Following a particularly cold snap in Michigan, USA, a farmer found some amazing ‘ghost apples’ hanging from his trees.

Andrew Sietsema / FACEBOOK

While they might look like glass ornaments on a Christmas tree, they’re actually made out of ice! The photos were made by farmer Andrew Sietsema while pruning some apple trees after freezing rain had hit the area. He has since dubbed the hollowed-out ice casings of unharvested apples “ghost apples”.

According to Sietsema, who manages a local orchard, the apples are of the Jonagold variety, “but we’ll call these Jona-ghosts,” he said in an interview for WOOD-TV. He later told TODAY Food that he estimated the ice to be about 1/2-inch thick, and that the apples indeed “felt like Christmas tree bulbs.”

But how exactly does something like this form?

Andrew Sietsema / FACEBOOK

Susan Brown, Professor of Agriculture and Life Science at Cornell University, explained to TODAY Food that the flesh of some apples, particularly those that remain on trees late after the harvest season, will decay into almost an applesauce-like consistency. “The skin keeps it in like a filled water balloon,” she said.

Ghost apples are created when freezing rain settles on the fruit and ices over immediately, creating a frozen coating that initially encases the whole fruit. Apples have a lower freezing point than water, so when it gets a bit warmer the apple defrosts before the ice does and the rotten apple mush seeps out from the bottom of the icy sphere, leaving the impression of an icy ‘ghost’ behind.

These images were taken while parts of America were experiencing a polar vortex, which is when a spinning pool of cold air causes temperatures to dramatically drop. Apparently, these kinds of conditions are also needed for the phenomenon to occur.

Photo: Andrew Sietsema / FACEBOOK

“I like the term that was coined ‘ghost apple,’” Brown said. “What I love about this story and the excitement it generated is that it showcases all the hard work our apple growers do in all weather conditions. The fact that Andrew Sietsema was out in freezing conditions, tending to his trees, allowed him to capture this beautiful occurrence.”