The HDD concept (base temperature 18.3 °C) is used to evaluate first the impact of climate on energy demand for space heating in Mexico City, and second to determine long-term changes in this impact that may have occurred due to increasing influence of the heat island phenomenon during recent decades. The city experienced a marked increase in population (from 8.7 mill in 1970 to 17.0 mill in 1990), while it more than doubled its urban area (from about 500 km2 to 1200 km2).
During the cool season (Nov-Apr) when anticyclonic weather prevails, the UHI attains its maximum intensity. Located in an elevated valley (2250 masl) in the tropics (lat. 19.5° N) the climate of Mexico City is characterized by cool to cold nights, during this period and temperatures may drop to around near freezing (depending on the observation site relative to the UHI position). Therefore it is not surprising that potential energy demands for space heating as estimated by heating degree-days (HDD) in the capital of Mexico (about 1025 HDD in 1955) are around double those for Los Angeles (384 HDD) but one third less than some U.S cities on the Eastern Seabord (HDD ~1300, Landsberg, 1981).Not withstanding this situation, only occasionally energy for space heating is used in the city except perhaps during cold nights when electric heaters (blankets) are shortly used. On the other hand since insolation is abundant, temperatures rise at a steep rate during this season reaching comfortable levels in the afternoon. Results show that as the city grew, potential energy demand for space heating in downtown Mexico city has markedly decreased from 1024 HDD in 1955 to only about half (458 HDD) in 1995 while this reduction has been more drastic (to one fourth) in the suburbs during the same period. At the same time urban/rural thermal contrasts have increased from around 6° to 10°C. It is hypothesized that the observed changes in HDD may be linked to changes in the intensity and extension of the UHI of the city.