It is well established that the heat island phenomenon has an influence on the intensification of convective rainshowers as they travel across the city. These intense rainfall events that are observed in an urban area are connected to more intense updrafts (Sato N. and M. Takahashi, 2003). Prevention of floods and inundation of cities caused by the high intensity of rain fall are of major concern in urban planning. Mexico City has experienced a very rapid growth in population in recent decades. Consequently the urban extent in the capital conurbation has grown an additional 24,161 ha (or 2,416 ha/per year) during the 1990 decade (Garza, 2007). The drastic change in land use has induced significant changes in (among others) convective precipitation. A significant positive trend in the frequency of intense rain showers in the capital city for period 1941 to 1996 has been found (Jauregui, 1996). This phenomenal rate of urbanization has most likely affected the urban precipitation regime of the mega city. Preliminary results show a positive significant trend for period 1941-2007 for Tacubaya station (urban). Other urban stations with shorter data periods located in the Mexico City region show also a positive trend in the frequency of intense precipitation (greater than 30 mm/hr). During the recent period of 1985-2007 the urban heat island has grown accordingly in extent and intensity. The establishment of an automatic network of 25 meteorological/air quality stations since 1986 has allowed a more detailed description of the urban changes. It is speculated that this change may be in no small part related to this drastic land use change observed in the capital city.

Key words: heat island, urban precipitation, Mexico City.