Blood rain

Blood rain, also called red rain, is rain that falls tinted red, pink or brown because it carries particles from somewhere else. Most of the time the particles are mineral dust lifted from a desert. Less often they are the spores of an alga, or the droppings of insects. Before anyone looked through a microscope, a shower of blood was read as an omen.

Dust is the commonest cause. A thin film of iron oxide is enough to turn pale clay red. In a record kept at Montseny, in Catalonia, from 1983 to 1994, 38 red rain events brought African dust to the site. Two of them, in November 1984 and March 1991, delivered 62% of all the dust that fell in the 11 years, and the average annual deposit was 5.3 g per square meter. Back trajectories of the air traced the dust to three regions: the Western Sahara, the Moroccan Atlas and central Algeria. By X-ray diffraction the dust was mostly illite, then quartz, smectite, palygorskite, kaolinite, calcite and dolomite.^[1] The calcite made the rain strongly alkaline. The share of a year's precipitation that fell as red rain tracked the year's mean pH (r = 0.71), and red rain brought 34% of the calcium that the holm oak forest at the site needed to grow.^[1]

In the Iliad Zeus sends drops of blood from the sky twice: in book 11, before a day of slaughter, and in book 16, for his son Sarpedon, who is about to die.^[2] Later writers kept lists of such portents. A survey of medieval cases found blood rain recorded in chronicles across Europe,^[3] among them Gregory of Tours's account of blood on people's clothes in the Paris region in the year 582.^[4]

Fabri de Peiresc (1580 to 1637), a magistrate and naturalist of Aix-en-Provence, heard of a rain of blood in his city and proposed butterflies as the cause.^[4] He is better remembered for astronomy; with the engraver Claude Mellan he produced early maps of the Moon.^[5]

Red rain fell on and off over Kottayam and Idukki districts in Kerala, India, from July 25 to September 23, 2001, staining clothes pink, and yellow, green and black rain was reported as well.^[6] A meteor burst was the first suggestion. After microscopy, a report by the Centre for Earth Science Studies and the Tropical Botanic Garden and Research Institute, commissioned by India's Department of Science and Technology, concluded that the color came from airborne spores of Trentepohlia, a green alga that coats tree bark and walls in an orange film of carotenoid pigment.^[6]

Two researchers at Mahatma Gandhi University offered a different reading, that the particles were living cells brought by a comet. Their paper of 2006 carries the words "possible extraterrestrial origin" in its title.^[7] A study of 2015 identified the alga in rain samples as Trentepohlia annulata, a species found in Europe as well as India.^[8]

A different rain of blood fell in a flow cell in late 2018, when a founder of demonpore, inc. cut himself with a razor blade and bled into the flow cell of one of the company's adjustable pores. His account of that day is titled "rain of blood"; the story is told in Rain of blood (demonpore). Red cells are counted one at a time, in a more orderly way, by the Coulter counter behind a complete blood count, an instrument in use since the 1950s.

• Rain of blood (demonpore)
• Complete blood count