At sunset

Two often dramatic phenomena are occasionally seen at sunset: the green flash and the Chinese lantern sun. Special atmospheric conditions are necessary for both. The green flash deserves to be divided into two distinctly different types. The more common type arises when a distant sharp horizon feature like a mountain profile cuts sequentially the vertical spread of the solar disk from normal atmospheric refraction, thus allowing the green to be seen after the longer wavelengths are cut off, the blue portion being attenuated by absorption in the long atmospheric path. A much more dramatic green flash is seen primarily in the tropics. It generally lies distinctly above an ocean horizon and appears a moment after the last reddened direct image of the upper limb of the sun has disappeared. It can be surprisingly bright. The mechanism appears to be the ducting of sunlight in a moist layer lying above a warm sea.