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Why it's hot in the city

December 24, 2009 By Katharine Sanderson This article courtesy of Nature News.

Heat wave in Baltimore made worse by hot air from Washington DC.

While most of Europe and much of North America are currently in the middle of a cold snap, researchers trying to understand a heatwave that hit the northeastern United States in 2007 have found that Baltimore's sweltering temperatures were exacerbated by the presence of its neighbours, Washington DC and Columbia.

Meteorologist Da-Lin Zhang at the University of Maryland in College Park and his colleagues used a three-dimensional meteorological model to investigate how weather and temperature change over time across the Baltimore and Washington DC region. Baltimore sits about 100 kilometres northeast of Washington, with Columbia about halfway between them.

Zhang wanted to look at the urban heat-island effect, a well-known phenomenon where built-up areas become much hotter than surrounding rural areas in warm weather. His models included data showing what the land in this large area was used for. The roughness of the surface — whether it is covered by trees or water or buildings — affects the weather at the surface and he was able to put detail down to 500 metres into the model.

Then, using data for 7–10 July 2007, when the area was hit by a heatwave, Zhang ran the models to simulate the weather at that time.

This heatwave was "horribly hot, dangerously hot," according to Russell Dickerson, an atmospheric chemist and a co-author of the paper, which is published in Geophysical Research Letters1.

Stagnant heat

The models confirmed that it had been hotter in Baltimore than in Washington — Baltimore reached 37.5 °C while Washington basked in a relatively balmy 36.5 °C — and that the air quality over Baltimore was also much worse, with ozone levels of 125 parts per billion (ppb) compared with 85 ppb over Washington. The US Environmental Protection Agency recommended maximum for ozone at ground level is 75 ppb. The amounts of small particulate pollutants over Baltimore were also higher than over Washington.

"The heat-island effect was worse in Baltimore than in Washington DC," says Dickerson, and this can't be explained simply by the physics of the buildings and road surfaces in Baltimore, he adds. The effect has to be a consequence of the weather dynamics in the area.

The prevailing winds over the region in July 2007 were southwesterlies. There was also an easterly breeze coming off Chesapeake Bay to the east of Baltimore. The model showed that the hot air from the two heat islands of Washington and Columbia was moved up to Baltimore by the prevailing wind, where it stagnated because of the incoming bay breeze, making temperatures soar and pollution stay put over the city, which was also subject to its own heat-island effect.

When Zhang and his team ran a model in which Washington was bulldozed and replaced by trees, things got cooler in Baltimore. The heat-island effect there was reduced by 25%, and the city was cooler by 1.25 °C.

"The urban heat island is a well-known phenomenon, but was thought to be quite local," says Gabriele Curci, who studies air quality and pollution transport at the University of L'Aquila in Italy. He suggests that this work will help to decide where best to place air-quality monitoring balloons.

"It's good to know that [the urban heat island] is not only a local effect," says Susanne Grossman-Clarke at the Global Institute of Sustainability, Arizona State University in Tempe. "Urban planners only think about the local aspect of the heat island," she adds. "The city is in the context of other cities."

Dickerson agrees that in the light of this work, planners should pay more attention to outside influences on urban heat islands. And they might — the research was supported in part by Maryland's Department of the Environment to determine how planting trees might help to improve air quality, Dickerson says. Zhang is currently working in China to advise on that country's heat-island problems.

"A little state like Maryland can't do much to help global climate change, but there are things you can do to mitigate the adverse impacts of climate locally," says Dickerson.


  1. Zhang, D.-L. et al. Geophys. Res. Lett. 36, L24401 (2009).


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