The climate of northern Mexico is semi-arid/arid (BS and BW). Extremes of temperature are characteristic of this environment. During summer heat waves are not infrequent. In the 1990’s the population has markedly increased in urban areas located in the border region of the country. The impact of heat waves is most severe on the weakest parts of the population, eg. the elderly, the very young. Another high risk group are the seasonal workers. Every summer unacclimated migrants coming from central Mexico suffer the rigors of sunstroke when walking across country through the Mexico-US border, some of them dying from dehydration and heat exhaustion. The urban poor which constitute a large proportion in those cities are particularly vulnerable to the impacts of heat waves. Only a limited number of the population have some access to air conditioning. Since morbidity/mortality statistics are scarce and often not easily available, impacts of heat waves on the population are difficult to assess. In addition there is difficulty in identifying impact of heat waves since causation of most human health disorders is multifactorial. However some signals of this impact appear in the newspapers such as an increase in gastrointestinal disorder (due to ingestion of contaminated water) and dehydration of infants.

Using temperature records for the summer months of 1950-90 a characterization is made of the hygrothermal environment prevailing in 8 middle and large cities of northern Mexico during summer. Estimation of the impact of heat waves is made by applying a heat index which involves the combined effect of high temperature and humidity (NOAA-2000, site http://www.crh.noaa.gov/arx/heatindex.html) in the shade and light wind.

In general, in some localities the low humidity prevailing in summer during the afternoon hours reduces (up to a certain point) the risk of sunstroke in spite of the high air temperature. Observed afternoon mean values of the heat index range between 32 to 40 °C (90 – 105 °F) which is likely to induce disorders (such as sunstroke, heat cramps and heat exhaustion) for people in higher risk groups (NOAA, 2000). During the late evening heat index values remain usually between 29-33 (82 – 92 °F) inducing fatigue. Only around daybreak the hygrothermal environment is within the comfort range (between 24 to 30 °C heat index values).

For some cities in the border region (eg. Hermosillo, Mexicali) the frequency of days with temperatures in the range of 32-37 °C increased during the last decades of the XXth century. In the hottest month typically two heat waves occur.

Analysis of long-term series of extreme temperatures shows that in large cities of the border region both maximum and minimum temperatures are increasing. This may be due not only to the accelerated rate of urbanization observed in the border cities in recent decades (eg. heat island) but also to global warming.