From IFLS:
There have been several videos going viral this week, showing that snow that doesn’t melt when put into contact with a lighter or a blowtorch. Some videos even show black charring on the snow. To many of these people posting the videos, this appears to be “proof” of chemtrails or that the government is responsible for covering the country in a white mystery substance.

This isn’t a problem with mystery chemicals, it’s a problem with not understanding chemistry. Everyone knows that the three most familiar states of matter are gas, liquid, and solid. Typically, to transition from a solid to a gas, the solid must melt into a liquid, and then vaporize into the gas. However, there is another phase transition known as sublimation that takes a solid directly to a gas. (think: dry ice)

Regular snow can sublimate below the freezing point under certain pressure conditions. This typically happens with cold, dry air, like what is found during the winter. Because this winter has been much drier than previous years for the East Coast, sublimation is achieved more easily.

Though a snowball may seem like a dense unit, it is still just a collection of particles with a high amount of surface area. When these particles come into contact with a much high heat source (like the blowtorch) it is energetically favorable to just sublimate directly into a vapor.

As for the black marks left on the snow? That’s just residue from the lighter/blowtorch/match, as the fuel did not burn cleanly and completely.

More info: http://on.doi.gov/1eDFEAS