The photo above showing a heart-shaped mid-level cloud fringed with iridescent colors was captured above Zabikow, Poland on July 30, 2010. Photographer: Dariusz Dorosz
Diffraction of sunlight by minute cloud droplets is responsible for the lovely pastel hues. Because the droplets are so tiny (1/10 to 1/1000 the size of raindrops), light waves that interact with them aren’t refracted nearly as much as they’re deflected — in all directions. Iridescence or irisation is generally observed in clouds that have formed relatively recently since then their droplets are more likely to be quite small and of more uniform size. Iridescent clouds are found within about 20 degrees of the Sun, so make sure to protect your eyes whenever looking for them.