for National Geographic News
During winter storms many city folk may praise warmer downtown temperatures for keeping the streets snow and ice free.
But urbanites ought to take steps to curb this phenomenon before localized temperature differences become a global weather problem, a meteorology expert says.
Tightly packed streets, parking lots, concrete buildings, and dark roofs absorb sunlight all day, explained Dale Quattrochi, a geographer with the NASA Marshall Space Flight Center in Huntsville, Alabama.
Quattrochi says that the absorbed sunshine keeps cities 1° to 10°F (0.56° to 5.6°C) warmer than the surrounding countryside.
When Earth is viewed from space with equipment that maps surface temperature, urban areas appear as islands of heat.
Quattrochi speaks about the urban heat-island effect in a Pulse of the Planet radio broadcast today. (The radio program and this news story are partially sponsored by the National Science Foundation.)
In winter the extra warmth may be just enough to prevent snow from sticking to the streets, even when the air temperature hovers near freezing.
But in summer the effect can bump up energy costs due to air conditioners and can even cause cities to create their own weather, Quattrochi said.
According to Quattrochi, city streets and rooftops pump warm air into the lower atmosphere long after the surrounding countryside has cooled. This is called the chimney effect.
"When hot air rises, it condensesit forms clouds," Quattrochi said.
"[The chimney effect] is enhanced through a period of time, and as it keeps pumping hot air into the atmosphere, clouds keep building and building, and pretty soon it starts raining."
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