When you think about islands, you probably think about Hawaii or Jamaica. You probably don’t think about Phoenix, Arizona. Phoenix has all the sand of a beachside resort, but it is not surrounded by water. Still, Phoenix is a type of island. Scientists call it an “urban heat island.”

Phoenix is a city surrounded by desert and farmland. That’s the “urban” part of the island. But the city is also hotter than the area around it. It is an island of heat.

How can this be so? Cities are made of buildings, roads, and parking lots. These structures absorb more heat from the sun than things like plants or dirt. They also hold onto the heat longer. When the desert cools down at night, the city stays pretty hot. You can often feel the heat radiating off of walls and pavement.

With summer temperatures well over 100 degrees Fahrenheit, people in Phoenix don’t need more heat! Hotter weather means people use more energy and more water. These are two resources we need to conserve.

Tony Brazel is a climatologist at Arizona State University. He has been studying the Phoenix heat island for more than 20 years. He looks at the difference between city temperatures and rural temperatures. This difference is called the “heat island magnitude.” Brazel also studies what causes this difference, and how we can reduce it. “We’ve found it can be 12 to 15 degrees Fahrenheit higher in the city than in rural areas. On one night we measured a 22-degree difference between the Southeast Valley agricultural area and Sky Harbor Airport,” Brazel says.

Agricultural areas are cooler than natural desert, Brazel says. This is because farms have a lot of water and plants. When water evaporates, it lowers the temperature of whatever is around it. For example, your body cools off when your sweat evaporates.

Some of the landscape water evaporates directly. Some of it evaporates through the leaves of plants in a process called transpiration. Plants also provide shade that further cools the ground. “You get a larger heat island magnitude if you compare the urban site to an agricultural area,” says Brazel. “There’s a smaller effect if you compare it to the natural desert.”

Even within the city, there are temperature differences. Some neighborhoods are just plain hotter than others. Brazel and other ASU scientists are conducting a study to find out which neighborhoods and built-up areas are hottest, and why. Working with ASU’s Decision Center for a Desert City, the scientists are looking at water use and nighttime temperatures. They are using a computer model and satellite surface temperatures of different neighborhoods—some quite dry, others with a lot of greenery. “The key is what we call impervious surfaces—roads, parking lots, buildings. Asphalt and concrete are heat-retaining materials that store energy and at night are still warm,” says Brazel.

On the other end of the spectrum is vegetation—good old green plants. The sun’s energy goes into evaporating the water that these plants receive, not into the ground. The researchers are finding out that neighborhoods with more plants and more water (such as swimming pools) are cooler than neighborhoods without these features. “This creates a dilemma,” says Brazel. “On the one hand, we are a desert with limited water resources and we want to conserve water. On the other hand, we have a heat island problem, and using more water may create a cooler environment.”

So far, the findings suggest that you can get significant cooling effects from adding a small amount of vegetation to areas that don’t have much. But if you go into a neighborhood that already has a lot of vegetation and add even more, you don’t get a very big boost in cooling. In fact, you can remove some of the plants from these areas without making them hotter.

Phoenix is not the only urban heat island in the world. Brazel says that all of the world’s largest cities—such as Mexico City, Tokyo, and Los Angeles—have a major heat island effect. “What surprises people is that even small places can have heat islands. Even a shopping center can be substantive. When you change the surface, you change the temperature virtually overnight,” says Brazel.

Adding plants is not the only way to lower temperatures in cities. There are other ways to cool the heat island. Choosing building materials that don’t absorb a lot of heat is one way. For instance, buildings could use light-colored, cooling roof materials. And cities could pave streets and parking lots with less heat-absorbent paving materials.

Researchers with ASU’s National Center for Excellence in SMART Innovations are studying how new paving materials can help the environment. They paved an ASU parking lot with pervious pavement. It allows water and air to pass through it. Pervious pavement absorbs less heat than regular pavement. It also reduces water pollution by eliminating runoff, and improves safety by preventing water from pooling on the surface.

The scientists have installed heat and moisture sensors on the parking lot. They are comparing the data with information from regular parking lots to find out how much effect the new pavement has.

The way buildings are arranged can also have an effect on heat islands. Brazel talks about “urban canyons,” created when you place tall buildings along narrow streets. The buildings provide shade and create cooling wind patterns. “The city of Phoenix was planned long ago,” says Brazel. “It has wide streets with few trees. Now, ASU and the city are trying to make it pedestrian-friendly. We’re trying to create a sustainable city.”