Why did some historical rain look red?

In Odd Showers, Gibb gives different explanations for different red-looking falls, rather than one cause for them all^[1].

• Siena, 1860–61: Rain, and some snow, appeared faintly red. Gibb reports an examination that identified organic colouring matter, probably from an algae-like plant; he does not present the precise source as certain^[2].
• Near Aix-la-Chapelle, 1608: A reported “shower of blood” was attributed to red fluid released as chrysalides became butterflies. The stains were reported where butterflies reached, not in the city centre^[3].
• Several red-snow reports: Analyses described fine earthy material containing minerals such as silica, alumina and iron oxide. Gibb suspected volcanic ash in four nineteenth-century cases, rather than establishing that origin as certain^[4].

These are the book’s historical accounts and proposed explanations, not modern confirmation of each event.

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