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AFTERGLOW, a broad high arch of radiance or glow seen occasionally in the western sky above the highest clouds in deep ening twilight, or reflected from the high snowfields in mountain regions long after sunset. It is caused by very fine particles of suspended dust in the high atmosphere exercising a scattering effect upon the components of white light. After the eruption of Krakatoa in 1883, a remarkable series of red sunsets appeared all over the world. These were consequent on the spread by atmos pheric currents of huge quantities of fine volcanic dust.