What are urban heat islands and what can we do to mitigate them?

The Big Apple is one of many cities around the world that is impacted by the urban heat island effect, a phenomenon where urban areas are warmer than their surrounding counterparts.

They call New York City a concrete jungle. The Big Apple is one of many cities around the world that is impacted by the urban heat island effect, a phenomenon where urban areas are warmer than their surrounding suburbs. Speaking of concrete, when air temperatures rise during a heat wave, the many miles of concrete streets, sidewalks, walls and roofs in cities amplify that effect.

As any city dweller already knows, cities are replete with tall buildings, roads, and other infrastructure, which absorb and re-emit the sun’s heat way more than natural landscapes such as fields, forests and water.

It's all the more important that we understand urban heat islands given how many people around the world live in cities. According to Climate Central, more than half of the world's population and 80% of Americans live in cities. Even established cities, like Phoenix and Las Vegas, are expanding rapidly, and they are already known for blistering heat throughout much of the year.

"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 [and] the more densely populated you are," Dr. Andrew Pershing, the director of climate science at Climate Central, told AccuWeather.

You might wonder how we disentangle the effects of urban heat islands from climate change. Well, evidence of warming across the planet has been seen in the oceans, too, where urbanization is not a factor, and at weather stations in rural areas. "While humans are creating new urban heat islands, this does not explain the warming trends that scientists have recorded," according to Climate Central.

Cities by the numbers and mitigation

Climate Central analyzed the urban heat island effect across 44 American cities and created an urban heat island index (UHI) to estimate how much hotter these cities are due to the built environment. The study concluded that 41 million people live in census tracts with an UHI index of 8°F or higher—or about 55% of the 74 million people included in the study.

The most impacted cities included New York City (9.5°F), San Francisco (8.8°F), Chicago (8.3°F), Miami (8.3°F), and Seattle (8.2°F).  Maps of each city revealed how different land use and urban growth patterns influence urban heat island hotspots. Some cities, like Bakersfield and Tulsa, were marked by a heat intensity peak in the urban core. Other cities, like the sprawling Los Angeles and Dallas, have a heat intensity that spreads throughout city limits.

There are ways to mitigate the urban heat island effect. One is by planting street trees. There are also cool roofs and pavements. According to Energy.gov, a cool roof is designed to reflect more sunlight than a conventional roof, absorbing less solar energy. Think of it like when your parents told you to wear light-colored clothing on a warm, cloudless day. Conventional roofs can reach temperatures of 150°F or more on a bright, hot afternoon. Under the same conditions a cool roof could stay more than 50°F (28 °C) cooler.  

Cool pavements work in a similar way and remain cooler than conventional pavements, bringing down the temperature of the pavement and surrounding air. As an added benefit, according to the EPA, these materials can also reduce runoff during rainstorms and improve visibility at night. One example of these "cool" efforts in action is in Phoenix, where the city's Street Transportation Department painted over roadways in a lighter grey color in order to create cool corridors.

Results of the first year of the Cool Pavement Program showed that surface temperatures using the lighter color were between 10.5 and 12 degrees cooler than typical Phoenix street pavement during afternoon hours.

Last summer, the city of Phoenix celebrated 100 miles of city streets with cool pavements, and that number is expected to rise in the coming years.

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