Snow does funny things sometimes.

I’m not talking about the wet snow you can build snowmen from, or the cold snow on warm ground that produces depth hoar. I’m talking about plain, dry, snow that has built up over a period of weeks or months on surfaces exposed to the air, like a picnic table or a fence rail. Fresh-fallen, it’s like powder – but it doesn’t stay that way.

Every snowflake has parts that stick out, and other parts that are concave. While the snowflake is falling through supersaturated air, the points that stick out attract the most moisture, and grow fastest. Not so once it has settled to the ground. Then the points tend to evaporate, and the moisture collects in the concave areas. Where two flakes touch, an ice bridge will form between them. But undisturbed snow is still mostly air, with a three-dimensional, lacy structure.

This year's festoon. You can see the hanging part through the lattice, but the base is still attached to the top rail. (Yes. the window needs cleaning, but from the outside.)

Sometimes that lacy structure is even flexible. I have a lattice with 2x4s on top. We don’t have much wind, and every winter the snow slowly builds up on the 2x4s. 4” wide and 18” high is not unusual by early spring. As the air warms and the sun comes back, it begins to sag a little, and once it starts to sag sideways gravity takes over. The lacy structure continues to hold together, and if the snow is held to the top rail only by its weight, part of it may even slip off the top rail, while remaining held up by the rest.

This morning, Feb 18 2012

I don’t think it will hang as far this winter. It’s started to bend over, but we had a thaw at the very beginning of the winter, and the base of the snow is glued to the top rail by ice. Late yesterday a tear developed between the base, which is still stuck to the top rail, and the top, which now hangs at the bottom of the festoon. I took a photo this morning. Anyone want to make a guess on how long it will hold together until the whole thing tears awat?