Blood rain

Blood rain or red rain is a phenomenon in which blood is perceived to fall from the sky in the form of rain. Cases have been recorded since Homer's Iliad, composed approximately 8th century BC, and are widespread. Before the 17th century it was generally believed that the rain was actually blood. Literature mirrors cult practice, in which the appearance of blood rain was considered a bad omen. It was used as a tool foreshadowing events, but while some of these may be literary devices, some occurrences are historic. There is now a scientific consensus that the blood rain phenomenon is caused by aerial spores of green microalgae Trentepohlia annulata.

Recorded instances of blood rain usually cover small areas. The duration can vary, sometimes lasting only a short time, others several days. By the 17th century, explanations for the phenomenon had moved away from the supernatural and attempted to provide natural reasons. In the 19th century, blood rains were scientifically examined, and theories that dust gave the water its red colour gained ground. Today, the dominant theories are that the rain is caused by red dust suspended in the water (rain dust) or by the presence of micro-organisms. Alternative explanations include sunspots and aurorae.

Occurrences of blood rain throughout history are distributed from the ancient, to the modern day. The earliest literary instance is in Homer's Iliad, in which Zeus twice caused a rain of blood, on one occasion to warn of slaughter in a battle. The same portent occurs in the work of the poet Hesiod, writing around 700 BC; the author John Tatlock suggests that Hesiod's story may have been influenced by that recorded in the Iliad. The first-century Greek biographer Plutarch also recounts a tradition of a rain of blood during the reign of Romulus, founder of Rome. Roman authors Livy and Pliny record some later cases of blood rain, with Livy describing it as a bad portent.

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