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July 15 -- 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, it's no surprise that metropolitan areas can see higher temperatures than their rural counterparts.
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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 Fahrenheit higher than outlying areas with more vegetation and less development, according to the National Integrated Heat Health Information System.
In a report on Wednesday, Climate Central explained an index to evaluate the intensity of urban heat islands and then applied it to 159 U.S. cities.
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 higher on average.
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The study also included large cities such as Chicago, Charlotte, N.C., Portland, Ore., and Richmond, Va.
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 footprint, according to the study.
Meanwhile, cities such as Houston and Fresno, Calif., 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.
"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," Climate Central Director of Climate Science Dr. Andrew Pershing told AccuWeather broadcaster Geoff Cornish.
The population density was one of the top six factors that researchers considered while creating the index. 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, albedo contributed the most.
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."
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 can not 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 socioeconomic 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.
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 Climate Central report, Chicago broke into the top 10 cities with high intensity scores.
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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, the National Oceanic and Atmospheric Administration 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, adding that evidence of warming has been seen in the oceans, where urbanization is not a factor, and in weather stations in rural areas.
"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."
