Friday, November 5, 2010
You might have to look at this photo for a moment before you figure out what it is, but that is an underside view of snowpack sliding off a roof. That's in slow motion, of course - the snow wasn't visibly moving when I took the photo. You could almost call it glacial speed, except it's certainly quicker than most glaciers move.
The dorm has a metal roof, which helps remove the snow by providing a slippery surface when there's a little bit of melt. It reminds me of earthquakes, actually, because the forces build up invisibly and then release their energy with no warning. The snow sits on the roof quietly, and then suddenly the friction can no longer hold the weight; you hear a sudden soft rumbling as a strip of snow slides down the roof and then a whump whump whump as it all falls to the ground.
But as I said, the snow in the photo wasn't moving that fast. This is a porch roof on the north side of the dorm and it doesn't get much sun. It's also smaller, so there's less weight pushing the snow downhill and it just creeps slowly instead of sliding off in a rush.
It looks like about a foot of snow slid off the roof during the day, halted when the temperature dropped, and began again the following day, when I took the photo. But during the night, the weight of the overhang was just enough to bend it downward without breaking off, leaving an unconformity of sorts that's obvious because of the way the roof structure leaves grooves in the snow. Would have been cool to have a time-lapse video of the process, but it's also just as cool to see the effects frozen (heh heh) in place.
You can see a couple more photos of sliding ice and snow here. I think I like these because of their similarity to geological and evolutionary processes: always at work, but only visible after a certain amount of time has passed.