In the 1950s, Dr. Cressman developed the first program that could produce accurate and reliable forecasts prepared by computer, which led to a monumental change in how weather is predicted and brought meteorology into the computer age.

As director of the Weather Service from 1965 to 1979, Dr. Cressman expanded the number of local weather radars, developed a nationwide weather radio network, and introduced systems to provide early warnings of tornadoes and flash floods.

"He really, truly was a giant in meteorology," said Richard Hallgren, who succeeded Dr. Cressman as National Weather Service director. "Worldwide, he was extraordinarily well-known. He is one of the few who have contributed to so many things."

In the late 1940s, meteorologists were among the first scientists attempting to harness the new technology of computers. They had little sustained success until Dr. Cressman was named director of the federal Joint Numerical Weather Prediction Unit in Suitland, Md., in 1954. Using an IBM 701 computer, he recorded the weather conditions at equally spaced points around the world, then devised a program that allowed the computer to produce forecasts derived from the cumulative data.

"At the time, that was a major breakthrough," said Ron McPherson, executive director emeritus of the American Meteorological Society. "Before that was done, forecasting was mostly an art" based on extrapolations from hand-drawn weather charts.

"When computers came in, forecasting became much more of a science," McPherson added. "It started, literally, a revolution in forecasting."

Dr. Cressman was born in West Chester. He and a boyhood friend, who also became a meteorologist, took an early interest in weather to determine when snowstorms would provide good opportunities for sledding.

After graduating from Pennsylvania State University, he studied meteorology in a military course at New York University, then served as a forecaster with the Army Air Forces.