The Kerala red rain phenomenon was a blood rain (red 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.1 Yellow, green, and black rain was also reported.234 Coloured rain was also reported in Kerala in 1896 and several times since,5 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 Trentepohlia 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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The brownish-red solid separated from the red rain consisted of about 90% round red particles and the balance consisted of debris. The particles in suspension in the rain water were responsible for the colour of the rain, which at times was strongly coloured red. A small percentage of particles were white or had light yellow, bluish grey and green tints. The particles were typically 4 to 10 µm across and spherical or oval. Electron microscope images showed the particles as having a depressed centre. At still higher magnification some particles showed internal structures.
Some water samples were taken to the Centre for Earth Science Studies (CESS) in India, where they separated the suspended particles by filtration. The pH (acidity) of the water was found to be around 7 (neutral). The electrical conductivity of the rainwater showed the absence of any dissolved salts. Sediment (red particles plus debris) was collected and analysed by the CESS using a combination of ion-coupled plasma mass spectrometry, atomic absorption spectrometry and wet chemical methods. The major elements found are listed below. The CESS analysis also showed significant amounts of heavy metals, including nickel (43 ppm), manganese (59 ppm), titanium (321 ppm), chromium (67ppm) and copper (55 ppm).
Louis and Kumar used energy dispersive X-ray spectroscopy analysis of the red solid and showed that the particles were composed of mostly carbon and oxygen, with trace amounts of silicon and iron. A CHN analyser showed content of 43.03% carbon, 4.43% hydrogen, and 1.84% nitrogen.
J. Thomas Brenna in the Division of Nutritional Sciences at Cornell University conducted carbon and nitrogen isotope analyses using a scanning electron microscope with X-ray micro-analysis, an elemental analyser, and an isotope ratio (IR) mass spectrometer. The red particles collapsed when dried, which suggested that they were filled with fluid. The amino acids in the particles were analysed and seven were identified (in order of concentration): phenylalanine, glutamic acid/glutamine, serine, aspartic acid, threonine, and arginine. The results were consistent with a marine origin or a terrestrial plant that uses a C4 photosynthetic pathway.