Heat Island Effect Warms Tucson Faster as City Grows

Sept. 19, 2000

If you think Tucson's hotter than it used to be, you're right.

The city's average temperatures are 11 degrees Fahrenheit warmer than they were in 1910. About half of that rise - or 5 1/2 degrees - is due to the "urban heat island" effect says University of Arizona geographer Andrew C. Comrie.

More than two-thirds of this increase - or about 3.8 degrees - occurred in the last 30 years as the city's population doubled.

Comrie will talk about the urban heat island effect this week at the Southwest Weather Symposium in Tucson. The results of the research he and his students have conducted in this area will be published in the October issue of the Bulletin of the American Meteorological Society.

Scientists have studied the urban heat island effect at least since 1833, when it was documented in London, Comrie noted. Cities form heat islands because they retain more heat through the night than surrounding rural environments. Mid-town areas in large cities can be as much as 10 to 15 degrees Fahrenheit warmer than the peripheral areas, he said.

Heat is retained in concrete, asphalt, buildings and other structures. At night, these man-made heat sinks give off the heat they have stored during the day to the surrounding air, keeping temperatures higher. The same phenomenon can be observed in micro-climates in the desert areas around Tucson, where large rocks retain heat and are warmer to the touch than the surrounding soil.

Students, mostly freshmen, in Comrie's spring 1999 undergraduate introductory course, "Our Changing Climate," analyzed long-term temperature data recorded at two stations in central Tucson and four rural stations located 24 to 30 miles north, south, east and west of Tucson.

While the general regional warming trend of the past 30 years caused temperatures to rise at all stations, temperatures recorded by stations inside the city rose faster and higher. Temperatures in Tucson have risen more than a 1/10th of a degree on average each year during that time. The heat island effect is strongest in March and weakest in November, the researchers learned.

Comrie's students also measured and mapped temperatures across the city at sunrise on Feb. 13, 1999. This gave them a one-day "snapshot" of warmer and cooler patches around town, including areas warmed by the heat island effect.

"Overnight and sunrise are the best times to look for any urban heat island," Comrie said. "The coldest daily temperature occurs at sunrise, after the ground has had the entire night to cool off. Therefore, the air that sits over the city, especially if the weather is calm, tends to be warmer than air over the surrounding rural areas."

Professor Emeritus William D. Sellers, of UA atmospheric sciences, and a graduate student analyzed cold-air-drainage and urban-heating patterns in Tucson during the winter of 1985/86. They mounted a thermometer on a car and drove a few major streets on clear, calm mornings throughout the winter, eventually mapping temperature gradients over most of the city. To no one's surprise, their composite picture showed the heat island effect in Tucson.

To map the Tucson heat island pattern, Comrie's students mounted sun-shielded thermometers on four university cars. They simultaneously drove each car through a quarter of the city, recording temperatures during the first hour or so after sunrise.

Using these data, they produced a color map illustrating how surrounding mountains, intersecting rivers and washes, and local winds complicate the city's heat island pattern. The map can be viewed at http://geog.arizona.edu/~comrie/heat/

Metro Tucson sits in a broad, flat valley surrounded by the Santa Catalina Mountains to the north, the Rincon Mountains to the east and the Tucson Mountains to the west. During the night, the Santa Cruz River and its tributaries (Rillito Creek, Canada del Oro, Tanque Verde Creek, Pantano Wash and many smaller dry river channels and washes) drain cold air from the mountains across the valley floor.

On calm days, breezes waft from the northwest up the Santa Cruz, Rillito and Pantano washes. On calm Tucson nights, the breezes reverse and blow gently down the same channels from the southeast. All this air sloshing in and out complicates the Tucson heat island pattern, Comrie said.

Cold-air drainage from the Santa Catalina Mountains obscures any potential urban warming patterns in northwestern Tucson. But air is warmer over the western, central and northeast parts of the city. The map shows hot spots in southwest and northeast areas of Tucson. These hot spots correspond with broad sandy riverbeds that show up in satellite imagery as warmer land surfaces.

The students also found that central Tucson was about 3 1/2 degrees F hotter than surrounding rural areas. This effect may be due to urban heating, Comrie said.

"I did this study to get the story on Tucson urban warming, and also to involve undergraduates in research," Comrie said. "Class is a lot more fun for the students and the instructor if all are actively involved in mutual learning."

Combining real-world research with teaching signals a shift in how research universities view their changing role - a trend that increasingly links students, faculty and other campus researchers in a common pursuit of learning, he added.

Comrie designed the course so that early in the semester, students learned to use spreadsheet software for simple calculations and graphing. He assigned writing assignments in the style of scientific journal articles to encourage students to think logically and critically about each part of the research.