@mpgA pileus cloud lit up iridescent near Flagstaff, Arizona, right after the sun went down. The colors come from diffraction: tiny ice crystals or small water droplets each scatter sunlight on their own. For the spectrum to show, light has to move through high-altitude cirrus made of ice crystals, or through altocumulus, cirrocumulus, lenticular, and cirrus clouds. And the crystals need a particular alignment before they refract that light into the full spread of color.

@mpgA pileus cloud lit up iridescent near Flagstaff, Arizona, right after the sun went down. The colors come from diffraction: tiny ice crystals or small water droplets each scatter sunlight on their own. For the spectrum to show, light has to move through high-altitude cirrus made of ice crystals, or through altocumulus, cirrocumulus, lenticular, and cirrus clouds. And the crystals need a particular alignment before they refract that light into the full spread of color.