Red rain, also known as blood rain, looks downright apocalyptic. Rain falls red, forming red puddles and red streams on the ground. Throughout history, philosophers, scientists, and ordinary folk alike have sought to explain the seemingly inexplicable phenomenon.
Records of red rain date back to the 7th century BC. Classical literary figures like Homer, Plutarch, and Geoffrey of Monmouth referred to it as a bad omen. In the Middle Ages, witnesses saw it as a warning from God, since it reportedly rained red before the Black Plague began in the mid-14th century.
In the 17th and 18th centuries, scientists from France and Italy made the first attempts to explain it more rationally. They considered a variety of theories — animal causes, volcanic ash, and dust dispersal. By this time, historians had recorded a couple of hundred cases of red rain.
Explosion, then red rain
On July 21, 2001, residents in the Malabar region of southern India heard a loud explosion and saw a bright flash of light. A torrent of thick red raindrops followed shortly after. It stained eyewitnesses’ clothes and eventually evolved into heavy flooding of the villages and larger towns. This went on for two months.
A similar event occurred in Sri Lanka in 2012 after a series of meteorites entered Earth’s atmosphere around the same time as the rain.
Red rain flooding in Kerala. Photo: Earth Science Stack Exchange
This suggested a possible connection between red rain and meteorite activity. Some believed that the loud thunderclap and flash of light indicated a meteor event. Two scientists with Mahatma Gandhi University, Godfrey Louis and A. Santhosh Kumar, posited a “cometary airburst” theory. They suggested that the colored rain came from “fragmentation and atmospheric disintegration of a fragile cometary body that presumably contains a dense collection of red cells.”
According to historical reports, around 36% of cases of red rain coincided with meteorite sightings. However, the academic pair’s theory fell out of favor because of their inaccurate methodology. Also, it did not explain how the debris could rain down for two whole months.
Another popular theory proclaimed volcanism as the culprit. The July 2001 case of red rain coincided with volcanic ash from the Mayon volcano in the Philippines. Theoretically, it could have drifted to southern India.
Red rain flooding Sri Lanka. Photo: Asrilanka.com
The lichen hypothesis
A study by the Indian government found that the red cells in the water measured 10 micrometres, had a pH of 7, and included metallic elements like silicon, nickel, titanium, manganese, chromium, and copper. It also contained 43.03% carbon, 4.43% hydrogen, 1.84% nitrogen, and great amounts of a lichen species called Trentepohlia.
This species grew significantly during heavy rain, prompting its spores to release into the air. Elements in the spores then reacted with elements in the raindrops to turn reddish brown. Mystery solved.
Sometimes, rain can take on other colors. Yellow, blue, green, and black rain make rare appearances. The reason for these different colors remains unclear. Some speculate that the same lichen can react with other elements in the air to produce different hues.
