Urban heat island

An urban heat island (UHI) is a built up area that is hotter than nearby rural areas. The annual mean air temperature of a city with 1 million people or more can be 1–3°C warmer than its surroundings.

The temperature in Atlanta is 5 to 8 degrees Fahrenheit higher than outlying areas, and this excess heat produces increased rainfall and thunderstorms.

This temperature gradient can gradually increase; for example, from 1945 to 1990 San Antonio has apparently gotten 3°C warmer than nearby New Braunfels.

Urban heat islands are caused by the "urban canyon effect" in which sunlight absorbed and reflected off tall buildings is trapped and causes increases in air temperature in the surrounding area. Escaping heat from building interiors, manufacturing, and other sources is a secondary source contributing to urban heat islands as well. In addition, eoads, sidewalks, parking lots and buildings in urban areas absorb and retain heat during the day and radiate it back into the air.

Heat islands cause a variety of health and environmental issues. An analysis in 2021 by Climate Central, a group of independent climate researchers, for example, determined that UHI's contribute to increased risk of respiratory illnesses, heat exhaustion, heat stroke and heat-related deaths. They also increase the energy consumption needed to air condition homes and buildings, which leads to more air pollution.

The effects of UHI's are not evenly distributed across physical areas or across urban populations. The Climate Central report, for example, suggested that "“Discriminatory housing practices like redlining along with other socioeconomic factors mean that communities of color are often in areas with fewer trees and parks and thus are exposed to higher urban heat.” The Climate Central citing the Environmental Protection Agency (EPA) report suggested also that there were important indirect effects on aquatic life as well. Very hot pavement and rooftops can heat up stormwater runoff, which drains into sewers and raises water temperatures as it flows into streams, rivers, ponds and lakes, according to the Environmental Protection Agency. In addition to health issues, very hot pavement and rooftops can heat up stormwater runoff draining into sewers eventually raising water temperatures in streams, rivers, ponds and lakes, according to the EPA.