Description: As I took a walk a couple of days after freezing rain left ice slicks on trees in my neighborhood, I found that the ice coating certain smooth trees with very smooth bark had interesting regular patterns of lines. The lines were regularly spaced along each branch (see 10cm long pocket knife for scale). The branches were closer to vertical than horizontal, a few inches thick, and covered in ice on only the north facing side. I could not feel a bump or crack where the lines were: it seemed quite smooth, but the lines splayed out as they neared the edges of the ice (close up on left).
My best guesses are either that as the water froze, the expansion of the ice relative to the tree made compression fractures (which might explain the fact that they were too tight to feel), or that as the day warmed up the tree expanded faster than the ice, putting the ice under tension, thereby cracking it (which better explains the splaying pattern at the edges of the ice since the ice on the south sides must have melted before the cracks formed).
Page creator's name: Mickey von Dassow
Page creator's contact info: Contact through the IGoR contact page.
cracks ice pattern-formation physics trees winter
Created: 12 Mar 2014 17:49
Updated: 12 Mar 2014 18:03
Comments: 19 Dec 2014 23:06
#Comments: 4
Cool!
Thanks!
I'm an amateur wood microtomist. I have made slides of 2500 + or - species. It has taken 8 years of tree and wood study so far ( hasn't stopped…never will). I've never found any info that says that trees expand with temperature increase. With all of the snow in the background, I'd say you've been in a freezing or near freezing environment for awhile? My guess is that the cracks and the splaying pattern at the ends is probably due to wind. I have bent unfrozen twigs of live trees before and always get a seam with some raised areas at the ends of the seam. I've seen the same results when bending (or trying to bend) a piece of copper tubing with out a spring bending tool. It leaves a seam with raised "bumps" at the ends of the seam or crease. Your ice cracks seem, to me, to be wind caused and the end splaying has something to do with the bending of a cylinder.
Thanks for your comment!
That's a very good idea. I wish I had paid attention to the wind direction, and wish I'd kept some notes on the weather at the time. I think I know the pattern you're talking about, though I'm not quite sure whether what you're describing is quite the pattern I'm thinking of.
Most materials expand with temperature, but the expansion is often very small. According to the engineering toolbox, wood expands lengthwise by just ≈3*10-4% per degree C. However — something I hadn't considered before — it says ice expands a lot more (51*10-4% per degree C). But then, if the cracks formed while the air was above freezing, the ice should have stayed at constant 0 °C (as it melted) while the tree warmed up (on the ice-free side). Then again, the cracks could have formed during a period of cooling (so the ice would shrink faster than the tree).
However, the wind-based bending that you propose might better explain the splaying at the end of the cracks.
Freezing rain and snow are pretty rare in my area, but if it happens this year, I'll try to watch those trees and keep some decent notes.