It’s an occurrence that is noticeable if you’ve ever been out and about in Phoenix in the middle of summer after sunset; though the sun might have gone down, temperatures remain hot into the night.
This phenomenon is known as the urban heat island effect, and according to State Climatologist Nancy Selover, it has a lot to do with how cities are developed.
In natural and rural areas where surfaces are made of dirt or soil, the sun hits the surface and heats up the molecules at the top layer. However, because the molecules that make up these surface materials are separated by gaps of air, the heat is only usually absorbed in the first couple of inches, Selover said.
Make no mistake. Those initial layers can get hot, with surface temperatures reaching up to 160 or 170 degrees on bright and sunny summer days.
But the story changes somewhat when the sun sets.
“When the sun goes down, the heat, which is in the very top of the soil, starts being released into the atmosphere,” Selover said. “That heat dissipates very fast. So within 15 minutes of the sun going down you could walk barefoot in the sand across the desert and it (wouldn’t be) a problem.”
That’s not the case, however, for urban environments. Unlike soil, sand, grass or agricultural areas, urban building materials are made up of tightly bonded molecules — i.e. molecules that aren’t cushioned by gaps of air between them. Because of this, heat that strikes the top of these surfaces is conducted well below the initial layers, sometimes traveling as far down as 12 inches or farther.
Just like in undeveloped and rural areas, pavement or asphalt will release heat stored throughout the day into the atmosphere when the sun sets. But the process is slow, and by sunrise the next day, not all the residual heat from the previous day has been released.
This effect results in rising evening temperatures.
“Everyone can notice this,” Selover said. “When you walk past a block wall at night, you can feel the heat radiating off of that block wall. So that makes the air at night much warmer than if you have a native desert surface.”
Other surfaces that create a similar effect include concrete and stone. Though the urban heat island doesn’t have a significant impact on daytime temperatures, it does have a measurable impact on nighttime, or minimum, temperatures — the coolest point of the day before the sunrise.
“In an urban area, you’re going to see those nighttime temperatures rising as we continue development of the city,” Selover said. “Daytime temperatures are going to rise a little bit because we have climate issues... but the minimum temperature is pretty much something that we control when we develop the city.”
According to Selover, in Phoenix the average daytime temperature has increased about 2.5 degrees Fahrenheit since the 1970s. By comparison, the minimum temperature has increased by seven degrees. The 4.5-degree difference, she noted, can be attributed to the abundance of paved surfaces in the city.
Unlike Maricopa County’s largest city, however, communities in Pinal County have the opportunity to avoid a similar fate as they continue to grow.
While Phoenix may have swapped out much of the agricultural land that once surrounded it for concrete developments, areas in Pinal County like Casa Grande, which historically consisted heavily of irrigated agricultural and natural desert landscapes, don’t have to suffer the same fate, Selover said.
It all depends on how development is built out, she noted.
One of the best ways to avoid a noticeable urban heat island like the one Phoenix experiences is mindful development. Selover recommends abstaining from clustered development and maintaining green spaces, including natural desert landscapes and irrigated agriculture, between developments.
Design concepts that retain less heat and can help circumvent the issue include open fencing around yards, which permit breezes to pass through, as opposed to block fencing, she noted.
Despite the ongoing growth, however, cities like Casa Grande may be headed in the right direction when it comes to building out in ways that produce a low heat island signal.
In Casa Grande, for example, temperatures for the area don’t show a significant increase over time to indicate a strong heat island effect, Selover said. As long as it avoids overly urban or clustered development and maintains a mix of natural desert and irrigated agriculture in between, the city should continue to be in good shape, she said.
But there are even small steps homeowners and developers can take as well. Keeping the area around a home or subdivision cooler during the summer months sometimes comes down to something as simple as including greenery in surrounding landscaping. That could be grass, but it could also include things like native desert vegetation or trees that provide ample shade.
“Anytime you can put shade on a surface, you’re going to keep that surface from getting as hot. So that’s always a good idea,” Selover said.