Six U.S. cities feature some of the country’s most intense urban heat islands, according to a report.
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.
These cities, along with Houston, San Francisco and even relatively small places such as Burlington, Vt., feature some of the country’s most intense urban heat islands, according to a report by the independent research group Climate Central.
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.
Most intense heat islands
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:
1. New Orleans: The Big Easy had the nation’s most intense heat island because of an abundance of dark, impermeable surfaces.
2. Newark: The report cited the city’s impermeable surfaces, building height, and population density as key factors.
3. New York City: For the same reasons as Newark.
4. Houston: According to Kinder Institute Research, nearly 187,000 football fields of impervious surfaces were added to the metro area from 1997 to 2016.
5. 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 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.
For the estimated 83% 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 U.S. 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.
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.
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.