west of Boulder. I’ve seen these colors many times, and they are quite dramatic through sunglasses. To get a particular color like those in these clouds requires that light at the wavelength corresponding to that color is scattered by the ice particles in the clouds much more efficiently than light at other wavelengths. Seeing these colors suggests therefore that the ice crystals are all nearly the same size and that that size is several times larger than the wavelength of the scattered light (about 0.5 microns). This is a case of diffraction where there is a resonance between the wavelength of light and the size of the particle.
November 30th, 2006 at 9:00 am
The pastel (or “irridescent”) cirrus clouds also seem to require strong updrafts and cold temperatures. The waves driven by flow of air over the mountains can have very strong updrafts that will drive formation of large numbers of ice crystals. Competition for vapor amongtst the large number of crystals limits there growth and keeps them small enough for the diffraction effect to work. If the clouds are high and cold, the vapor available for condensation is very limited, which further limits their growth. The wave clouds in these photos did indeed form at unusually high altitudes and low temperatures. The particles in irridescent cirrus need to be much smaller than those in typical cirrus.