By Dane Gambrell
July of 2019 was Boston’s hottest month ever recorded, with an average temperature of 78.5 degrees Fahrenheit. While temperatures continue to rise around the globe, the city of Boston also stays significantly warmer than the surrounding countryside.
This phenomenon, known as the urban heat island effect, is caused by the replacement of a rural area’s vegetation and soil, which provide moisture and shade, with roads, buildings, and other infrastructure. These impermeable surfaces retain more heat, leading to higher air and surface temperatures that remain warm into the night. The Environmental Protection Agency estimates that the average air temperature for a city of 1 million can be 1.8–5.4 degrees Fahrenheit warmer than its surroundings.
Last summer, a citizen science project led by Boston’s Museum of Science mapped this effect block-by-block. Using handheld thermometers, a team of 50 volunteers collected geotagged temperature and humidity data for the areas of Boston, Brookline, and Cambridge, Massachusetts. Readings were collected for each location during the morning, afternoon, and evening.
These measurements were then used to create a series of interactive web maps showing the results. The first map shows the air temperature collected at 3 pm for each area.
As expected, the warmest areas are the city’s historic downtown and surrounding neighborhoods. The less densely-populated areas to the south and east and cooler by comparison.
This map is similar, but shows a hotter city overall compared to the pure air temperature. Several coastal areas, such as the Squantum neighborhood and the islands in Dorchester Bay, are warmer than shown on the air temperature map, perhaps due to higher levels of humidity.
A third map plots the change in temperature between the afternoon and early morning. This map shows that the hot, dense areas in the city center retain the most heat, while the surrounding areas cool off the most overnight.
Similar data collection initiatives are underway in 24 cities across the world, including Honolulu, Hong Kong, Doha, Qatar, and Hermosillo, Mexico. Such precise and accurate data can help municipal leaders to make more informed decisions about how to allocate city resources. In cities like Washington DC, Baltimore, and Richmond, Virginia, researchers have identified a correlation between impoverished neighborhoods –which often lack green space– and higher temperatures. These are also the populations that could benefit the most from resources like community cooling centers and better electrical infrastructure to help keep the air conditioning running.
Urban heat islands do not occur due to a lack of vegetation alone, but as a result of additional factors such as building heights, canopy cover, and street widths. Identifying where warming occurs is crucial to help planners understand why it happens, and how future cities can be designed to mitigate heat risks.