Is Your Community Stranded on an Urban Heat Island?

May 23, 2018
Green roofs help reduce urban heat, as well as to control stormwater. The U.S. Environmental Protection Agency’s Region 8 headquarters in Denver has had a green roof for more than a decade, one of the first of its kind in the state. Photo: U.S. EPA

TipSheet: Is Your Community Stranded on an Urban Heat Island?

As summer approaches, it’s getting warmer. But did you know summer heat is worse in the city? It’s science. You can write about it. And your city can do something about it.

Urban heat matters because some 80 percent of the U.S. population lives in cities (more or less, depending on how you define a city). That could well be most of your audience.

There are many bad things that happen because of urban heat, from ozone (smog) pollution to sweaty discomfort. Urban heat affects people’s quality of life. Or death. Heat is actually the number one weather-related killer of people in the United States

The so-called “urban heat island effect” is not a new bugaboo, and not every city is affected the same way. But in many cities, air temperature will be several degrees Fahrenheit higher than the surrounding countryside, and as much as 12 degrees higher in the worst cases.

And it is getting worse. One recent study, for instance, found that loss of trees was making urban heat islands hotter.

Culprits include blacktop, waste heat, weather

Besides lack of trees, another culprit is blacktop.

You have felt the heat radiate off of a blacktop street as you cross it on a hot summer day. Many man-made surfaces absorb more heat from the sun than natural surfaces — like soil covered with plants. The dark colors of blacktop, concrete, tar roofs and other urban land coverings are less reflective.

The mass of pavement means it stores more heat in daytime, only to release it at night.

Waste heat from human activities is another cause of urban heat. You turn your air conditioning on … and the heat from inside is pumped outside, heating up the rest of the world. It’s a vicious circle: when people turn on A/C because it is hot, that actually makes it hotter outside.


A shortage of trees is a major cause

of heat islands, and planting more

(along with taking care of the ones

you have) is part of the solution.


But many other human activities — building heating, industrial operations, electric utilities and automobiles among them — also produce heat. These heat sources are concentrated in urban areas. If you look at a weather map that shows temperatures at different stations, you will see the numbers go down outside the city.

Meteorology matters, too. Urban heat islands are usually more pronounced at night — as the heat absorbed by urban surfaces is radiated back to the air. If there is a wind to blow heat away, that helps. When urban air is held in basins (e.g., cities surrounded by mountains), that can make things worse.

Atmospheric inversions and smog (which is worsened by heat) make things worse, too. Normally, air near the ground is cooler, and heat is dissipated by convection; in an inversion, an upper layer of warm air prevents this cooling. And smoggy air absorbs more heat than clear air.

If you want to check out your local heat island, start looking at sources that display weather info graphically.

Google “temperature map” and add the name of your city or metro region. You will find displays on your local TV news sites, some local newspaper sites and national sources like The Weather Channel, AccuWeather, Weather Underground or the National Weather Service. Spoiler alert: Urban heat islands also happen in the winter.

Your metro area may already be taking measures that could take the edge off of urban heat. You can inquire with your city or county, or check in with metro regional agencies like a council of governments.

Solving the urban heat problem

Here are some approaches that may help with heat islands.

  • Trees, please. A shortage of trees is a major cause of heat islands, and planting more (along with taking care of the ones you have) is part of the solution . Does your municipality have a tree program? How well is it funded (they often aren’t)? Are there incentives against removing trees? Is there a planting program? One helpful source is the Urban Tree Foundation. Or you may want to check out the journal Urban Forestry & Urban Greening.
  • Greener roofs. A green roof is an engineered surface on top of a building that actually consists of soil with plants growing on it. It helps with stormwater, as well as heat. Not every building can support one, and doing it right is important. Fortunately, there is a lot of experience to learn from. It’s become an industry. The Canadian group Green Roofs for Healthy Cities is one good source. It has a directory. Federal sources include the General Services Administration and the U.S. Environmental Protection Agency.
  • Lighter roofs. The choice of roofing treatment and materials makes a huge difference for urban heat islands. Of course, other concerns like cost and durability matter, too. One of the most common materials — asphalt shingles — can be made with extra reflectivity. These “cool shingles” also help homeowners chill. Aluminized paint can help metal roofs. There are also cool membranes — it gets technical pretty fast. The Energy Department is one source of info about cool roofs.
  • Energy efficiency. Air conditioners, cars and other machinery can be engineered to produce less heat for the same amount of benefit. Lowering heat emissions does not have to mean sacrifice, although it may mean investing in a newer model. Energy efficiency ratings are available for most appliances (as are mpg ratings for cars). EPA’s Energy Star program has been a very successful and well-received non-regulatory program, which leaves many wondering why President Donald Trump wants to eliminate it.

These are only some of the solutions. Although municipal planning and action makes a huge difference in mitigating urban heat islands, individual action matters a lot. You may have noticed that many of the “solutions” cited above depend on individual consumer choice.

That’s also why explaining the problem — and its solutions — is an especially worthwhile project for journalists.

* From the weekly news magazine SEJournal Online, Vol. 3, No. 21. Content from each new issue of SEJournal Online is available to the public via the SEJournal Online main page. Subscribe to the e-newsletter here. And see past issues of the SEJournal archived here.

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