Abstract

JAUREGUI, Ernesto. The heat spells of Mexico City. Invest. Geog [online]. 2009, n.70, pp.71-76. ISSN 2448-7279.

The warning of urban air has been documented to increase in intensity and area as cities grow (Oke, 1982). As the cities grow the so called "heat island" tends to increase the risk of more frequent heat waves as well as their impacts (IPCC, 2001). Threshold values to define a heat wave vary geographically. For the case of Mexico City located in a high inland valley in the tropics, values above 30° C (daily maximum observed for three or more consecutive days and 25° C or more as mean temperature) have been adopted to define the phenomenon. These events occur at the end of the dry season during March to May when afternoon relative humidity is quite low (~20%) and thus reducing the stress. Maximum temperature data from the Observatory of the National Meteorological Service were used. Results show that during the second half of the XXth century the frequency of heat waves as defined above has doubled from 6 events/decade to 16/decade in the 1990s with a marked increase in the last third of the last century when population of the city grew from 8.5 to 18.5 million (CONAPO, 2000). During this time the average urban/rural contrast grew considerably from about 6° C to 10° C (Jáuregui, 1986). While these heat waves may be considered as "mild" they receive attention from the media and prompt actions by the population to relieve the heat stress. Application of heat indices based on the human energy balance (PET and PMV) result in moderate to strong heat stress during these events. Because climate change is expected to raise nighttime minimum temperatures more than daytime highs (as suggested by the IPCC, 2001) urban heat islands and their related heat waves are likely to be a significant health concern in days to come in large urban centers especially in the developing countries.

Keywords : Heat waves; heat island; tropics.