PHOENIX — Researchers at Arizona State University have found that poor neighborhoods are less insulated from the heat that radiates through Phoenix every summer.
The researchers report in a new study that conditions in less affluent neighborhoods — from high-density housing and the lack of trees and landscaping to converging freeways — help create pockets of extreme heat that persist day and night. In fact, for every $10,000 that an area's income rises, the average outside temperature drops one-half degree. In wealthier neighborhoods, lush yards and trees help cool the air more quickly after the sun goes down, shortening the hours of the heat waves.
The disparities present threats more serious than just discomfort on a hot day, the study says. The densely developed nature of the hottest areas also means more vulnerable people — the elderly, children and the homebound — live in the neighborhoods where the risk is greatest.
"It's an environmental-justice issue," said Darren Ruddell, a geographer who led the study. "The people who are most vulnerable are also living in the worst conditions. It's a double whammy."
The researchers said they hope their findings will spur discussions about better managing land, water and energy use, factors that will grow more important if the predictions of climate-change models come true.
"If we can identify the areas most at risk, we can try to help them," Ruddell said. "We could redesign neighborhoods, build cities differently, improve warning systems and ultimately reduce our vulnerability to heat."
Cities usually feel the effects of the urban heat island at their core when buildings and roads absorb energy during the day and then release it slowly after the sun sets. The phenomenon keeps temperatures higher at night and speeds the warm-up the next day.
What Ruddell and sociologist Sharon Harlan depict in their study is more like a string of heat islands in Phoenix, with cooler neighborhoods sandwiched between the hottest areas.
One of the key differences is land cover. The hottest neighborhoods were the most barren, with homes surrounded by asphalt or dirt. Slightly cooler were xeriscaped areas — where drought-resistant plants are used in landscaping — and cooler still were areas in natural desert.
The coolest neighborhoods were planted with grass, shrubs, flowers and shade trees.
Ruddell and Harlan focused on a 96-hour heat wave in July 2005. Using historical climate data and weather records, the researchers established 113 degrees as a so-called threshold level to define an extreme heat wave. The highs in the July 2005 period ranged from 113 degrees to 116 degrees.
Working with actual temperature readings and interviews with residents in the targeted neighborhoods, the researchers determined how many hours the residents were exposed to the hottest temperatures.
In the most barren urban neighborhoods, residents were exposed to almost 22 hours of the intense heat. In the lushly landscaped areas, residents suffered just four hours of intense heat.
Some of the neighborhoods with the most hours of extreme heat were far from the inner core, where the heat island typically develops.
"These are increasingly stressed fringe areas that are not as comfortable because of sparse landscaping and poor land quality," Ruddell said. "And they're mostly middle-class neighborhoods."
The ASU study also revealed how severely heat can target lower-income residents and subject them to heat-related illnesses.
The researchers divided the 40 neighborhoods they studied into three categories based on heat-intensity hours and overlaid demographics. They found that median household income in the hottest neighborhoods was less than half of what it was in the low- and medium-intensity areas. People in the high-intensity areas were more likely to be minority, and they were older than the people in the coolest areas.
"Wealth can buy options that let people change their indoor and outdoor environments," Harlan said. "They can bring in more landscaping; they can run the air conditioning; they can move to cooler neighborhoods."
Harlan and Ruddell are part of a larger ASU research team working with a $1.4 million National Science Foundation grant to study urban residents' vulnerability to heat. The findings could lead residents to rethink ideas about which landscape is best for Phoenix and how planners can better design neighborhoods to stop heat islands from developing.