What is in this picture? Grout building up on industrial equipment? A melting screw and bolt? Seaweed on a recovered bit of the Titanic? I'll give you a hint. It's very, very small.

What you see there is ice building up on ice. The large "screw and bolt" structure is a snow crystal. The small weedy things growing off of both ends of the snow crystal are rime.

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You regularly see rime build up on trees and fences. Tiny drops of liquid in the atmosphere come in contact with already-frozen liquid on a surface and freeze to it. The result is an almost fluffy-looking, uneven build up of ice. These tiny droplets don't confine themselves to big structures. As crystals of snow come down through the atmosphere, they pick up rime on their surfaces. Enough rime, and the crystal is completely obscured under a formless mess. At this point, it forms dry, hard little pellets called graupel. They look like very small packing peanuts.

Take a look at a completely rimed-over crystal here.

Snow crystals free of rime tend to cause small avalanches. A little bit of rime on snow allows the crystals to cling together, and so is considered safer. There is a downside. The adhesion of the pellets lets huge layers of snow build up that, when they go, all go together. Death by ice peanut.

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[Via The Relation of Crystal Riming to Avalanche Formation in New Snow, Rime and Graupel.]