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On the eve of the fall of the so-called Aztec empire a tornado swept around the besieged capital-seemingly a rare omen, except that records document subsequent tornadoes in Mexico City.
The Basin of Mexico, artificially drained since the seventeenth century, was originally a closed hydrological basin. The lacustrine plain has an average altitude of 2240 m and is surrounded by elevated volcanic ranges (3000-5500 m) on the east, south, and west and by low discontinuous ranges on the north (see, e.g., Sanders 1976). The basin's high altitude combines with a tropical location (19.5°N) to produce a temperate climate with rainy summers and dry winters. This rain regime seems to have prevailed during the whole historic period despite the relatively large fluctuations in the total annual precipitation (see Jáuregui Ostos 2000; O'Hara and Metcalfe 1997; Therrell et al. 2006).
At the dawn of the sixteenth century the Basin of Mexico was occupied by a chain of shallow lakes and inhabited by Nahuatl-speaking peoples distributed in dozens of altepetl. Literally meaning "water(s) and mountain(s)", "altepetl" can be understood as city-state although the equivalence is not exact (Lockhart 2004). Estimations of the total population in the Basin range from 800,000 to 1.2 million people, with one quarter of them living in the twin altepetl of Tenochtitlan-Tlatelolco (Sanders 1992). These were founded in the first half of the fourteenth century in two small islands in the western side of the central lake, Texcoco. Drought conditions prevailed at that time, but the climate soon became wetter and the Tenochca (or Mexica) and Tlatelolca flourished under relatively moist conditions (O'Hara and Metcalfe 1997). Two centuries of development and draining works transformed the two islands into a single one of about 6 km in north-south direction and 4 km in east-west direction (see Fig. 1).
The Nahuas, like nearly all early civilizations with an economy based on agriculture, were meticulous observers of astronomical and meteorological phenomena. Their achievements in observing the motion of celestial bodies, though not as impressive as those of the Maya, are well documented (see, e.g., Aveni 2001). The surviving pictorial manuscripts also give evidence of a careful register of meteorological phenomena such as rains, winds, and storms (e.g., Therrell et al. 2004). The start and end of...