D.C. heat stagnates Baltimore’s air

By Frank Roylance
Baltimore, MD (January 8, 2010)- Hot air rises off the capital’s buildings, pavement, blows north and gets trapped here. 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.

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.
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.
“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. 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 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.
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. “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.”
The UM study was funded by the Maryland Department of the Environment, and published last month in the journal Geophysical Research Letters.
Related Resources:
Baltimore Sun- D.C. heat stagnates Baltimore’s air
Urban Heat Island Task Force Notes (contact ACT if you’re interested in joining)