Hot in the city

Friday, September 28, 2012
by Diane Boudreau

Phoenix, Arizona

Summertime rolls into Central Arizona with as much welcome as a pimple on prom night. While people in other parts of the country head outdoors for picnics and baseball games, Phoenicians hole up in air-conditioned buildings. Temperatures soar above 100 degrees Fahrenheit for months at a time.

Phoenix is located in the Sonoran Desert, which is known for its hot, dry climate. But the Phoenix metro area gets an extra helping of heat because it is an urban heat island.

But the heat island isn’t exactly the same everywhere. Some neighborhoods within the city are hotter than others. The map above shows surface temperatures taken by satellite on a July day. You can see that temperatures vary across the metro area.

The reason for these differences has to do with land cover—the things people build or plant on the ground. Darren Ruddell, a spatial scientist at the University of Southern California, studied air temperature and land cover in 40 different Phoenix-area neighborhoods in July 2005. He worked with Arizona State University (ASU) sociologist Sharon Harlan and computer modeler Susanne Grossman-Clarke. They divided land cover into four categories:

• desert (undisturbed natural land)

• xeric (homes with “desert landscaping”)

• urban (business and industry)

• mesic (homes with mostly grass landscaping)

land cover t ypes

They found that urban and xeric landscapes were hotter than mesic and desert areas. This might come as disappointing news for people trying to conserve water by xeriscaping their yards. Ruddell says that xeriscaping might not always be the best environmental decision.

“You change the landscape and then these neighborhoods become hotter. So people use the air conditioner more and for longer periods,” he says. “And water is used in generating the electricity that runs the air conditioner.”

In some inner-city neighborhoods, neither grass nor desert landscapes are common. Instead, these areas sport layers of concrete or simply bare soil. These are the hottest neighborhoods of all.

Scientists used airplane and satellite images to find out what kinds of land cover are used in different parts of metro Phoenix. They also looked at information from the U.S. Census to learn about the socioeconomic status (SES) of people living in those neighborhoods. SES describes the social and economic position of a family or neighborhood compared to others. It is often measured as a combination of family income, education level and occupation. 

They found a correlation between socioeconomic status and temperature. A correlation measures how strongly two things are related to each other. As SES gets lower, temperatures get higher. As SES gets higher, temperatures go down.

Unfortunately, people in low-SES neighborhoods may not have many resources for coping with the heat. For instance, they may not be able to afford air conditioning. They are also more likely to work outdoors in jobs like construction or landscaping. These things expose them to more heat.

“It’s an environmental justice issue,” says Harlan. “The people who are most vulnerable are also living in the hottest conditions.”

Phoenix at sunset

The scientists are also looking at how the heat affects human health. They want to find out who is at the most risk from the heat, so they can help community members create solutions. For example, cities might add more grass and trees to cool low-SES neighborhoods.

Redesigning neighborhoods is not as simple as it sounds, however. Growing trees requires time and money, but time and money don’t grow on trees. A lush landscape requires water, which costs money, and time to care for it.

Another possibility is improving heat wave warning systems. “Right now, the heat wave warning systems aren’t that effective,” says Ruddell. “They’re applied at really broad scales. With the information that we have we could be a lot more effective. We could identify areas that are vulnerable and set up aid stations there.”

Harlan adds: “Designing better social and environmental policies for cities should help reduce the intensity of urban heat islands as well as reduce human vulnerability to heat.”