These strange explosions along the East Coast that mystified thousands of people were sonic booms from military aircraft, the Navy said yesterday in releasing its official report on the phenomena.

Weather conditions were just right, according to a Naval Research Laboratory spokesman, to funnel the sonic booms made on supersonic training flights from sea to shore, rattling doors and windows on land and, in one case, blowing a fuse on a power line.

He said the aircraft pilots were not breaking any rules as they touched off the booms but apparently did not realize that the special atmospheric conditions required extra precautions.

"I am satisfied now" that the booms heard along the coast from New Jersey to South Carolina in December came from military aircraft, siad the spokesman, Alan Berman. Berman, director of the laboratory, gave his explanation at a Pentagon news conference.

He said that after spending about $100,000, mostly for computer studies, and tapping experts in the field from Jan. 5 until this week, the laboratory concluded that aircraft sonic booms were the only explanation.

Cleared as possible cuprits, he said, were methane gas explosions; nuclear blasts; missiles and spacecraft, and the Concordo supersonic transport.

One key club in the lab's detective work, Berman said, was that the reported explosions occurred almost always during weekday working hours, not on weekends. The team corrolated the booms with training flights.

"Every military aircraft was operating within a legally established military warning zone," he said. This presumably is why the Pentagon did not link the booms to training flights, not realizing the special weather conditions.

"The booms apparently bounced off warmer, high altitude air, which deflected the sound to areas 100 to 200 miles away from where the aircraft were flying," the Pentagon said in an official statement summarizing the lab's investigation.

"Normally," the statement continued, "sonic booms travel downward in a path 15 to 30 miles wide. That is why commercial and jet aircraft avoid supersonic flight within 30 miles of land.

"The December events, however, and several more that were reported later along East Coast points as far north as Nova Scotia, were sensed much further from their source than is usually the case because of the unusual enhancing effect of the winter weather, particularly the temperatures and speed of air mass layers, on the propagation of the resulting sound and shock waves."

While clearing the Concorde of causing the booms along the U.S. East Coast, Berman said the study suggested the transport was responsible for explosions reported in Nova Scotia.