PHILADELPHIA - Last Sunday was not only the first full day of winter. For reasons that remain a deep mystery, it was also the first full day of the deadliest season.

To the surprise of even those scientists who documented the phenomenon, far more people die in winter than at any other time of year - and not necessarily while shoveling snow.

In a study of 28 U.S. cities, rates of death from all causes were found to be more than 20 percent higher in January than in August, regardless of climate. Whether Philadelphia or Phoenix, Minneapolis or Miami, the pattern held.

"People die not because it's cold, but because it's winter," said Robert E. Davis, a University of Virginia environmental scientist who was the report's lead author.

His finding, and the puzzle it presents, caused a hubbub at the International Congress of Biometeorology in the fall in Kansas City, Mo., where the study was released. For a week, 200 researchers from around the world convened there to consider potential cause-and-effect connections between weather and health.

Going back four millennia, ancient medicine was linking weather to well-being - laying the foundation for the latter-day science of biometeorology. It has been well-accepted in Europe for 50 years.