Chapter 4 - The Built Environment
Part 5 - Urban Heat Island Effect
What is Urban Heat Island?
Urban heat island (UHI) is a term used to describe the increased temperature of city centers compared to rural surroundings. UHI is a result of high anthropogenic activity in densely populated areas. UHI is caused by an increase in impervious surfaces like asphalt, concrete, and buildings which have replaced the natural landscape and vegetation. During the daytime built surfaces absorb heat causing the surface temperature to increase. Typically, built surfaces have low heat emittance, therefore during nighttime heat is released causing not only an increase in day temperatures but at night as well (Fruh, 2010). In addition to the surface heat absorption/release, anthropogenic activities like space heating, air condition, transportation, cooking and industrial processes also introduce heat into the urban centers (Wilby, 2008).
Urban heat island does not only affect temperature. Due to the unique geometry of urban centers, the built environment can obstruct wind speed and reduced convective heat losses. The most important determinants of UHI have been summarized into eight different controls, which effect UHI magnitude and structure (Wilby, 2008):
- UHI intensity decreases with increasing wind speed.
- UHI intensity decreases with increasing cloud cover.
- UHI intensity is greatest during anticyclonic conditions.
- UHI intensity is best developed in the summer or warm half of the year.
- UHI intensity tends to increase with increasing city size and/or population.
- UHI intensity is greatest at night.
- UHI may disappear by day or the city may be cooler than the rural environs.
- Rates of heating and cooling are greater at rural sites than the city.
- urban heat island (UHI)
- heat wave
- climate change
- thermal wavelengths
- green roofs
- cool roofs
- cool pavements
- surface and near-surface air temperatures
- USEPA. "Heat Island Effect | U.S. EPA." US Environmental Protection Agency. 29 Mar. 2011. Web. 18 Apr. 2011.
(By Kiersten Lee/Gregory Möller)