The Blood Rain of 1901 and the Investigative Lens of Cosimo De Giorgi

On 10 March 1901, the skies of Europe turned red – from Rome to Sicily, all the way to Lecce. There, the rain brought down a yellowish-red dust that settled on the leaves of the garden in front of the house of Cosimo De Giorgi (1842–1922), physician and meteorologist. It was not the first time such a phenomenon had been observed. Already at the end of the eighteenth century, Giuseppe Maria Giovene (1753–1837), meteorologist from Molfetta, had offered a “rational” explanation against superstition: the reddish dust falling with the rain, he argued, came from the Sahara, carried northward by the winds. This “African hypothesis,” later echoed in the nineteenth century by the astronomer and science popularizer Camille Flammarion (1842–1925), had by then become almost a commonplace. De Giorgi, however, was not content to accept it. A man of method and systematic observation, he collected samples, examined them under the microscope with the help of his assistants, and published his findings in two instalments in the Corriere Meridionale.
The result was surprising: among the grains were visible the siliceous shells of diatoms – microscopic aquatic organisms incompatible with a desert origin. Moreover, by applying the law of the Dutch meteorologist Buys Ballot (1817–1890), De Giorgi demonstrated that, on that day, the winds had blown from the east-southeast, not from the south. The dust, therefore, had not come from the Sahara but from the Balkans and Asia Minor. It was a true reversal of perspective: the so-called “blood rain” was not African sand but fine dust lifted from the moist soils of the East, swept up by cyclones and carried across the Adriatic before falling on Apulia as coloured rain. The explanation – later confirmed by other scholars such as Napoleone Passerini (1862–1951) – revealed De Giorgi not only as a positivist meteorologist but also as an investigator of the clouds, capable of reading in faint traces the hidden truth of the skies. Thus the red rain, once interpreted as a divine sign or mysterious prodigy, became for the Lecce meteorologist an opportunity for useful science – a phenomenon to be understood in order to protect agriculture and public health. Through this episode, De Giorgi and his observatory emerged as a crucial node in an international network, a place where meteorology became both knowledge of the heavens and care for the land.

___ Stefano Daniele & Francesco Paolo de Ceglia