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[1] 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 for some historical descriptions include red aurorae being described as having the appearance of rain or blood.