It’s nearly Christmas in Boston, and beyond a light dusting several weeks ago, no snow graces our only recently chilly city. For one, it hasn’t been cold enough, we regularly have afternoons in the 40s (Fahrenheit, 5-10 Celsius) so snow wouldn’t last long anyway. If it had snowed earlier in the season it’s possible it would be cold enough for snow now. Why is that, you might wonder? Clean, white snow reflects sunlight back into the sky, preventing it from heating the ground. Snow can have a reflection coefficient near 0.9 if it is fresh. Either way, I’m crossing my fingers for a white Christmas.