Made in the shade: ASU team crunches data on how best to cool urban areas

By

Scott Seckel

It’s debatable what can kill you faster in an Arizona summer: the sun or the electric bill.

Anyone owning a home can recite the litany of summer woes. The dawn patrol to cut the lawn before the really bad heat hits. The power bill the size of a BMW payment. The neighborhood stroll abbreviated by solar assault.

Anything that lives knows the answer to all of that is shade. From fish to people, getting out from under the solar blast is the key to comfort.

ASU studies on microclimates and urban climate have measured for the first time precisely how much cooler different elements can make an environment — information highly useful to architects and urban planners.

“The reason you would want those detailed numbers is if you’re doing design work in an urban area,” said Ben Ruddell, associate professor in the Polytechnic School, part of the Ira A. Fulton Schools of Engineering. Ruddell and his colleagues have worked on several studies on shade and urban climate. “In the past that design work has not been evidence-based, but now we can tell you exactly what the effect is going to be on that microclimate.”

Trees or sails? Grass, gravel or concrete?

“Name your materials and we can give you the numbers,” he said.  “We now have the data to tell them exactly what techniques to use. … We’ve got the data; we’re open for business. Give us a call.”

Shading helps cool the landscape underneath it. It also helps reduce home energy use and create beneficial microclimates for growing different types of plants. The type of shade doesn’t matter much: trees, shade sails, ramadas and pavilions all have roughly the same effect, according to researchers.

“The main effect is keeping all that solar energy from impacting you or your house,” Ruddell said. “Shade is very effective at cooling off what’s underneath.”

If homeowners have an environment where they can keep the sun from hitting the house, they can save significantly on their energy bill, said Nancy Selover, research professor in the School of Geographical Sciences and Urban Planning and state climatologist.

Shade structures, awnings, vine-covered trellises will all work. Rooftop solar panels will intercept the sun while air flows beneath them to cool the attic.

“It doesn’t just have to be a tree,” Selover said. “Whatever you can do to keep the sun from hitting the surface.”

How much shade is enough shade?

“If I have a 1-acre plot of land, what percentage do I need shaded?” Selover said. “Unfortunately, you need a large percentage of shade. If you only have a little bit shaded, it’s not going to be helpful.”

Ariane Middel, assistant research professor in the School of Geographical Sciences and Urban Planning, is working on studies of how much trees affect human comfort.

“It’s pretty hot here, and summers are pretty miserable,” she said. “In terms of thermal comfort what’s even more important than temperature is the radiated environment. It’s the solar radiation that determines how comfortable you feel. We looked at the impact of trees on thermal comfort.”

Middel, Ruddell and three colleagues measured temperatures and heat stress in the sun and under five trees through four seasons in three typical Phoenix area neighborhood types: mesic (lawns and lush trees), xeric (desert landscaping), and oasis (a mix; think putting greens in gravel beds found in master-planned communities).

They found that naturally mesic neighborhoods are cooler because of the grass and trees. There was little difference between the xeric and oasis neighborhoods.

“The little grass patches didn’t make a difference,” she said.

Researchers found during the mid-afternoon heat being under a tree means being 8 degrees more comfortable than standing in the sun.

Homeowners should plant trees by the front porch or around seating areas in the back yard.

“If you’re going to plant trees, you want to plant them in locations where they make a difference — where people are,” Middel said.

There needs to be more shade in places where people are outside, said Ruddell, like business districts, around mass transit and over playgrounds.

Ruddell has a paper in review with a colleague from Texas Tech. One of the clearest findings is that shade plays a huge role in keeping playgrounds safe. Kids are more vulnerable to heat than adults are, and many playgrounds aren’t shaded.

“It needs much more attention than it’s getting,” he said. “We have taken readings in excess of (194 degrees Fahrenheit) on surfaces kids would play on and touch. To put that in perspective, that temperature is far in excess of the standard for factory workers to touch anything. … That’s hot enough to burn you, and certainly hot enough to make your uncomfortable.”

Homeowners should be reminded that Salt River Project will give free shade trees to qualifying homeowners.

“Planting trees on the south side of your house and the southwest side of your house will lower your energy bill,” he said.