Why does a plate of steel melt when shot with a giant frikkin laser, even though the air temperature is only 70^o F? ;)
It's just a less intense version of that.
There are 3 ways heat transfers into things (some might say only two). Conduction - contact with something hot (put snow in your mouth and it will melt). Convection - Hot air or water etc flowing over something (use a hair dryer on snow and it will melt, though slower than you might expect). Radiation - in simple English, heat waves. (this is why bonfires in winter carnivals still feel warm even though the air temperature is freezing cold).
The air does conduction or convection heat transfer. The Sun does radiant heat transfer. So the heat transfer from the air is not a way to melt snow if the air temperature is below freezing, but the heat from the Sun might be enough.
If the air temperature is *closer* to freezing at 32^o F, but still below freezing, that will help get the sunlight heat over the treshhhold.
You can get a boost by putting black colored powder on the snow, to absorb more of that radiant heat.
Just a placeholder until someone better provides an answer:
Major forms of heat transfer are:
Convection, conduction, advection, radiation.
Heat going from the atmosphere to the snow would be generally be conduction or convection, depending on whether the air is moving.
Radiation works via electromagnetic waves. No solid medium is required. The sun can heat the snow via electromagnetic waves without being in contact, either direct or indirect via atmosphere.
There are probably some inaccuracies here, first post on this sub. :/