Cities and urbanized areas are generally hotter than the surrounding countryside. In some instances, much hotter. This discrepancy, known as the urban heat island (“UHI”) effect, occurs for a number of reasons, but it can largely be attribute to the greater presence of asphalt and concrete in urban areas. A recent study done by researchers at the University of Georgia analyzed land uses in the 50 largest metropolitan areas of the country and not only validated this idea, but provided evidence of a direct relationship between the contiguity of developed parcels of land and the UHI effect.
Generally speaking, when an urban area has a greater contiguity of developed parcels of land it experiences a more extreme UHI effect. The researchers found that increasing the urban spatial contiguity by 10 percent is predicted to increase the minimum temperature average annual urban heat island intensity by between .3 degrees Celsius and .4 degrees Celsius. Simply put, the study shows that parcels of open, natural land have a huge impact on lowering the concentration of heat that builds up in urban, developed areas.
This probably doesn’t come as much of a surprise as walking across that huge parking lot in the middle of summer sometimes requires several water breaks. Trees not only provide direct shade from sunlight, but prevent the absorption and release of heat by concrete throughout the day. Not only do grasses and other plants absorb much less heat than asphalt and concrete, they also release cooler, moister air through transpiration. In fact, some cities, mainly in the western and southwestern parts of the country, have temperatures that are cooler than the surrounding landscapes partially due to this occurrence.
While the study validates the existing scientific consensus that developed land causes hotter air temperatures than undeveloped land, the real value in the study is what it apparently disproves: there is no statistically significant correlation between population and urban heat island intensities. Meaning areas with lower population densities and areas with higher population densities could see the same temperature deviations from their surrounding natural landscapes.
At the end of the paper the authors take the opportunity to note that this research shouldn’t be looked at in a vacuum when being used to craft policy. While the UHI effect in leafy suburban settings may be much lower than in denser urban areas, those denser urban areas are less auto-centric and offer many benefits including more active lifestyles and improved overall air quality. The research supports the idea that strategically placing natural landscapes in denser, urban environments can greatly mitigate the intensity of the urban heat island effect.
As an example of how we can achieve the benefits of urban living while also reducing urban heating, the authors point to our own Savannah. The strategically placed squares and parks of historic Savannah is the exact type of land use pattern that would seemingly mitigate the urban heat island effect. These heavily-vegetated spaces and tree-lined streets greatly reduce the contiguity of developed land and promote a cooler urban climate without limiting the benefits of a denser built environment.
Seemingly small things go a long way. The lack of a comprehensive tree ordinance in Louisville, KY is hypothesized as a reason why that city exhibited the third most intense UHI effect of the 50 largest cities. The abundance of trees here in Atlanta is perhaps one reason why our metro area fails to exhibit an intense UHI effect.
Miami, second on the list in terms of cities with the greatest UHI effect, may have it’s own zoning problems to blame. The authors hypothesize that the tall buildings in the city may create a wall that prevents cool breezes from the Atlantic from reaching much of the urbanized area. Transitional height planes could come to the rescue. While we’ve spoken about this zoning tool several times in other articles with regards to protecting the aesthetic integrity of areas it could also be used in the context of the UHI effect. Perhaps Miami and other coastal cities could benefit from implementing a transitional height plane that allows buildings to gradually get taller as they get farther from the coastline thereby allowing greater air circulation, which could reduce heat and pollution.
The creation and protection of sensible policies and zoning rules that promote more open space is critical for a number of reasons, including, as we now know, the mitigation of localized heating. The research tends to show that it is better policy to allow for or demand the creation of many, smaller tracts of open space throughout an urban area than to only allow for or create several larger parks or open spaces. Not only does it give more people easier access to parks and open space, but it provides the dis-contiguity that is needed to promote cooling.
See Debbage, Neil and Shepher, J. Marshall, The Urban Heat Island Effect and City Contiguity, Computer, Environment, and Urban Systems 54 (2015) 181-194 <http://www.researchgate.net/publication/280883456_The_Urban_Heat_Island_Effect_and_City_Contiguity>