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Just after sunset near Flagstaff, Arizona, a developing storm cloud caught the last light of day in a rare way. As the Sun dipped below the horizon, its rays passed through a thin veil of high-altitude ice crystals above the storm. Instead of a simple glow, the light split and spread into shifting bands of color that rippled along the cloud’s edge. From the ground, it looked like a painted wave suspended in the sky. This wasn’t fire or aurora. It was cloud iridescence...an optical effect that appears when sunlight diffracts through tiny, uniformly sized droplets or ice crystals. The swirling shapes came from upper-level winds stretching the cloud layers while the color followed the light. Moments like this don’t last long. As the Sun dropped lower and the angle changed, the spectrum faded back into gray, leaving behind only the memory of a cloud that seemed to glow from within.