This paper discusses a meteorological phenomenon that occurred in the Basin of Mexico in August 1521. The only known description appears in Book XII of the Florentine Codex, which is an account of the Spanish conquest of Mexico written in Nahuatl in the mid-sixteenth century. According to it, just before the fall of Mexico-Tenochtitlan a heavy storm accompanied by a whirlwind struck the basin. The whirlwind hovered for a while above Tlatelolco, Tenochtitlan's twin city, before moving to the lake where it disappeared. Analyzing the account in the context of Nahua culture and contrasting it with contemporary European descriptions of tornadoes and waterspouts shows that the described phenomenon was a tornado. This conclusion receives further support from eighteenth-and nineteenth-century pictorial and written evidence showing that tornadoes do occur in the territory now occupied by Mexico City. Since the tornado of Tlatelolco predates the Cambridge, Massachusetts, tornado of 1680, it represents the earliest documented tornado in the Americas.
Departamento de Oceanografía Física, CICESE, Ensenada, México