Residents in southern Pandeglang, Indonesia, were left stunned as the sky turned a deep red on 18 December. A video of the rare phenomenon went viral, but Indonesia’s Meteorology, Climatology, and Geophysics Agency (BMKG) confirmed it was a normal atmospheric effect called Rayleigh scattering. BMKG explained the red sky happens when sunlight travels a longer distance through the atmosphere at sunset, allowing only longer wavelengths like red and orange to be seen. Rain, high humidity, and dust a