Red rain in Kerala

The Kerala red rain phenomenon was a blood rain event that occurred from 25 July to 23 September 2001, when heavy downpours of red-coloured rain fell sporadically on the southern Indian state of Kerala, staining clothes pink. Yellow, green, and black rain was also reported. Coloured rain was also reported in Kerala in 1896 and several times since, most recently in June 2012, and from 15 November 2012 to 27 December 2012 in eastern and north-central provinces of Sri Lanka.

Following a light microscopy examination in 2001, it was initially thought that the rains were coloured by fallout from a hypothetical meteor burst, but a study commissioned by the Government of India concluded that the rains had been coloured by airborne spores from a locally prolific terrestrial green alga from the genus Trentepohlia. An international team later identified the exact species as T. annulata.

The coloured rain of Kerala began falling on 25 July 2001, in the districts of Kottayam and Idukki in the southern part of the state. Yellow, green, and black rain was also reported. Many more occurrences of the red rain were reported over the following ten days, and then with diminishing frequency until late September. According to locals, the first coloured rain was preceded by a loud thunderclap and flash of light, and followed by groves of trees shedding shrivelled grey "burnt" leaves. Shriveled leaves and the disappearance and sudden formation of wells were also reported around the same time in the area. It typically fell over small areas, no more than a few square kilometres in size, and was sometimes so localised that normal rain could be falling just a few meters away from the red rain. Red rainfalls typically lasted less than 20 minutes. Each millilitre of rain water contained about 9 million red particles. Extrapolating these figures to the total amount of red rain estimated to have fallen, it was estimated that 50,000 kilograms (110,000 lb) of red particles had fallen on Kerala.

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