Sister Cities, Separate Styles: Residential Roof Albedos in San Diego, U.S.A and Tijuana, Mexico

Quantitative recording of residential roof colors revealed that out of the 1,347 roofs counted in San Diego, California, U.S.A., only 15% were light colored (white or light gray). Of the 1,666 roofs counted in Tijuana, Baja California, Mexico, 33% were light colored. Given the warm and sunny climate of the region, dark colored roofs add to the thermal loading on individual residences and likely contribute to the overall urban heat island effect. Although the climate in this region currently is considered "pleasant," climate models predict a substantial increase in the frequency, magnitude, and duration of heat waves in the region by 2050. The number of days over 36^oC is expected to increase 6-fold by then.

Despite their proximity, median income in San Diego is ~ 75,000, with some living 15% below the poverty level. In Tijuana the median salary is ~ 25,000, with 30-40% of the residents living below the Mexican poverty standard. In Tijuana, and in low-income housing in San Diego, residential thermal loading can have negative health effects on residents. Heat stress morbidity and mortality is highest among the elderly and those with pre-existing disease. In light of global warming and the associated efforts to limit energy usage, a move to increase roof albedo appears to be a low-cost, low-tech approach to this problem.

One suspects that cooling costs have been less of a concern in the richer city north of the border. Perhaps style trumps cost - Mediterranean faux terra cotta roofs are a common theme in San Diego. Regardless of the differences in the median salary between these sister cities, both cities currently have a high percentage of dark colored roofs.

Satellite images were superposed with a 100-meter grid system, the intersections of which were used as targets; point counting the feature under the target provided sampling areas, appropriate for studying albedo. Colors were classified as: white, light gray, dark gray, light brown, dark brown, light blue, dark blue, light green, dark green, terracotta, red, and black. Light colored here was defined as white and light gray. Further studies will examine such other albedo factors as housing age, roof slopes, roof materials, and home building quality standards for the two regions.