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A new weather forecast to warn the public that hot, killer air masses are on the way is being developed for the nation's 44 largest cities.

The first such warnings were issued in June for Philadelphia and, forecasters say, may have saved lives."(June's) hot air mass (in the Midwest and East) was one of the worst the country can get," said University of Delaware climatologist Laurence Kalkstein, who developed the forecast.

More than 670 people died in 22 states and the District of Columbia during the heat wave. Of that total, nearly 400 died in Chicago.

In a five-year, $1 million research program funded by the Environmental Protection Agency and the National Weather Service, Kalkstein determined that air masses become killers when they become very hot, humid, relatively cloudless, windy, move slowly and bake a city for three or more days.

Kalkstein calls these air masses Maritime Tropical Oppressives (MTOs) and said they form between June 1 and Sept. 21.

"The largest number of MTOs occur in midsummer, but the deadliest happen in June and early July when people aren't used to oppressively hot weather yet," he said.

Virtually all of these weather systems form in the Southeast and Midwest and move to the Eastern Seaboard.

The primary factor of the deadly air masses is temperature, Kalkstein said. An air mass becomes a killer when it causes thermometer readings above 92 degrees. Deaths climb as the temperature soars and continues for several days.

"As the number of hot days increase, so does the number of deaths," he said. "The largest death tolls occur on the second, third and fourth days of the heat wave."

Dew point temperatures above 70, which cause high humidity, also contribute to the deadly stew, as do winds above 15 mph, which dehydrate humans.

Scattered cloud cover or clear skies also contribute by allowing solar insolation to bake city buildings and streets. Air pollution plays a minor role in the mix, Kalkstein said.

"If you get all these factors simultaneously, you've got a deadly air mass," Kalkstein said.

Kalkstein issued his first killer air mass warning for Philadelphia on Thursday, July 13, and predicted the oppressive heat would cause 20 deaths on that Friday and 29 on Saturday. Philadelphia's actual death toll during the two days was 29.

Kalkstein said the lower-than-anticipated number of fatalities may mean the new forecast saved lives. But there's also a chance the Philadelphia death count didn't include fatalities in the city's suburbs.

The new warning system will be triggered by National Weather Service forecasts indicating a hot air mass is moving toward a city, Kalkstein said.

Kalkstein's research group will then advise the local metropolitan health department to issue a hot air mass watch 48 hours before it's scheduled to arrive.

The health department will be advised to issue a hot air mass warning 24 hours before the potentially deadly system is due. Further warnings will be announced as the air mass rolls in, Kalkstein said.

"A level one warning will indicate the heat may kill one to four people; a level two warning that it may cause five to 14 deaths, and level three that deaths may exceed 15," he said.

Kalkstein said similar air mass watch and warning systems will be designed for Atlanta and Kansas City next year, and all American cities with populations over 1 million will get the warnings in 1997.

"Eventually, we'll develop a computerized system that will quickly warn all cities that a dangerous air mass is heading their way," Kalkstein said.