Manhattan Heat Waves Sign of City Scorchers to Come

Jeremy Hsu

Date: 30 July 2010 Time: 09:31 AM ET

The dog days of summer, uh, nights are here. New Yorkers living in Manhattan suffered nighttime temperatures 10 to 15 degrees Fahrenheit higher than in New Jersey or Long Island during the first July heat wave, according to an area-wide network of sensors.

The difference arose because of Manhattan's urban heat island effect, researchers said. Energy demand, air quality, asphalt surfaces and exhaust fumes all prevent the city from cooling off as fast as the surrounding areas. The same holds for other metro areas.

These heat waves could become more frequent in cities, scientists say, mainly as a result of global warming and the increase in high-rises and other buildings.

Hot summer nights

"While surrounding suburban and green areas may perceive the same maximum temperatures, the built regions will perceive them for longer periods of time," said Jorge Gonzalez, a mechanical engineer at The City College of New York.

That explains why the nighttime heat lingered in Manhattan, even as both the city and surrounding areas had the same maximum daytime temperatures.

Such an effect makes sense in New York City and other large cities, said Kevin Trenberth, a leading climatologist at the National Center for Atmospheric Research in Boulder, Colo., who did not take part in the research.

But he expressed some surprise about such a huge night-day temperature difference between Manhattan and the outlying areas. He added that such measurements depend on factors such as placement of the sensors, as well as the extreme conditions in July.

"These city effects are most apparent when winds are very light," Trenberth told LiveScience. "That occurs in heat wave conditions, rather than in regular conditions when you have sea breezes."

Perhaps only Central Park might offer relief with its trees and lawns, which hold onto moisture and divert part of the heat into energy for evaporating that moisture. By contrast, rain that falls on New York pavement runs off into the gutters and sewers.

"Water acts as the air conditioner of the planet," Trenberth explained.

Manhattan's heat island effect may also lead to so-called split storms, such as those this summer that drenched some Long Island communities and left other villages untouched. The city essentially acts as a barrier to storm fronts that create concentrated storms in scattered areas.

More heat to come

Frazzled New Yorkers can expect such heat waves to become more frequent and intense because of climate change warming the northeastern United States, Gonzalez noted. Manhattan's urban heat island effect will also become magnified as new buildings go up.

"To mitigate these effects, landlords and policymakers should strive [for] greening the cities with urban parks and vegetated roofs, and motivate construction and retrofits that are thermally light and reflective to the sun when possible," Gonzalez said.

Aerosols, or fine particulates of solids and liquids, may also play havoc with the severity and frequency of storms. Urban areas such as Manhattan typically produce many such aerosols.

In the long run, Gonzalez and his colleagues plan to see how an expanding city affects its regional climate. The researchers rely upon the City College's New York City Meteorological Network, which includes several hundred ground-based sensors and roof-monitoring stations that collect temperature information.

"In the case of New York City, I'd expect that all the air conditioners and such churning away would easily be 20 to 40 times greater than the global greenhouse effect [for heating up the city]," Trenberth said.

Still, Trenberth cautioned that the exploding population of urban dwellers worldwide could lead the urban heat island effect to have a bigger global impact in the future.