Shallow Thoughts : tags : glacier

A Post-Christmas Snow

The morning after Christmas we woke up to a beautiful white world, with snow still coming down.

Shoveling is a drag, but still, the snowy landscape is so beautiful, and still such a wonderful novelty for ex-Californians.

This morning we awoke to much the same view, except the snow was deeper -- 8-12 inches, quite a lot for White Rock.

We also had the usual amusement of Roof Glaciers: as the mat of snow gradually slides off the metal roof, it hangs off the edge, gradually curling, until finally the weight is great enough that it breaks off and falls. Definitely an amusing sight from inside, and fun from outside too (a few years ago I made time-lapse movies of the roof glaciers).

And then, this being New Mexico, the sun came out, so even while snowflakes continued to swirl down we got a bright sunny sparkly snow vista.

Yesterday, the snow stopped falling by afternoon, so Raspberry Pi Club had its usual Thursday meeting. But the second storm came in hours earlier than predicted, and driving home from Pi Club was a bit icy. I wasn't looking forward to the drive up to PEEC and back tonight in a heavier snowstorm for our planetarium talk; but PEEC has closed the Nature Center today on account of snow, which means that tonight's planetarium talk is also canceled. We'll reschedule, probably next quarter.

Happy Holidays, everyone, whether you're huddling inside watching the snow, enjoying sunny weather, or anything in between. Stay warm, and walk in beauty.

Studying Glaciers on our Roof

A few days ago, I wrote about the snowpack we get on the roof during snowstorms:

It doesn't just sit there until it gets warm enough to melt and run off as water. Instead, the whole mass of snow moves together, gradually, down the metal roof, like a glacier.

When it gets to the edge, it still doesn't fall; it somehow stays intact, curling over and inward, until the mass is too great and it loses cohesion and a clump falls with a Clunk!

The day after I posted that, I had a chance to see what happens as the snow sheet slides off a roof if it doesn't have a long distance to fall. It folds gracefully and gradually, like a sheet.

The underside as they slide off the roof is pretty interesting, too, with varied shapes and patterns in addition to the imprinted pattern of the roof.

But does it really move like a glacier? I decided to set up a camera and film it on the move. I set the Rebel on a tripod with an AC power adaptor, pointed it out the window at a section of roof with a good snow load, plugged in the intervalometer I bought last summer, located the manual to re-learn how to program it, and set it for a 30-second interval. I ran that way for a bit over an hour -- long enough that one section of ice had detached and fallen and a new section was starting to slide down. Then I moved to another window and shot a series of the same section of snow from underneath, with a 40-second interval.

I uploaded the photos to my workstation and verified that they'd captured what I wanted. But when I stitched them into a movie, the way I'd used for my time-lapse clouds last summer, it went way too fast -- the movie was over in just a few seconds and you couldn't see what it was doing. Evidently a 30-second interval is far too slow for the motion of a roof glacier on a day in the mid-thirties.

But surely that's solvable in software? There must be a way to get avconv to make duplicates of each frame, if I don't mind that the movie come out slightly jump. I read through the avconv manual, but it wasn't very clear about this. After a lot of fiddling and googling and help from a more expert friend, I ended up with this:

avconv -r 3 -start_number 8252 -i 'img_%04d.jpg' -vcodec libx264 -r 30 timelapse.mp4

In avconv, -r specifies a frame rate for the next file, input or output, that will be specified. So -r 3 specifies the frame rate for the set of input images, -i 'img_%04d.jpg'; and then the later -r 30 overrides that 3 and sets a new frame rate for the output file, -timelapse.mp4. The start number is because the first file in my sequence is named img_8252.jpg. 30, I'm told, is a reasonable frame rate for movies intended to be watched on typical 60FPS monitors; 3 is a number I adjusted until the glacier in the movie moved at what seemed like a good speed.

The movies came out quite interesting! The main movie, from the top, is the most interesting; the one from the underside is shorter.

	Roof Glacier
	Roof Glacier from underneath.

I wish I had a time-lapse of that folded sheet I showed above ... but that happened overnight on the night after I made the movies. By the next morning there wasn't enough left to be worth setting up another time-lapse. But maybe one of these years I'll have a chance to catch a sheet-folding roof glacier.