Daytime heat rises like a 'Phoenix' at night

It wasn't all that long ago that you could skip across the patio and splash into the pool in many Phoenix area backyards. But now, it's hard to do that without the threat of scorching the bottom of your feet.

According to the US Census Bureau, as the population soars in America's fastest growing city, the temperature is rising, too. Research shows that the average summer nighttime temperature in Phoenix has jumped 10 degrees in the last 40 years.

"It used to stay in the upper 70s and low 80s at night. Now, it's not uncommon to see 90," says Randy Cerveney, a climate researcher at Arizona State University (ASU) in Tempe, Ariz.

The increase in temperatures is driving up energy costs, pollution levels, and concerns in what was once a sleepy, medium-sized town just a few hours drive from the Grand Canyon. Phoenix, researchers say, has become a 'heat island'. Like a rock next to a campfire, the sprawling web of freeways, skyscrapers, strip malls, and parking lots retains the daytime heat, when temperatures commonly rise above 110 degrees F. Because there is less desert foliage and open space than there used to be, there is nowhere for the heat to go after the sun goes down. The blacktop, which swelters and takes your breath away with radiated heat during the day, retains the heat at night.

"When you start putting a city on the landscape with cement, asphalt, hot roof tops, and so forth, then that energy gets absorbed during the day, and at night when the wind calms down, the structures will retain the heat and conduct it back to the atmosphere. It's definitely getting hotter," says Anthony Brazel, a climatologist and geographer at ASU, who is part of a multimillion dollar National Science Foundation project designed to study of the impact of humans on the ecosystems they live in.

Temperatures in big cities across the country are on the rise, but not to the degree to which they are soaring in Phoenix. "The weather controls this stuff," says Dr. Brazel. In Las Vegas, for example, where the population is also booming, the wind blows more, and there is often more rain than there is in Phoenix, both of which help to dissipate the heat, allowing nighttime temperatures to drop.

Researchers are also studying people's effect on the ecosystem of Baltimore, where, they say, the average daytime temperatures are going up, but there's still enough vegetation left to help cool the city at night. "Phoenix dwarfs the change that's occurred in Baltimore over the last 20 years," says Brazel.

By some estimates, developers are paving over the Sonoran desert in the American Southwest at the rate of an acre an hour, and critics of such unplanned, sprawling growth say it's threatening not only the wildlife that inhabits the area, but also the future of the desert itself.

"I think the heat island phenomenon is just one more consequence of sprawl and unplanned growth in the Phoenix area," says Sandra Bahr, president of the Grand Canyon chapter of the Sierra Club. The city of Phoenix is lagging behind other cities in addressing the problem. Chicago and Salt Lake City, for example, both of which are much cooler than Phoenix, have started planting rooftop gardens and shingling roofs with reflective materials in an attempt to stave off the heat and rising energy costs, says Ms. Bahr.

A study commissioned by the University of California at Berkeley, which looked at energy consumption in major metropolitan areas, estimates that Phoenix could save as much as $37 million a year in energy costs by using reflective tile shingles on building rooftops.

Phoenix city officials insist that while they have not mandated any changes such as rooftop gardens or reflective tiles, they insist they are doing the most important thing that can be done to reduce the heat island effect: planting trees. They offer as evidence of their success a "Tree City USA" designation by the National Arbor Day Foundation 13 years in a row. City planning director Dave Rickert says officials are not likely to mandate any changes, because keeping one of America's hottest cities as cool as it can be is common sense.

(c) Copyright 1999. The Christian Science Publishing Society