Having to start a fire on top of snow is one of those things that you never really think about, until you have to do it. In areas with little snow, you can always dig down to the soil surface to build your fire. But if the snow is very deep, you lack digging tools, or you don’t have time to dig, building a fire on top of the snow is the best option–if you do it right. You can certainly start a fire literally on top of the snow, but it won’t last very long as the snow will melt and douse your coals.
So how can you build a fire on top of frozen water? You’re going to need a “raft.”
In an emergency, use whatever material is available to form your raft. Man-made materials, like a piece of metal, would work. But, again, remember that the hot metal will melt the snow, so you’ll need to stack some logs beneath it. If you don’t have a piece of metal handy, the raft can be made of wood–dead, rotten, or green. This last choice is the best of the foraged materials, because live wood has enough moisture in it to keep it from burning for a few hours, but not so much moisture that it puts out your burning sticks or bed of coals. You can use whole chunks of wood, or split the wood in half to create a very flat raft. Obviously, the whole pieces will last longer than split wood. Replace the raft, or pieces of it, as needed.