Why Boston Brings the Heat
Boston can feel notoriously hot in the summertime — and it’s partially the city’s fault. While the climate influences how people develop and use a city, a city also affects regional climate conditions, such as cloud cover, precipitation, air temperature, and wind speed. Within the city, the shapes, spacing, and orientations of buildings also create microclimates that can vary even in the distance of a few meters.
These factors sometimes culminate in the urban heat island (UHI) effect. This phenomenon occurs when the urban air temperature is higher than that of the surrounding rural environment. The effect can be especially apparent at night when cities cool down much more slowly as structures and roads release all the heat they absorbed from the sun that day.
Various potential causes combine to create the UHI effect. Building and street surfaces absorb and trap short-wave radiation from the sun by reflecting between each other. Heat can also be intercepted by buildings that obstruct surfaces from the sky, radiating the heat back into the city landscape. At the same time, air pollution in the atmosphere absorbs and re-emits long-wave radiation, while traffic, heaters, and industries release anthropogenic heat. Evaporation is also decreased because cities are made of less permeable materials and vegetation compared to rural areas.
A 2012 study in the journal Resources, Conservation and Recycling proposes several design principles for Dutch cities to mitigate their UHI effects. These strategies, which can be applied to other regions, aim to diminish accumulated heat and apply cooling techniques.
For example, increasing vegetation can help cool the environment actively by evaporation and transpiration, as well as passively by shading surfaces that would otherwise absorb short-wave radiation. This means plants like trees and grass can help cool areas through shade and released water. The four different types of vegetation in urban areas are urban forests (parks), street trees, private green in gardens, and green roofs or façades. Though street trees are typically spread out, their large quantity adds up to a significant impact on temperature within the city. On a sunny day, a single tree cools with the power of 20–30 kW, which is equivalent to that of more than 10 air-conditioning units, according to the study.
On a sunny day, a single tree cools with the power of 20–30 kW, which is equivalent to that of more than 10 air-conditioning units.
Results from a 2016 study by Boston University researchers in Environmental Research Letters corroborated this approach by quantifying the interactions between urban vegetation and UHIs in the Boston metropolitan region. The researchers found that, on average, land surface temperatures in Boston were around 7 degrees Celsius warmer and the plant growing season was 18–22 days longer compared to adjacent rural areas. But for parks and other vegetation patches in Boston’s urban cores, they observed similar temperature and growth timing patterns to the rural areas. These findings suggest that urban vegetation patches provide a significant ecosystem service and societal benefit in offsetting the UHI effect at the local city scale.
Discovering and implementing these strategies is important because excessive heat can have serious effects on health and well-being to some populations in particular beside general discomfort. Based on a 2015 study in the International Journal of Disaster Risk Reduction on social factors that reduce resilience of Bostonians, the city’s Climate Ready Boston project identifies social groups especially vulnerable to the impacts of extreme climate and weather events, including heat.
Some age groups like older adults and children are physically more vulnerable to extreme heat. Many people of color, who make up 53 percent of Boston’s population according American Community Survey data, may have limited English proficiency and consequently limited access to information about the dangers of extreme heat or resources such as cooling centers. Additionally, people of color often live in more densely populated urban areas which are at higher risk of impacts from the UHI effect.
With the uncertainty surrounding future climate conditions, cities like Boston must continue adapting their environments to mitigate the effects of urban warming and protect the populations that live within them.
DOI:10.1016/j.ijdrr.2014.12.001
DOI:10.1016/j.resconrec.2011.06.004
DOI:10.1088/1748–9326/11/5/054020
