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