Trees, bicycles, and swing sets sprouted growths of prickly ice needles.

To walk around Boulder this week was to explore a fantastical world of glassy polyps and crystalline sea urchins. Bone-chilling temperatures and freezing fog had infiltrated the city, causing surfaces to sprout growths of delicate, prickly ice needles.

"Woke up this morning and Mother Nature left us a gift," tweeted Neal Lurie on Tuesday. "Never seen frost like this in Boulder." Here's what he spied:

Daily Camera reporter Charlie Brennan lays out the meteorology behind the explosion of icy spines:

The consensus was that Boulder had been blanketed with a spectacular sampling of both hoar frost and rime ice.

"Water in liquid form can be a tiny water droplet in fog before freezing," Henson said, explaining hoar frost. "It is looking around for something to adhere to and freeze onto.

"As soon as it runs into a tree branch or hood of a car, it freezes. As those accumulate, they can form these really beautiful structures."

With temperatures expected to rise into the 50s, it's unlikely Boulder residents will encounter another lasting bout of ice needles this week. But they'll always have the enchanted memories:

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