BARCELONA, Spain - A Spanish-American scientific team is monitoring ice events in the United States this winter following research on a baffling phenomenon first detected here almost three years ago.

They're not watching for ordinary ice storms or slick roads, but incidents involving "megacryometeors," great balls of ice that fall out of the clear blue sky - possibly due to global warming.

"I'm not worried that a block of ice may fall on your head," said Dr. Jesus Martinez-Frias of the Center for Astrobioilogy in Madrid. "I'm worried that great blocks of ice are forming where they shouldn't exist."

Heads, however, have very nearly been cracked by megacryometeors, a term coined from "mega" which means big, "cryo" for ice, and meteor, the extraterrestial debris that streak through the atmosphere. Most weigh 25 to 35 pounds, but one whopper found in Brazil tipped the scales at 440 pounds.

Ice balls have punched holes in the roofs of houses, smashed through car windshields, and whizzed right past people's heads. Last winter, an ice chunk which eyewittnesses described as "half the size of a car" ripped through the roof of an automobile dealership in Lawrenceville, Ga.

Incidents like those may be just the beginning, according to Dr. David Travis, who does research on atmospheric conditions that foster megacryometeor formation. He is chairman of the department of geography and geology at the University of Wisconsin-Whitewater.

"If megacryometeor formation is linked to global warming, as we suspect, then it is fair to assume that these events may increase in the future," Dr. Travis said.

Dr. Martinez-Frias pioneered research on megacryometeors in January 2000, after ice chunks weighing up to 6.6 pounds rained on Spain out of cloudless skies for 10 days. A government scientific research agency thought the ice might be extraterrestial, from a comet, and asked him to investigate.

At first, scientists thought the phenomenon was uniqe to Spain.

During the last three years, however, they've accumulated strong evidence that megacryometeors are a gloabl event, Dr. Travis said. They've documented ice balls falling from cloudless skies everywhere from China to the United States and studied about 20 events outside Spain.

More than 50 falls have been confirmed, and researchers believe that's a small fraction of the actual number, since most may hit unoccupied areas or melt before discovery.

Dr. Travis said there appears to be a seasonal pattern to megacrymeteor falls, with most occurring in January, February, and March.

"I am anxiously waiting to see what will happen this winter," Dr. Travis said. "We'll be keeping a lookout, and we want to make people in every state aware and ask their help. We strongly encourage eyewittnesses to preserve samples, in a freezer if need be, and contact us."

Researchers had ice samples from the 2000 incidents to analyze, thanks to quick-thinking eyewittnesses who kept the material cold.

Dr. Martinez's team quickly ruled out obvious explanations.

The ice balls, for instance, were not frozen water from toilets flushed on jetliners. The ice contained no human waste and none of the blue disinfectiant used in airplane toilets.

Air traffic control records showed that no planes overflew the areas near the ice falls, so the ice was not shed from aircraft wings or fuselages.

Chunks of debris from a comet? Comets, after all, are composed partly of extraterrestial ice. Again, lab tests showed that ice in megacryometeors had the distinctive chemical signature of ice in ordinary terrestial hailstones.

When sawed in half, they also showed the physical profile of hail stones.

Hail forms in air currents, called updrafts and downdrafts, in thunderstorms. The updrafts carry droplets of supercooled water which freeze into ice particles. More supercooled water droplets hit the particles as winds toss them around. The water freezes instantly, and the hailstone grows in size, layer by layer.

The stronger the winds, the more the hailstone gets tossed around, and the bigger it grows before dropping to the ground at speeds of 90 miles per hour. Most hailstones weigh a faction of an ounce, with 27 ounces the U. S. recored.

Megacryometeors showed the telltale onion-skin layering seen in hailstones, also contained dust particles and air pockets found in hail.

"These occurrancves are not the result of hoaxers, either." Dr. Travis said. "There are too many similarities in the atmospheric conditions associated their occurences that hoaxers would have no knowledge or interest in."

That leaves monster hailstones forming in a cloudless sky, a notion that defies more than a century of research on hail formation.

"Scientists are naturally reluctant to say something never can happen," noted Dr. Charles Knight, a hail expert at the University Corporation for Atmospheric Research, a university consortium in Boulder, Colorado. "But oh, dear. I would be tempted to say 'never' on this."

Dr. Knight said he has reviewed scientific papers published on megacryometeors, and thinks the explanation, which involves unusual atmospheric conditions possibly linked to global warming, is wrong.

Although global warming involves higher temperatures on Earth's surface, it actually means colder conditions in the stratosphere, the uppermost layer of the atmosphere, Dr. Travis said.

He has linked megacryometeor events to unusual conditions in the "tropopause," the boundry between the troposphere (the lower atmosphere) and the stratosphere. Located five to nine miles above the surface, the tropopause marks the limit of clouds, and is important in the development of storms.

Global warming may be making the tropopause colder, moister, and more turbulent, Dr. Travis said, careating conditions in which ice crystals can get tossed around in air currents, and grow like ordinary hail stones in thunderclouds.

First Published December 8, 2003, 2:53 p.m.