Growing up around Washington, D.C., it’s a relief to now live in a city laid out in a coherent fashion, with numbered streets increasing along a cardinal directional axis and being intersected at sensible right angles. Which is why it pains me to say that physics brings some bad news for my fellow gridded city-dwellers: our streets may be set up to swelter.
Research published this month in Physical Review Letters found that the way a city’s streets and buildings are arranged strongly relates to the urban heat island effect, with orderly, gridded cities experiencing greater nighttime heat build-up than cities with a less tidy layout. This information could be useful to planners in areas that are urbanizing quickly in the age of climate change, but other experts say we should be careful not to oversimplify things.
The urban heat island effect is a well-known phenomenon: cities are heat traps compared with the surrounding countryside, thanks largely to impervious surfaces like concrete and asphalt that absorb heat during the day and re-radiate it at night. The effect can add more than ten degrees Fahrenheit to the temperature, increasing the risk of dehydration and heat stroke on hot days.
To explore the relationship between urban heat and a city’s layout, the MIT researchers behind the new study adapted a model normally used to study the atomic scale structure of condensed matter. They combined it it with temperature data captured both in a city and in the surrounding countryside, and satellite imagery showing building patterns, for 22 U.S. cities. The model was also applied to previously collected data on a larger group of cities worldwide.
The analysis revealed that cities have distinct “textures” that can be likened to the structure of crystals (OK condensed matter physicists), and that more ordered cities like Chicago experience a larger heat island effect than disordered, “glass-like” cities, like Los Angeles. The differences appear to relate to fact that buildings can absorb heat re-radiated from other buildings that face them directly, according to a press release. They also appear to relate to the sky view factor, which describes how obstructed the sky is from the ground.
“Our analysis thus suggests that city texture plays a key role in determining its response to heat radiation phenomena and points to urban design parameters that can be modulated to mitigate UHI [urban heat island] in planning and retrofitting of cities,” the researchers wrote.
Scott Krayenhoff, an atmospheric scientist at the University of Guelph who was not involved with the study, called it “a very interesting application of spatial analysis techniques to the field of urban climatology.” However, he had some issues, including the fact that paper “appear[s] to mix surface and air temperature [urban heat islands]” which do not easily relate to one another, and that other factors like materials and heat release by buildings “are likely to play important roles or even dominate in other contexts.”
James Voogt, a geographer at the University of Western Ontario who also wasn’t involved, told Earther in an email that we also want to take into account vegetation, which “has a large potential to modify urban temperatures.”
Many cities are already onto this. L.A., for instance, recently implemented a goal of reducing the urban-rural heat differential three degrees by 2035, which will be accomplished in part by planting trees.
Caveats aside, the study’s findings are important to consider, as climate change drives temperatures upwards and planners need to figure out how to dial the urban heat bubble down. Maybe, in addition to more trees and painting major thoroughfares white, more open and fluid designs can sometimes help, too.
I still don’t think D.C. has an excuse for being a disorderly mess.