If you’ve ever walked through New Orleans, Newark or New York City on a hot summer day, you’ve probably felt warmth radiating off the buildings or witnessed the heat mirage permeating off pavement and rooftops.
Urban heat islands are zones of elevated temperatures that form from high concentrations of dark, heat-retaining surfaces like asphalt and concrete. They can run as much as 15 to 20 degrees hotter than surrounding rural areas.
Climate Central dug into the characteristics of 158 cities around the United States to rank their heat island intensity.
The most intense heat islands, ranked
To calculate heat island intensity, Climate Central evaluated how much of the sun’s energy the cities were reflecting, or their “albedo,” their percentage of greenery, population density, building height, and the average width of streets (which affects how much heat is trapped near the ground), among other factors. Each city was assigned a score reflecting the difference in temperature between the city and its less developed surroundings.
They found the top five most intense heat islands in:
- New Orleans: The Big Easy had the nation’s most intense heat island because of an abundance of dark, impermeable surfaces.
- Newark: The report cited the city’s impermeable surfaces, building height, and population density as key factors.
- New York City: For the same reasons as Newark.
- Houston: According to Kinder Institute Research, nearly 187,000 football fields of impervious surfaces were added to the metro area from 1997 to 2016.
- San Francisco: The city traps heat mainly because of building height, impermeable surfaces and population density.
It’s not just big cities that have intense heat islands. Smaller cities that effectively retain heat, such as Burlington, Fresno and Salinas, Calif., and Erie, Pa., also made the top 20.
“The impacts of urban heat island can be very disparate, so our report was a way for us to remind people that heat is not impacting everyone equally,” said Jen Brady, a senior data analyst with Climate Central.
Surprisingly, no cities in the typically scorching desert Southwest made the cut; places like Phoenix and Albuquerque had relatively low scores. That’s not because they aren’t hot but because their surrounding communities aren’t that much cooler since their desert landscapes absorb and retain a lot of heat.
Washington, D.C., which had one of the top 10 most intense heat islands in a Climate Central analysis conducted in 2014, did not make the top 20 this time but had a rather high heat island index score of 6.3 degrees.
“It’s important to remember that each score is an average for the entire city, and certain neighborhoods or areas of a city will likely be cooler or hotter, depending on vegetation and other factors,” the report stated.
When Washington’s heat island was mapped in 2018, research found temperatures varied across sections of the city by as many 17 degrees.
Why it matters
For the estimated 83 percent of the U.S. population that lives within cities and urbanized areas, the higher temperatures mean more heat-related illnesses and increased energy demand and associated costs. Higher temperatures can also reduce water quality and increase air pollution.
Heat is the leading weather-related cause of death in the United States and affects people unequally. Higher risk categories for heat illness include children and people over 65, as well as those with chronic health conditions such as obesity, heart disease and diabetes. Outdoor workers and athletes training outside are also at greater risk. Low-income communities that may lack air conditioning or green spaces are also particularly vulnerable.
The climate change connection
Heat islands are a local-scale phenomenon and not caused by global climate change, or the increase in temperatures from the buildup of greenhouse gases in the atmosphere. But since global climate change is increasing the intensity of heat and other weather events, heat islands are also being intensified, raising temperatures even higher.
Humans are creating new urban heat islands, but this does not explain the global warming trends being observed, including in the oceans where urbanization is not a factor and in weather stations in rural areas. In certain cities, where the characteristics of the urban area haven’t changed for a long time, the continuation of rising temperatures offers clear evidence that warming is compounded by global climate change.
“A good example is New York City, where the weather station has been in Central Park since the 1800s, and we know New York City hasn’t expanded because it has physical limits being surrounded by water,” said Climate Central’s Brady. “Yet New York City is still seeing a steady increase in temperatures. So we know that’s not the urban heat island effect; there’s something else going on.”
Reducing the heat island effect
Many communities are working to reduce urban heat island impacts using short- and long-term strategies. In the short term, approaches focus on protecting vulnerable populations from excessive heat by opening cooling centers, establishing heat information hotlines and deploying neighborhood teams to check in on older adults.
“We need to begin conversations about what cities can do and get through these acute heat periods,” said Kim Cobb, a climate scientist at Georgia Tech. “We can get ahead of the scramble and mobilize a community through its connective tissue.”
Long-term cooling strategies, which focus on changing or adapting building materials and introducing more green spaces, can be more complicated and costly, but a number of cities are undertaking such measures. They include:
- Increasing tree and vegetative cover
- Installing green roofs
- Installing cool — mainly reflective — roofs
- Using cool pavements (either reflective or permeable)
- Using smart-growth practices
The National Oceanic and Atmospheric Administration has supported community-led urban heat island mapping campaigns in a growing number of cities every year, partnering with universities and environmental and social justice groups to study how heat risk aligns with important dimensions of local vulnerability, including race and ethnicity, income, access to air conditioning, and heat-related illnesses.