Tree-trunk sized icicles have been spotted all over Saskatoon this winter.

Among the biggest was a monstrous icy slab, found clinging to the side of an apartment building. It had to be taken down with sledgehammers and chainsaws.

But just how do these three-storey-high behemoths form?

We asked CBC meteorologist Johanna Wagstaffe. Here's her explanation of the phenomenon.

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Giant icicles like these have been spotted around Saskatoon. (Dan Kerslake/CBC)