How these 5 cities became the most intense urban heat islands

There's a reason you can cook an egg over pavement on a hot, sunny day.

Pavement, concrete, bricks, blacktop, parking lots and buildings all absorb and retain heat during the day, then radiate the heat back out. And with cities in no short supply of pavement or a large mass of buildings, it's no surprise that metropolitan areas can see higher temperatures than their rural counterparts.

But in many cases, this isn't a difference of just a few degrees. Neighborhoods in highly developed cities can experience mid-afternoon temperatures that are 15 to 20 degrees higher than outlying areas with more vegetation and less development, according to the National Integrated Heat Health Information System.

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Climate Central released a report on July 14, 2021, detailing how the group's scientists and researchers created an index to evaluate the intensity of urban heat islands and then applied it to 159 cities across the nation. In the top five, New Orleans, Newark, New York City, Houston, and San Francisco scored as having the most intense urban heat islands, ranging from 7 to 9 degrees Fahrenheit higher on average.

The study also included large cities such as Chicago, Illinois (7 degrees); Charlotte, North Carolina (7 degrees); Portland, Oregon (6 degrees); and Richmond, Virginia (6 degrees).

Cities in the Midwest and Northeast were more compact, historically built-out environments, with taller buildings, which added to the intensity of their urban heat island footprints, according to the study. Meanwhile, cities such as Houston and Fresno, California, scored higher due to a large percentage of impermeable surfaces in their city's topography.

"Perhaps surprisingly, many cities in the extremely hot Southwest scored lower on the index," the report noted. "Their relatively low scores are largely because their surrounding areas have temperatures similar to city temperatures."

It added that despite the lower scores, that doesn't mean the cities aren't experiencing heat impacts. Instead, it emphasizes that the city's surrounding area, made up of desert or rock, are naturally hotter due to a lower albedo.

The Manhattan skyline is seen from the observatory of the Empire State Building in New York City on Jan. 12, 2022. New York City is ranked among the top five U.S. cities with the most intense urban heat islands, according to Climate Central. (AP Photo/Ted Shaffrey)

"You can think of an urban heat island as essentially anything that humans do that make the environment around us hotter, and many of those things get amplified the more people you have living together, the more densely populated you are, which obviously that's going to take place a lot in an urban environment," Dr. Andrew Pershing, the director of climate science at Climate Central, told AccuWeather Assistant Chief Broadcast Meteorologist Geoff Cornish.

Population density was one of the top six factors that researchers considered while creating the index for the study. The others included albedo, percentage of greenery, building height and the average width of streets and irregularity of the city. Out of all of these, though, albedo contributed the most.

The sun shines near the Space Needle in Seattle on June 28, 2021, when Seattle and other cities broke all-time heat records, with temperatures soaring well above 100 degrees Fahrenheit (37.8 Celsius). Washington's largest city is also an "urban heat island," a phenomenon that pushes up temperatures in areas covered in heat-retaining asphalt and concrete. (AP Photo/Ted S. Warren)

Albedo can best be explained as to why wearing dark clothes in the summer can be warmer than wearing lighter clothes. It measures whether a surface reflects the sun's heat, like a white shirt, or absorbs it, like a black shirt.

"You think about something like blacktop or ... black roofing materials, they're gonna absorb a lot of the sun's energy, and then they're going to hold onto that heat and they're going to radiate it back to you all day long and into the night," Pershing said. "And so that's gonna have a big factor in terms of increasing the heat in the environment around you."

In addition to the previously listed factors, human-created heat emissions also played a role in the heat intensity index. Pershing found that, ironically, air conditioning units had their own role to play in contributing to heat islands. The study notes that the units from urban buildings can add 20% more heat to the outside air compared to regular summer weather.

"Things like air conditioners actually are dumping heat out into the alleyways and into the streets, and so that's contributing to the urban heat island effect as well," Pershing said. "And so you, on a really hot, muggy night, you're gonna run your air conditioner. That's going to be additional heat that's accumulating overnight."

Extreme urban heat is considered a public health threat as it amplifies air pollution and creates dangerous conditions for people who work outside or live in buildings without air conditioning. With about 85% of the U.S. population living in metropolitan areas, it's fair to say this cannot only impact a vast majority of Americans, but that it already has.

Studies have found that past discriminatory housing practices such as redlining -- a now-illegal practice in which mortgage lenders denied loans or insurance providers restricted services to certain areas of a community -- along with other socio-economic factors, have resulted in communities of color often ending up in areas with fewer trees and parks. This has resulted in entire neighborhoods experiencing higher urban heat intensity than other neighborhoods within the same city, in turn resulting in the latter communities being disproportionately exposed to urban heat.

Beachgoers enjoy a hot afternoon at Virginia Key Beach Park in Miami on Feb. 4, 2022. (AP Photo/Rebecca Blackwell)

In the past, the urban heat island effect has been noted to have contributed to the intensity of heat waves, preventing cities from cooling down during the nighttime. During the Chicago heat wave of 1995, while the actual death tolls across black and white communities were almost identical, the mortality rate for the black communities of the city were roughly 1.5 times higher than their white counterparts. In the report, Chicago broke into the top 10 cities with high intensity scores.

Outside of heat waves, areas of elevated temperatures don't just mean that the air is a little hotter, but that the area carries an increased risk of respiratory illnesses, heat exhaustion, heat stroke and heat-related mortality, according to the NIHHIS.

With this life-threatening heat, what does that mean about the data we collect surrounding it?

According to Pershing, NOAA takes into account the urban heat island effect.

Even if a weather station were to find itself in in an area impacted by the urban heat, he added, it wouldn't record the steady trends currently seen driven by climate change. Instead, it would appear as a jump in the record, going from essentially a cooler period to a warmer period.

"While humans are creating new urban heat islands, this does not explain the warming trends that scientists have recorded," the report states. It adds that evidence of warming across the planet has been seen in the oceans, where urbanization is not a factor, and in weather stations in rural areas.

In this July 24, 2018, photo, a jet comes in for approach over downtown Phoenix as temperatures exceed 100 degrees in the morning hours. Already devilishly hot for being in the Sonoran desert, Arizona's largest city is also an "urban heat island." Phoenix officials say they are tackling urban warming, monitoring downtown temperatures, planting thousands of trees and capturing rainwater to cool off public spaces. (AP Photo/Matt York)

"We have weather stations like New York City that have essentially been urban heat islands for 150 years that also show warmings as well," Pershing said.

While the topic of heating cities and health condition risks is heavy, Pershing and the study noted that there are solutions within reach.

"I'm actually really excited about this study because we spend a lot of time talking about negative things and scary things with climate change, and this work really points to clear actions that we can take to make our lives better," Pershing said.

The study not only points to certain short and long-term solutions, but also solutions specific to a city's rating on the intensity index.

"This process also points to really clear solutions that in these cities we could actually go in and diagnose what are the factors that are driving your urban heat island score, and you have the potential to lower your score by doing things like planting trees, changing roofing materials, building green roofs and things like that," Pershing said.

"Cool roofs" was a long-term solution listed in the report, referring to creating surfaces that are engineered to be highly reflective. The installation of a cool roof would also result in less transferring of heat to the building below, meaning a cooler building and less energy going toward air conditioning. The report also mentions "cool pavement," or whitewashing roads and sidewalks, though it noted that the method isn't always enough to make a significant difference depending on how the sunlight hits the pavement.

"I think there's a lot of really interesting creativity around it," Pershing said, adding that the changes can make a city "a much richer place, and a much more pleasant place for people to live in."

Additional reporting by AccuWeather Assistant Chief Broadcast Meteorologist Geoff Cornish.

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