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Residents look at flooding outside their building in the Queens borough of New York on Wednesday. ((Shahrzad Elghanayan/Associated Press))

Summer thunderstormsget worse when they hit cities, two U.S. researchers say.

Using data from theextreme storm that produced record flooding, rainfall and lightning in Baltimore on July 7, 2004, the scientists concluded that the city's buildings increased the rainfall byabout 30 per cent.

There was a much as 15 centimetres of rain in two hours and "parts of Baltimore experienced as many lightning strikes in the space of two hours as they normally receive during the course of a year," Alexandros A. Ntelekos and James A. Smith said in a release Thursday.

Much of the lightning storm wrapped around the western edges of Baltimore and nearby Washington."It's as if all of a sudden the lightning can 'feel' the city," said Ntelekos, a graduate student in civil and environmental engineeringat Princeton University's School of Engineering and Applied Science. Smith is a Princeton professor of civil and environmental engineering.

They cited two factors for the strengthened storm:

  • The height and placement of buildings had a large effect by changingstorm winds. Tall buildings increase wind drag, creating "vertical velocities" or a boiling action thatincreasesrainfall.
  • Urban aerosols, particles in the air from industrial and vehicle pollution,have been thought to reduce rainfall, butthe research suggests the opposite.

Another factor thought to increase storm strength is the heat cities produce — urban areas are between 1 C and 3 C hotter than the surrounding environment.

But the urban heat had little effect in 2004 because high winds levelled temperatures.

Climate changewill bring more bad storms

An extreme storm like the one that hit Baltimore usually occurs only once in 200 years, but climate change is expected to make such events more frequent.

"The storm that occurred yesterday [Wednesday] in New York City is an example of the sort of event that we expect more of in the future," said Michael Oppenheimer, a lead author of the recent report from the UN's Intergovernmental Panel on Climate Change, and a Princeton professor.

The Baltimore research "may allow us to better cope with the future, warmer climate," andshouldserve "as a goad to policy-makers to act more urgently to stabilize the climate."

Ntelekos and Smith used data on lightning strikes, rainfall, clouds and aerosols,combined with analyses based on computer models of the atmosphere.

The Water Resources Research journal is reviewing a paper prepared by the two, and they are presenting some of their research this week at a workshop sponsored by a Princeton engineering research centre.