A conspiracy theory is floating around that someone other than Mother Nature is responsible for Atlanta's latest snowfall, spurred on by clips of people burning snowballs black. This video, using a lighter and some snow, explains why they're wrong — and explores the phenomenon of "snow that doesn't melt."
Phil Plait of Bad Astronomy explains how a snowball could blacken:
The black scorch marks are actually from the lighters themselves. Butane is a hydrocarbon, a molecule made up of carbon and hydrogen. When you burn it, the molecule reacts with oxygen in the air, breaking the bonds between atoms, and reforming new molecules. If the burning were perfect, all you'd have left is carbon dioxide (CO2) and water (H20).
But the burning is never completely perfect, and you get other stuff too. One thing that happens is that some of the carbon molecules reform into long chains, creating what we call soot. It's that stuff that's collecting on the snowball, not material from the snow itself!
He also then shows how that same snowball still melts down to water after being blackened, using timelapse photography. Check it out.