Differing Conditions

by Don Friedman on February 9, 2017


In honor of today’s snowstorm, I’ve provided a picture of the aftermath of our last (very minor) snowfall. If you look closely at the awning, you’ll see that a small amount of snow collected on the white-striped areas while nothing much stayed in the black-striped areas.

The obvious question is why the difference? Snow is not known for its careful decision-making and can’t tell the difference between a black stripe and a white stripe. There’s an obvious answer: the black stripes might be warmer than the white stripes, but I’m not so sure that’s correct.

A black piece of cloth (or, in this case, vinyl) is not inherently warmer than a white piece. The black piece absorbs more light (including infrared) than the white, but only if there is a source for that energy. In this case, the ambient air temperature was a little below freezing and it had been overcast all day. It’s possible that there was enough energy from the general overcast sky to slightly heat up the black, but that’s not a given.

Why else could this have happened? It’s possible that the surface roughness of the different colors is different. The amount of snow build-up on a sloped surface like the top of the awning is related, in part, to the friction between snow and surface. On a sufficiently rough surface – cedar shingles, for example – you can build up a lot of friction even if the surface is not quite at freezing. On a  slick surface such as glass, the temperature has to be well below freezing to get much snow at all.

The awning has lights inside for night-time use. They may not be evenly distributed – they would be less visible in the black-stripe areas than the white-stripe areas – and they may affect how the awning holds heat. Similarly, we don’t know that the different colors are equally attractive as landing spots for birds, and the dirt tracked by birds’ feet might, again, affect the surface roughness.

Without any real investigation, it’s not really possible to rank the likelihood of these theories and any others I missed. My intuition tells me that the birds are the least likely answer and heat from the sun, even on a cold overcast day, is the most likely answer, but intuition can easily be wrong. The lesson here is that even a structure as simple as an awning has multiple factors that lead to multiple possible explanations for any given action.

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