The Urban Heat Island Challenge – How to quantify this pressing concern for cities facing a changing climate

Sangiorgio, V., Fiorito, F., & Santamouris, M. (2020). Development of a holistic urban heat island evaluation methodology. Scientific Reports, 10(1), 1–13.

As summer approaches cities across the United States, the dread of sweltering heat, drenching humidity, exhausting commutes, and refuge to air conditioning may cross urban dwellers’ minds. Living in a city comes with extremes in temperatures, depending on the climate zone, whether that’s catching an ice storm in January or weeks of heat waves beating down in July. An important part of city infrastructure is being able to provide services for their inhabitants during these extreme weather events. Subsidized heating, cooling centers, mist tents, and access to these services is critical for helping to keep the health of the population stable and equitable. However, as cities face a changing climate, extreme weather, and the urban heat island effect (UHI) are likely to intensify meaning cities will have to be prepared to keep their populations safe.

Temperatures are higher in urban areas, as compared to nearby urban areas due to the urban heat island effect.
Urban Heat Island Effect

The UHI is a phenomenon in which urban areas experience warmer temperatures than surrounding rural communities. The UHI is caused by several factors including:

  1. Land cover type – most cities have a lot more concrete and impervious surfaces that hold onto heat as compared to rural areas
  2. Trees and vegetation – trees and vegetation help to cool temperatures and cities have less greenery as compared to rural areas
  3. Anthropogenic heat – or heat caused by a concentrated large population, think energy use, heat from transportation, industrial spaces
  4. Urban form – or city canyons that can affect temperatures and wind flow patterns making it feel hotter in a city

In addition to warmer temperatures, the UHI has a serious impact on the health and safety of vulnerable populations and an increase in cooling demand means that electricity consumption rises dramatically. Although the impacts of the UHI are devastating and well understood, measuring and quantifying the intensity of the UHI across different cities is challenging to do. Many methods for quantifying the intensity of the UHI are time and data intensive, complex, and resource consuming meaning not all cities can afford the time or cost spent on doing a full-scale investigation. A team of authors aimed to tackle the complexities associated with quantifying the intensity of the UHI by creating an analysis that would measure the influence of the variables leading to the UHI for a given city and would inform municipal leaders of the local UHI hazard for their city.

The team of researchers tackled this ambitious project by first completing a large data acquisition project, including the wide breadth of variables and data that influence the intensity of the UHI in different cities. They performed analyses to further define what is meant by the UHI, in addition to measuring the influence of each variable on predicting the intensity of the UHI using a number of validation approaches.

What are the causes?

Results from this exhaustive and novel study found 11 variables or criteria of most importance that lead to the intensity of the UHI for a given city. These 11 criteria had varying influence on the extent to which an urban area would feel the effects of the UHI. The authors found that the two most important criteria for quantifying the intensity of the UHI in an urban area were Albedo and Greenery. Albedo refers to the ability of surfaces in a city that come in contact with sunlight (think streets and rooftops) to reflect solar radiation and reduce heat in the surrounding areas. Greenery, as discussed above, refers to the amount of vegetation in a city, which is important for heat reduction because vegetation helps cool the surrounding air both by providing shade and reducing air temperatures through a process called evapotranspiration.

Plants have a special ability to help lower local temperatures. This can have benefit in cities experiencing high temperatures due to the urban heat island effect.

In addition to albedo and greenery, the authors found that city canyons, or areas between buildings, had a large impact on the intensity of the UHI. City canyons is a broad term that includes the width of the streets, building height, city canyon orientation, and irregularity of the city. Following city canyons, the authors found anthropogenic heat, or population density, to be an important predictor of the intensity of the UHI. Lastly, the authors found the impact of meteorological variables, such as clear skies and wind patterns to have a significant effect on the intensity of the UHI.

Moving Forward

By studying the impact and importance of these 11 criteria, the authors were able to produce an easy-to-use index for estimating the intensity of the UHI that is applicable to different cities. The authors explain that this analysis was performed for anticipating the intensity of the UHI during the summer and would need to be amended for use in cold climates. This research can be used to develop hazard maps of the intensity of the UHI both within a city through different neighborhoods and across different cities. This study has important implications for urban planners hoping to reduce the intensity of the UHI in their own city and targeting relief efforts for populations most at risk of negative health outcomes from excess heat exposure.

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Jessica Wright

Jessica Wright

I am a fourth year PhD. student in earth sciences at Boston University in the Department of Earth and Environment. My research focuses on urban infrastructure systems and energy transition policy, specifically focusing on the role of natural gas. I completed my undergraduate studies at Connecticut College in Biology and have worked with a lot of non-profits in and around the Greater Boston area on energy transition policy-making. I love to swim, do yoga, and travel!

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