Resumen
Urbanization has been the dominant demographic trend during the second half of the 20th century in México. In 2000 there were 69 cities with more than 100,000 inhabitants of which 9 of them exceeded one million population, totalizing 53.4 million. Using time series of mean monthly temperature for about a dozen available stations, this paper sets out to examine temperature changes occurring during the late 20th century. Since it is well established that urban warming is mainly a nocturnal phenomenon minimum temperature series were selected after a test for homogeneity. Trend analysis was applied to the minimum temperature series and a linear regression coefficient was obtained. Tests of significance were performed. Most of the positive trends proved to be significant (>90%). Although temperature trend variability amongst the individual ities was large (from 0.02ºC/decade to 0.74 ºC/decade) average temperature increase in large (=106 inhabitants) cities was (0.57 ºC/decade) considerably higher than that orresponding to medium size urban centers where on the average temperature increase was 0.37 ºC/decade. These temperature increases express not only the urbanization effect but also that due to global climate change (of the order of 0.07 ºC/decade) and natural variability. In concluding it may be said that increasing urbanization in México has originated a positive trend in urban temperatures which has implications for human comfort and health.