During winter and early spring the Mexico Basin (lat 19°N) is under the influence of modified cold air masses from Northamerica, causing temperatures to drop below freezing and snow on the surrounding mountain tops (700 to 2300 m over the valley plains). The frontal passages affect about once every week not only the area under study but most of the country Historical records show that during the Aztec Empire, severe winters were frequent and in 1454-59 a series of cold spells resulted in poor crops and famines.

Climate extremes in central Mexico are linked to meridional circulation patterns characterized by deep polar air penetration into tropical America associated to a development of winter anticyclones over the Northamerican continent.

Long-term records of extreme minimum temperature for two stations covering the XXth century were used to determine periods of cold winters. These stations are: the Tacubaya observatory located on the western suburbs of Mexico City on the SW corner of the basin. The other station is situated in the small city of Pachuca on the northern edge of the basin. Analysis of the two series showed three cold episodes in which cold winters were interspersed with years with near-normal winters.. The periods are 1897-1910, 1918-1941 and 1955-1966. During these cold events temperatures dropped to -4 °C (in the south) and -6 to -10°C (in the center and north portion of the basin). On occasions the cold spells were associated with light snowfall mainly on the mountains. Since no region is representative of regional/hemispheric conditions as suggested by Jones and Bradley (1992) the above-mentioned periods are not coincident in time with other cold episodes observed elsewhere, e.g. the 1939-42 severe winters in Europe and those occurring in Northamerica in 1977-79.

Synoptic conditions at the continental/hemispheric scale linked to some severe winters in the Mexico basin are discussed. The last two decades of the XXth century show a trend toward warmer winters suggesting the influence of increasing urbanization (e.g. the heat island) and perhaps, in a smaller measure, global warming.

Jones, P. and R. Bradley. 1992. Climate variations in the longest instrumental records. In: Climate since A.D. 1500. R. Bradley and P. Jones (eds.). pp. 246-268.