Urban hot spots
New York, Atlanta and Salt Lake City, United States
New York, Atlanta and Salt Lake City are poster cities for a phenomenon common to cities in industrialized nations: They create their own weather.
"When you replace soil and grass with concrete and asphalt, you alter the balance of energy that occurs at the earth's surface," says Dr. Robert Bornstein, a professor of meteorology at San Jose State University in California.
Altering the balance of energy has created what Bornstein calls "urban heat islands" where the temperature is 5 to 10 degrees hotter than outlying areas and generate their own rain. Working with graduate student Qinglu Lin, Bornstein found that cities also can change regional weather patterns.
When there is no wind, for example, heat builds up in a city, creating a low-pressure system that pulls air into the city. The air heats as it moves through the city to the center, then rises, cools and condenses into rain.
Studies have also found that the high-rise buildings in cities create a barrier that can redirect regional weather patterns.
"Forecasters told me they would see thunderstorms approach a city, slow down and a divergence of flow around the city," says Bornstein. "Then the flow recombines downwind."
The problem with urban heat islands, says Bornstein, is that "they create thermal stress in which people die. It's also more likely that high pollution levels will occur because heat creates the conditions for photochemical smog."
Higher temperatures, of course, create greater demand for air conditioning, which requires higher energy consumption and a deterioration in air quality. Remedial measures include planting trees and roofing buildings with reflective tiles, but the issue is one few cities have addressed.
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