Webtool helps cities mitigate localized heat 

Researchers at Arizona State University developed a webtool showing different temperatures in various parts of the Valley as cities in the region continue to address temperature disparities.

According to the webtool, the hottest areas of the Valley were concentrated in areas of south, central, north, and northwest Phoenix. The coolest areas of the Valley were concentrated in less populated areas on the metropolitan area’s edge, such as Buckeye and Fountain Hills. The map shows most of Scottsdale is cooler than Phoenix.

The Cool Region Webtool was developed by a clinical associate professor at the Sustainability School at  ASU, Katja Brundiers and Paul Coseo, program head and associate professor of landscape architecture, urban design and environmental design. The map can be filtered to find trends and correlations between heat and vulnerability with multiple environmental and economic factors.

Coseo said researchers used land surface temperatures from satellite imagery to create the map. He said land surface temperature differs more than air temperature because of different “built environments.” He said buildings and pavement act as “batteries” in the way they absorb heat.

“Pavement, concrete buildings all absorb a lot of the sunlight, and then I like to say, hold it as a battery, and then release that at night,” Coseo said. “It’s not air temperature, but it’s land surface.”

One example of a positive heat vulnerability correlation was between high land surface temperatures and households without a vehicle. Coseo said areas with people who use public transportation more often than cars are correlated to living in areas with higher land surface temperatures.

Brundiers said heat impacts health for those who live in households without a vehicle. She said lack of structural shade at transit stops and wait times factor into health complications for people who rely on public transit. There were 378 heat-related deaths in Maricopa County in the heat season of 2022, according to the Maricopa County Department of Public Health. Heat season usually runs from May through October. Of those deaths, 79% of them were outdoors.

Brundiers said the hottest areas also tend to be the areas with the highest amounts of pollution, which she said already affects children and the elderly who experience diseases like asthma. She said there are many more ways people are more vulnerable to heat than others, and the map is meant to encourage structural equity. She said they wanted to draw attention to different heat impacts and to invite municipal planners to think about heat equity.

“We called it our heat vulnerability map to draw attention to equity, because often we don’t think about heat through the vulnerabilities, and that different people have different vulnerabilities, and some are individual based,” Brundiers said. “But others are vulnerabilities due to structural injustices, like, you know, historic evidence of red lining that then pushes people to live in these places of high air pollution, and low, cooling amenities, etc.”

Coseo said this information could help people better understand what causes land surface temperatures to vary within cities.

“Whether that’s a city planner trying to understand how to build bus, stop, shade structures, or it could be community groups trying to advocate for those things so it could be a variety of different ways,” Coseo said. “It also could be just school kids wanting to learn more about their neighborhoods or learn more about the region and understand how heat affects either themselves or people in their neighbors in the region.”

Brundiers said Tempe incorporated a heat equity map into their city climate action plan in 2022. The plan says the city will address blazing and localized land surface temperatures on a city-scale, neighborhood-scale, and touch-scale.

City-scale solutions include using heat maps, like the Cool Region Webtool, to inform the city on where to place cooling centers and shade structures. The neighborhood-scale includes shading parking lots and planting trees in parks to create a cooler atmosphere. And the touch-scale reforms include heat-conscious selection of materials at playgrounds, in parks, and in building materials.

City of Tempe Climate Action Manager Brianne Fisher said using land surface temperature maps like ASU’s helps the city know where new infrastructure is needed the most.

Viewing these maps “helps us make decisions on where cooling interventions should be prioritized, where some of these resources need to go,” in order to help people who are most vulnerable to heat exposure, Fisher said.

She said the city is planting “desert adaptive trees” in hotter areas but said planting trees is a deliberate process. Fisher said it must be “the right tree in the right place,” with water usage and tree maintenance in mind.

“We don’t want to be in the position where we’re planting trees with high water use,” she said.

One way to conserve water and maintain plants that provide shade is what Fisher called green stormwater infrastructure, which takes stormwater runoff and conserves and harvests water to maintain plants with less potable water.

“Green stormwater infrastructure is a way to capture the rain that falls and to develop landscaping that is native and produces shade and ideally using less potable water to allow it to thrive,” she said.

The city of Phoenix has also employed heat mitigation strategies, but the city’s Director of Heat Response and Mitigation David Hondula said every specific location has different needs. He said the city plants trees, which he said directly cools the air through evaporation, but Phoenix is also using a technique using cool pavement.

Creating cool pavement is just changing the color of roads and roofs, because the city heats from the ground up, according to Hondula. The roads and roofs’ colors would change to a brighter and more reflective color to keep the ground as cool as possible.

However, he said there is no “free lunch” with any of these heat mitigation techniques. Hondula said higher reflectivity can lead to a hotter air temperature down the middle of a street. Despite that drawback, it is still a worthy investment, he said.

“For the most part, we don’t see a lot of people walking down the middle of our streets on our hot summer days at noon, one o’clock, two o’clock, so this result doesn’t discourage us from applying the cool pavement in general,” Hondula said.

He said there are exceptions, including in busy, high-pedestrian intersections and on playgrounds, where cool pavement would not be utilized because of the reflections.

Hondula said cities in the Valley are national leaders in local heat mitigation. He said Phoenix has the most miles of cool pavement of any city in the country and that Phoenix is the first city in the country with a public office devoted to heat mitigation.

“Our phone literally rings quite often with questions from other cities about how we’re doing some of the heat investments we’re doing,” Hondula said. “I do see our region as a leader, a leader that itself has as much to learn as well.”