The Urban Heat Island Effect (UHI) is when a city is significantly warmer than the outlying rural area due to the preponderance of concrete and asphalt surfaces that store heat during the day and release it at night. While urban heat islands exist in most large cities, the Phoenix metropolitan area has presented a special case for the study of this phenomenon because of its rapid growth over the last 30 years and the already high summertime temperatures experienced in the valleys of the Sonoran Desert.
The emergence and intensification of Phoenix's UHI represents an important stressor on humans in the city. CAP researchers found that the UHI varies greatly in space, mirroring the physical heterogeneity of the urban landscape. Variations in amounts and distributions of soil, impervious surface, and vegetation in urban and suburban areas can either exacerbate or ameliorate the UHI. Superimposed on this are spatially variable demographic characteristics of the human population. As a result, extreme temperatures are distributed unevenly among neighborhoods, with minority, low-income, and elderly residents at greatest risk for exposure to high heat.