Urban Heat Islands

Content on urban heat islands, heat impacts, and recruitment for the Urban Heat Island Mapping Campaign was developed for the City's Climate Protection Program through the UVA Climate Ambition Summit summer internship program in 2021. We’d like to acknowledge and thank Seth Evans, Grace George, Gavin Oxley, and Yulun Wu for their work. 

Drawing of an Urban Heat IslandImage Source:  American Forests

  1. Urban Heat Effects
  2. Local Impacts
  3. Reduce Urban Heat
  4. References

Heat and its effects on environments

How do urban heat islands form?  

During the day and night, rural and urban areas are subject to the same sun exposure, yet they have different temperatures that occur. This happens because the composition and geometry of these two areas are drastically different. An urban area is mostly made up of asphalt and buildings, amongst other opaque materials; in contrast, a rural area is going to have more green areas and typically fewer vertical structures. In an urban area there is the added factor of this phenomenon called an urban surface heat island as well as an urban atmosphere heat island. These two things are different as the surface and atmosphere can have differing temperatures for a variety of reasons. The surface heat islands are more dramatic and influential to humans during the day than the atmospheric ones are. These surface heat islands linger throughout the day and night and can have temperatures that are 1-7 degrees Fahrenheit warmer and 2-5 degrees warmer, respectively so. These urban heat islands form in large and small cities, as well as northern and southern ones, however, it does seem to be more dramatic in eastern cities.

Chart showing Variations on Surface and Atmospheric Temperatures across different land use types
Figure: Variations of Surface and Atmospheric Temperatures (source: U.S. EPA)

These heat islands form from a variety of reasons such as:

  • Reduced natural landscapes: vegetation and water bodies provide shade, transpire water from plant leaves, and evaporate surface water which cools the air. Hard, dry surfaces found in urban areas, such as roofs, sidewalks, roads, buildings, and parking lots provide less shade and moisture, leading to higher temperatures and less cooling effects. 
  • Urban materials: pavement and roofing tend to reflect less solar energy and instead absorb and re-emit the sun’s heat more than natural landscapes. Due to the absorption during the day, heat islands can build throughout the day and become more severe after sunset as the heat slowly releases from urban materials. According to the EPA, downtown areas can absorb and store twice the amount of heat compared to their rural areas during the day.
  • Urban geometry: dimensions and spacing of buildings within a city affect the urban material’s absorption of solar energy and the design of a city can block wind flow. This can combine with the issue nighttime cooling, in that the heat that is released by urban material gets trapped by the urban design.
  • Human activities: anthropogenic waste heat, such as vehicles, air-conditioning units, and industrial facilities. 
  • Weather and geographic location affects heat islands as well, for example, clear weather conditions allow more solar energy to reach the urban area while strong winds and clouds may suppress heat. Being located near large bodies of water can dramatically help with temperature moderation or having nearby mountains can block wind from entering the city.