Urban Heat Island

An urban heat island (UHI) is a metropolitan area that is significantly warmer than its surrounding rural areas. The modification of the land surface by urban development, which uses materials that effectively retain heat, is the main cause of the UHI. Paved streets, parking lots, and sidewalks, amount to about one third of the area of the average city and therefore have a significant contribution to the heat island effect. A secondary contributor to the UHI is waste heat generated by energy usage. Research into the effects of elevated urban temperatures on air quality, energy consumption, and human health has resulted in scientific, legislative, health, and municipal stakeholders implementing various strategies to mitigate this effect.

How to mitigate?

One method to reduce UHI is focused on retrofitting urban infrastructure, such as roofs and roads, with high-albedo or reflective materials. Albedo refers to a surface’s capacity to reflect solar radiation and is defined as the ratio of the reflected radiation from the surface to the incident radiation upon it. The greater the albedo (i.e. reflectivity), the less the radiative energy absorbed by the surface. Other mitigation strategies include the use of urban vegetation, such as shade trees, and pervious surfaces. Shade trees provide direct shade to buildings and pedestrians, while also improving the thermal environment through evapotranspiration processes and increasing albedo. Impervious surfaces lead to runoff of available moisture and limit the ability of cities to be cooled by evapotranspiration processes.

Where to learn more and who to contact?

Check out the Green Up info pages on Cool Pavement and Permeable Pavement

Environmental Protection Agency web page on urban heat island:

Lawrence Berkeley National Laboratory: