D.C. heat stagnates Baltimore's air

Hot air rises off capital's buildings, pavement, blows north and gets trapped here

  • While the scientists focused on a July 2007 heat wave in their study, the air in Baltimore was still thick in early August, making the skyline barely visible from West Bay Avenue.
While the scientists focused on a July 2007 heat wave in their… (Baltimore Sun photo by Kim…)
January 08, 2010|By Frank D. Roylance

People who argue that the hot air rising out of Washington makes things worse for the rest of us might have stumbled onto something.

Scientists at the University of Maryland, College Park have discovered that heat from the capital's buildings and pavement during the summer can stoke higher temperatures, humidity and dangerous air pollution downwind in Baltimore.

"It isn't a simple matter of, 'It's hotter in Washington, and that hot air blows into Maryland,' " said Russell R. Dickerson, a professor in UM's Department of Atmospheric and Oceanic Science and a co-author of the study.

The heat rising from Washington's urban "heat island" actually slows the prevailing winds headed toward Baltimore, reducing their beneficial cooling and cleansing effects, he said. Slowed further by cross-breezes off the Chesapeake Bay, the air stagnates over Baltimore. That can raise temperatures here 2 degrees to 3 degrees higher than they would otherwise be and worsen Baltimore's particulate and ozone air pollution.

The discovery "allows us to better model and understand the origins of heat waves and air pollution," Dickerson said. But more practically, "it helps us in planning land use, to try to mitigate some of those air quality problems" by, for example, planting or preserving upwind tree cover.

The UM study was funded by the Maryland Department of the Environment, and published last month in the journal Geophysical Research Letters.

Urban heat islands occur when buildings, rooftops and pavement absorb solar energy during the day and, unlike more natural landscapes, retain much of it overnight.

The absence of trees also prevents "evapo-transpiration" - the atmospheric cooling that occurs with the evaporation of soil moisture through trees. Motor vehicle traffic and air conditioning in densely populated urban areas also add heat directly to the air.

Heat maps of the region show hot spots along Interstate 270, a six-lane interstate with no trees, Dickerson said. There are no hot spots along the Baltimore-Washington Parkway, with its wooded shoulders and median.

Led by UM meteorologist Da-Lin Zhang, Dickerson and Yi-Xuan Shou, of UM College Park and China's National Satellite Meteorological Center, focused on a heat wave in July 2007, one of the worst local smog events in a decade.

From July 7-11, high temperatures ranged from 94 degrees to 101 in downtown Baltimore. Humidity reached more than 90 percent; winds dropped off and ozone pollution rose to "unhealthy" levels.

Although Washington is a bigger urban area with more people, it was cooler than Baltimore. At Reagan National Airport, temperatures ranged from 92 degrees to 98 degrees.

Ozone readings, spurred by higher temperatures and sunlight, reached 125 parts per billion in Baltimore, but only 85 ppb in Washington. The federal standard for ozone at ground level is 75 ppb.

Zhang plugged surface and satellite weather data into a three-dimensional weather model for the region. The complex computer program includes detailed information on land use, including the roughness and darkness of the natural and man-made surfaces, the presence of trees and bodies of water.

The model revealed that prevailing winds from the southwest slowed when they encountered hot air rising over Washington and Columbia. They stagnated over Baltimore when they ran into the city's own heat island, and a crosswind off the Chesapeake. The southeasterly bay breeze typically occurs on summer afternoons as hot air rising over the land draws cooler air off the bay.

Where they collide, Dickerson said, "You get an area of stagnation. For air quality and a heat wave, stagnant air is a really bad thing." Air monitors in Edgewood, northeast (and downwind) of the city, frequently report the area's worst air quality.

Higher temperatures cause fine particulates and ozone levels to climb. And such increases correlate with higher death rates.

"We are right on the ragged edge, in compliance" with federal standards, Dickerson said, "and the standards are going to get tighter."

When the UM scientists manipulated the computer model to see what would happen if Washington, its buildings and pavement were magically replaced by trees, the heat island effect in Baltimore was reduced by 25 percent. Temperatures dropped by as much as 2.7 degrees Fahrenheit.

Wishful thinking, perhaps. But there was a message in the exercise.

"While individual cities alone can do little to diminish the harmful impacts of global climate change, they can take steps to mitigate changes in local climate," the study concluded.

Zhang hopes to apply this principle in developing countries, like his native China, Dickerson said. "He hopes it will help them with urban planning, and avoid the mistakes we made. Tree-lined streets and islands in parking lots - it all will help."

In Maryland, he said, the study's findings suggest less costly possibilities for attacking urban heat problems.

"If you can prevent a cornfield from turning into suburbs, or make sure the suburbs have good tree cover, that's less expensive" than finding places in the city for trees," he said. "Or, if you buy up agricultural land and convert it back to forests, that's real cheap, and you will have substantial benefits for climate and air quality."


> Read Frank Roylance's blog on MarylandWeather.com

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