Sunlight, Snow, and a Twig
March 5th, 2012
We had a big snowfall last week and the snow has been melting for the last few days. I came across this little scene yesterday:
The direction to the sun is clearly marked by the twig’s shadow. What’s striking to me is the melt hole around the twig. Why is it shaped the way it is? The sunward face of the twig is warmed by the sun and then re-radiates infrared light that melts the snow – why didn’t it preferentially melt the snow in front of the twig? This appears to be a profound mystery. So here’s my attempt to explain this oddity.
Four factors are at work:
1. the incoming sunlight, which warms the front surface of the twig
2. conduction of heat from the front surface of the twig to the rear surface of the twig, and down the twig
3. convection via meltwater moving down the twig
4. convection via air warmed by the surface of the twig
None of these factors immediately explain the evidence. So I went for another walk, examining lots of twigs in the snow, and the answer became obvious: the long axis of the hole’s cross section falls directly underneath the twig. In other words, it’s the tilt of the twig that determines the shape of the hole. This in turn explains the process by which the hole is formed: frost forms on the twig during the night, then melts during the day. The meltwater flows partly down the twig, then at some point becomes so large and heavy that it drops off the twig. On nights with heavy frost, the meltwater drops near the top of the twig; on other nights, it flows a ways down before dropping.
Finally, there’s another observation to be made here, arising from the fact that I noticed this oddity in the first place. I don’t think that many people would have noticed it; why did I notice such a trivial phenomenon? I think that the answer lies in my process-heavy approach to thinking. I do not see the universe as a collection of things, I see it as a system of processes. For me, the universe is not an arbitrary collection of facts (like odd-shaped holes around twigs). Everything I see fits into the system of processes that I have learned over the decades. But this observation struck me because it did NOT fit into my system of processes. Something was awry and I had to set it straight.