No wonder it’s gonna be a hot time in the old town tonight. Cities can be anywhere from 2 to 20 degrees Fahrenheit hotter than nearby countryside, due to what’s known as the "urban heat island" effect. But when heat waves roll through, they interact synergistically with the urban heat island—boosting temperatures even higher than you might expect.
Researchers used a June 2008 heat wave in Baltimore as a case study. They compared temperatures downtown to those near the Baltimore/Washington International Airport—a residential, half-forested area. Using modeling software, they found that temperatures downtown weren't simply a sum of the urban heat island and the heat wave—they were three and a half degrees hotter than that. The results appear in the Journal of Applied Meteorology and Climatology. [Dan Li and Elie Bou-Zeid, Synergistic Interactions between Urban Heat Islands and Heat Waves: the Impact in Cities is Larger than the Sum of its Parts]
Urban areas are carpeted in asphalt and concrete, which don't hold water like soil and vegetation do. So while the countryside sweats during a heat wave, cooling itself through evaporation, the city just bakes. But researchers say there's a simple solution to beat the heat: install green roofs, and plant some trees. Which would help the concrete jungle live up to the second half of its name.
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