Weather Station

An Island in the Sun

by Lindsey Michaels

Is Phoenix a desert or is it an island? The answer depends upon your point of view. For Robert Balling, Phoenix is both.

Balling is an ASU climatologist. He studies weather trends. Most people know that Phoenix is a desert. Balling claims that it is also an island. The reason, he says, is something called "heat island effect."

To understand heat island effect, think of the city as an island surrounded by desert. That "island" consists mainly of roads, houses, and parking lots-all of which soak up and hold more heat than the natural desert does. As a result, the city gets hotter and stays hotter than the surrounding desert. The bigger the city grows, the hotter it becomes.

"The average temperature in Phoenix has climbed five degrees since the 1960s," Balling says. "That's due largely to the changes in surface structure that growth has brought." While Phoenix was once a small, heavily irrigated farming community, it is now the eighth largest city in the United States.

That means desert grass gives way to black asphalt. Cinder block foundations sprout up from land where coyotes roamed. It also means that both our lifestyle and weather have changed.

"Thirty or 40 years ago, Phoenix's summer nighttime temperatures often dropped into the '70s," Balling says. "People didn't need air conditioners to get to sleep." Today, night temperatures rarely drop below the mid 80s.

Sunlight is energy. In fact, it's mostly electromagnetic energy. The energy of sunlight is turned to heat when the light shines on objects. That heat is spread out and used up by evaporating water or by heating air.

When Phoenix was primarily a farming area, evaporating water used most sunlight energy. Today, the energy in sunlight is focused on heating concrete, bricks, asphalt, and air. Since concrete and asphalt hold onto heat longer than cactus and sand, they stay hot long after the sun goes down. Balling says this is one reason why both day and nighttime temperatures have climbed higher in Phoenix.

Does this mean Phoenicians are leading the global warming rush? No, insists Balling, who is a nationally recognized expert on such issues. Many people think that the Earth's average temperature is getting dangerously higher. They call this global warming and often point to pollution as the cause. Some scientists also have observed pollutants "eating holes" in a layer of the atmosphere, containing ozone, which helps protect us from sun's dangerous ultraviolet radiation.

Balling thinks that the global warming scare is wrong, though many scientists don't agree with him. Balling's research findings show it is the heat island effect that is causing higher temperatures in fast-growing cities like Phoenix.

Balling's research team has taken temperature readings in areas outside these big cities. Team members average those temperatures together. The averages suggest that the Earth's temperature has remained pretty much the same in recent years.

For instance, Phoenix's average temperature has risen by five degrees. However, the temperature in Carefree (a much more rural neighbor town) has remained unchanged. In fact, the average temperature in the entire state of Arizona as a whole has remained unchanged. That, Balling says, is because both Carefree and most of Arizona are still relatively building-free.

While the heat island effect obviously affects temperature, it also affects rainfall patterns. Hotter air often causes moisture to "boil off" before it can turn into rain. Hot city air is also tougher and more active than cooler desert air. That's why most storms split apart and slide off to one side or another when they reach the hot central Phoenix air.

Balling says that it is just easier for winds to go around that hot air, instead of trying to fight through it. As a result, during summer thunderstorms, areas on the edge of Phoenix-Glendale in the west and Mesa to the east-get most of the really heavy winds and rain.