Resumen
A spatial and temporal analysis of the air temperature in the boundary layer of Mexicali City, B.C. and rural surroundings was carried out. Several statistical tests were applied to a long-term database (1950-2000) to identify the temporal variability of the temperature; positive and statistically significant trends of the minimum temperature were observed, with a value of 0.66ºC/decade in the urban area, while in rural stations, smaller values were observed. Statistically significant results, but with a negative trend, were also observed with respect to the maximum temperature. When the spatial analysis was carried out, with a data base from a recent period (2000-2005), a nocturnal warm air mass in the urban atmosphere was present, and it was found that the maximum difference between the city and its surroundings occurs in winter with a value of 5.7ºC. These results suggest that urbanization, as well as in many other cities around the world, importantly affect the local climate and corroborate that in this city, an urban heat island has developed.