The extreme daytime temperatures of an urban heat-wave may be hard to ignore, but really, scientists say, it's the nights that kill you.

The tragedy of Paris in 2003 — the deaths of 5,000 vulnerable people in the first two weeks of August — spawned a raft of new studies on the subject of death-by-weather, a researchers presented their findings this week at the annual meeting of the American Geophysical Union in San Francisco.

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Half of us live in cities, and just 20 years from now 80 percent of us are expected to be urban dwellers. Cities are the lifeblood of the modern economic times, no doubt about it, but they are also the boiler rooms. And the rising frequency of lethal heat waves across the globe is prompting a more detailed look at the "urban heat island" effect.

Using satellite data, Ping Zhang and colleagues at NASA's Goddard Space Flight Center compared the different heat-generating signatures of thousands of cities around the planet, and their findings confirm that the strongest heat islands are the larger, most densely developed cities.

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This probably goes some way toward explaining why about 600 people died in sprawling, greener London during the same heat wave that killed 4,867 people in densely developed Paris. 

"Exposure to high temperature during several nights, especially consecutive nights, can double the risk of death for the most vulnerable people — people over 65 years of age or young infants, and also people with chronic health problems," researcher Benedicte Dousset told a news conference.

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The University of Hawaii researcher and French colleagues studied satellite-derived thermal images of Paris on a neighborhood-by-neighborhood scale, noticing subtle temperature difference depending on vegetation and development-density.

Average afternoon temperatures from Aug. 1-13 carried a heat-island effect of 18 degrees Fahrenheit, she reported, while average nighttime temperatures measured a 15-degree F heat island.

Dousset said a one percent increase of the "vegetation index decreases by 0.2 degrees C (about 0.4 F) the surface temperature of Paris in summer afternoons."

Studying the impact of air conditioning on the heat island of Paris, another team led by Cecile de Munck and French colleagues observed that air conditioning increases energy demand and the cooling systems themselves release heat onto city streets.

"It's a vicious circle," said de Munck, "temperature increase due to air condition will lead to an increasing air cooling demand."

Photo: People cool off in a Paris street, Saturday Aug. 9, 2003. credit: AP Photo/Franck Prevel; Surface temperature of Paris at night, courtesy B. Dousset, University of Hawaii.