IN March, 1835, Charles Darwin was exploring in the Andes. "On several patches of the snow," he wrote, "I found the Protococcus nivalis or `red snow' so well known from the accounts of Arctic navigators. My attention was called to it by observing the footsteps of the mules stained with a pale red, as if their hoofs had been slightly bloody."
The history books contain a myriad of references to what people used to think of as being "showers of blood". Homer, for example, relates how a "shower of blood" fell on the heroes of ancient Greece as a harbinger of death; Plutarch theorised that bloody vapours, distilled from the corpses of the slain after great battles, condensed in the clouds and fell to earth again as crimson rain; and in Italy in 1117, the showers of blood caused such a panic that a meeting, not of meteorologists but bishops, was held in Milan to consider what their origin might be.
For the most part, these scarlet visitations were ascribed to divine or satanic interference with the elements, and were interpreted as a sign of wrath in hell or heaven as the case might be. Some commentators, however, got closer to the truth. Venerable Bede, for example, writing in the 8th century, said that rain thicker than usual might become blood red "and so deceive the uninstructed". A later colleague suggested something similar, but hedged his bets: "What the vulgar call a shower of blood is generally a mere fall of vapours tinted with vermilion or red chalk, but when blood actually does fall, which it would be difficult to deny takes place, it is a miracle due to the will of God."
Nowadays, we recognise several more mundane causes for these happenings. The organisms described by Darwin, or others like Uredo nivalis or the algae Protococcus fluiahs, often have a reddening effect. There are also a few recorded cases on the Continent of the red deposits being, not meteorological but, rather unappealingly, the excrement left by a recent plague of butterflies. By far the most common cause, however, especially when such phenomena occur in Ireland, is that the rain is contaminated by a fine red dust, usually originating in the deserts of North Africa.
The dust is raised in sand storms and carried northwards by fortuitous winds aloft. It is then washed down to earth by a passing shower, and on the one or two occasions every year when this occurs, we wake to find our cars and other outdoor belongings shrouded in a dappled coat of fine red powder.