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Boston has the sixth hottest heat island in the US, new report says

Boston was ranked as having one of the hottest heat islands in the country.David L. Ryan/Globe Staff/file

City dwellers know all too well that a stroll through downtown Boston can be stifling on a summer afternoon. But a walk on the same day just outside of the city offers a significantly cooler experience — and a new report reveals just how staggering that difference can be.

Out of 159 cities evaluated by independent research group Climate Central, Boston was ranked as having the sixth hottest heat island in the US, according to a report released Thursday. Urban heat islands refer to zones of elevated temperatures that form from high concentrations of dark, heat-retaining surfaces like asphalt and concrete on paved roads, parking lots, tall buildings, and other features commonly found in cities. Heat islands can run as much as 15 to 20 degrees hotter than surrounding tree-lined areas, according to the report, and they disproportionately affect communities of color that have been subjected to racist housing policies.

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Climate change is making extreme heat events worse and more frequent, and by mid-summer 2021, communities across the country have already sweltered through record-shattering stretches of heat. Urban heat islands can exacerbate the impacts of these heat waves, the report said, compromising health and comfort, and often creating dangerous conditions for many vulnerable populations.

Climate Central created an index to evaluate the intensity of urban heat islands and applied it to cities across the US. The index measures type of land cover in each city, from greenhouse space to paved areas, and factors including building height, population density, and average width of streets.

Researchers ranked the top 20 most intense heat islands, and in addition to Boston securing the sixth most stifling spot, Providence, R.I., and Burlington, Vt., also made an appearance on the list.

1. New Orleans

2. Newark, N.J.

3. New York City

4. Houston

5. San Francisco

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6. Boston

7. Chicago

8. Miami

9. Baltimore, Md.

10. Providence, R.I.

11. Sacramento, Calif.

12. Salinas, Calif.

13. Burlington, Vt.

14. Bend, Ore.

15. Cleveland, Ohio

16. Detroit

17. Erie, Pa.

18. Fresno, Calif.

19. Lafayette, La.

20. McAllen, Texas

Cities in the Northeast like Boston, Newark, and New York have more compact, historically-built out environments with taller buildings, researchers said, adding to the intensity of their heat island footprint.

Cities in the typically sweltering Southwest, however, did not make the list, which may come as a surprise to some. Researchers said it’s not that these cities don’t face extreme heat, but rather their surrounding communities aren’t much cooler than the urban areas due to a desert landscape that absorbs and retains a lot of heat.

Smaller cities like Burlington that made the list might have also come as a surprise. According to the study, urban heat islands don’t only occur in large cities. Malls, parking lots, hospital campuses, and anywhere with more impermeable surfaces and fewer trees can create a heat island effect, according to the study, even if they are located in what might be considered a rural or suburban area.

Heat emissions can come from transportation, machinery, appliances, and the heating and cooling of buildings. During a heat wave, air conditioning urban buildings can add 20 percent more heat to the outside air, according to the report. Heat emissions also pile up when there’s a lack of vegetation. Plants help cool the air, so where there is less green, there is less evaporation, and thus, more heat.

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Tall buildings also account for the intense heat these urban areas experience. The shape an

d height of buildings can trap hot air, as well as prevent pollutants from dissipating, reducing air quality, the study said.

So why does this all matter?

Extreme heat is the leading cause of weather-related fatalities over the past 30 years, according to the National Weather Service. Extreme heat also makes air pollution worse, which amplifies existing health issues for people with conditions like asthma. The heat also makes it harder and dangerous for people to work outside, who are at risk of heat stress.

Low-income neighborhoods and communities of color also face disproportionate impacts of extreme heat. Research suggests that neighborhoods experiencing higher intensity urban heat islands were often the same ones subjected to discriminatory, racist housing practices known as redlining, according to the study. People living in homes below the poverty line also tend to live in areas that have less green space.

Extreme heat also leads to increased demand for cooling that strains the grid, drives up energy costs, and creates more greenhouse gas emissions, contributing to climate change.

What can cities do?

There are a number of short-term and long-term solutions to adapt to increasingly warming weather, as well as mitigate effects from urban heat stress. Here are a few outlined in the study:

  • Provide information and transportation to public places with air conditioning or cooling centers.
  • Public alerts, telephone hotlines, and neighborhood teams should look out for seniors and those with compromised health conditions.
  • Plant more trees
  • Install a green roof or a rooftop garden, which can provide shade and lower temperatures on roof surface and surrounding areas. Cool roofs are also an option, and made of reflective materials that remain cooler than traditional materials.
  • Cooling pavements, or whitewashing roads and sidewalks, is another way to cool things down in cities.
  • Considering the width of streets and the height of buildings when developing urban areas.
  • Switching to solar energy, which reduces carbon dioxide emissions as well as pollutants that are a threat to public health. Solar panels have also been shown to provide shade and reduce the heat island effect.

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Brittany Bowker can be reached at brittany.bowker@globe.com. Follow her @brittbowker and also on Instagram @brittbowker.