The Thermodynamics of Good Cities

You don’t need to travel extensively to realize that a hot day doesn’t feel the same everywhere. How does the same summer day that seems perfectly comfortable in the countryside or on a shaded boulevard feel so uncomfortably hot in a major city? Comfort is subjective, but more often, the heat you experience is the result of the infrastructure around you, which is itself the result of choices made by engineers and city planners. Therefore, as record heat waves and drought are becoming more frequent occurrences, it is worth evaluating the impacts of these choices from a scientific perspective.

The urban heat island effect

Over the past few years, the urban heat island (UHI) effect has grown in visibility. Many people are familiar with the concept, but several misconceptions remain. The UHI effect is the phenomenon where urban environments are often much hotter than rural areas. This effect is not solely due to building density, nor is it an inevitable consequence of urbanization. The primary factor contributing to this phenomenon is manmade surfaces like roofs and asphalt and their ratio to green spaces — like trees, shrubs, grass, and parks — and blue spaces, i.e., urban bodies of water (1).

Manmade surfaces store heat from the sun, as well as heat created by engines and air conditioning (AC) units, which increases daytime temperatures. Importantly, these surfaces emit this heat during the night, which limits nighttime temperature drops. The exact difference in temperature varies from city to city. One meta-analysis found that urban parks were, on average, 1 K cooler during the day. Another study of London parks found that a 3 K cooling radius extended 200 m from the parks’ boundaries. This effect becomes clearer when you compare the thermal characteristics of materials found in cities.

Cement, stone, and brick have relatively low specific heat capacities, around 1,000 J/kg-K (2). They are also dense and cover large expanses of land that are often exposed to direct sunlight. These characteristics allow them to absorb thermal energy quickly and release it slowly overnight, acting like a thermal battery. During hot days, cement can quickly become hot enough to cause burns. Compare this to water, which has a specific heat of ~4,200 J/kg-K and is far less thermally conductive. These qualities allow urban bodies of water to act as “heat buffers” that accept thermal energy without radiating it back to the surrounding environment. Water’s albedo — its ability to reflect light from the sun — is also much lower than that of concrete or asphalt.

The potential of green spaces

Even compared to water’s considerable cooling ability, the cooling power of green spaces is more substantial. Due to having a lower thermal conductivity and higher specific heat capacity, vegetation exhibits far lower surface temperatures than asphalt. Evapotranspiration, where vegetation releases water vapor when heated, cools the surrounding air and strongly contributes to greenery’s ability to keep spaces cool. Green spaces also passively decrease urban heating by reducing the need for mechanical cooling with AC units.

In addition, green spaces reduce flood danger and water waste, as precipitation soaks into the ground instead of evaporating on asphalt. Perhaps just as importantly, green spaces improve the aesthetics of urban environments and quality of life.

Effect of car infrastructure

As a reliance on cars requires extensive networks of roads and highways, car-centric infrastructure is a significant contributor to the UHI effect (3). Neighborhoods of single-family housing units frequently suffer from extreme urban heating despite their low housing density. Although many suburbs have extensive tree coverage, lower-income neighborhoods often suffer from a lack of green spaces. Minimum parking requirements mandate expanses of frequently unused parking lots around new developments. All of this infrastructure absorbs heat in a way that pedestrian-friendly areas and public transit infrastructure do not.

The solutions

Designing new houses and buildings to promote passive cooling mitigates the effects of the UHI for the building’s residents while reducing energy intensity. Improving and expanding pedestrian spaces and public transit infrastructure improves quality of life and reduces the need for more highways, arterials, and parking lots. However, to improve existing infrastructure in the U.S., advocates must use scientific understanding to argue for seemingly untechnical solutions. One would be hard-pressed to find a rigorous scientific analysis of the subject that doesn’t come to the simple conclusion that communities need more trees.

Still, if the UHI effect is a problem in your community, getting a tree planted can be easier than you think. Visit your city’s website or contact your local parks and recreation department, as many cities already have programs to plant trees on request. Where I live, Bostonians can request a tree at boston.gov. See if you can get a tree planted in your community and make your neighborhood a little cooler.

1. Kershaw, T., et al., “Utilising Green and Bluespace to Mitigate Urban Heat Island Intensity,” Science of The Total Environment, 584–585 (1), pp. 1040–1055 (Apr. 2017).
2. GreenSpec, “Thermal Mass,” https://www.greenspec.co.uk/building-design/thermal-mass (accessed Oct. 6, 2023).
3. Hoehne, C., et al., “Urban Heat Implications from Parking, Roads, and Cars: A Case Study of Metro Phoenix,” Sustainable and Resilient Infrastructure, 7 (4), pp. 272–290 (July 6, 2020).

This article originally appeared in the Emerging Voices column in the November 2023 issue of CEP. Members have access online to complete issues, including a vast, searchable archive of back-issues found at www.aiche.org/cep.