It sounded like a bowling ball crashed into his roof, but the noise came from underground

Every once in a while, it sounds like Santa Claus is making a hard landing on Claude Chiasson's roof in Cole Harbour, N.S.

The first time he heard the noise, Chiassonthought something had explodedas it smashed into his roof.

"I would describe the sound as like a large object falling on our roof. It sounded like a bowling ball being dropped," he told CBC Radio's MainstreetonWednesday.

The first few times it happened, he slipped intohis backyard to see if there was indeed abowling ball dented intohis roof. He couldn't find any damage, so he went inside and back to sleep.

But then he'd hear the "explosion" again. As it turns out, the sound came from underground.

He started paying attention to the conditions when it happened, and noted it had usually just rainedand then gotten very cold. He did some online detecting and discovered a phenomenon called frost quakes.

"It's loud enough when it occurs that you'll remember it," he said.

Lots of rain, followed by a rapid freeze

David Phillips, a senior climatologist with Environment and Climate Change Canada, said Chiasson has likely found the culprit.

He said conditions have to be just right to set off a frost quake.

"That ground has to be full of water, just brimming with water," he said. "Then you have to have a rapid freeze. There's no room for those molecules of water to go."

Halifax has had an exceptionally rainy winter, and all that water poured into the earth. With temperatures plunging to10 C from 7 C, that water freezes.

The water expands as it becomes ice and when it fills up the empty spaces, it starts pushing on the earth around it.

"The ice doesn't have any place to go, and the rocks that have water that has now frozen ... they break, they make this thumping sound, this booming sound,"Phillips said.

"They certainly can wake you up. People come out, wondering what it is trees falling on roofs?"

He said a frost quakehappens more often than people notice, and if it comesunder deep snow, it'll be muffled. If it happens during the day, people won't hear it amid the din. Wind can blow it away, too.

But on a still, cold night, the frost quake booms.

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