Emerging Voices: 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...