Answer: That's a good question, Chip! You know, I didn't know the answer for sure myself, but here's how I thought through the problem:
I know that if I move a match around, the flame always goes up. Gravity attracts downward, so maybe it is important. I then did a quick search on YouTube for a video of a flame in zero gravity to see if the flame might look different and sure enough, it did!
What we see is that the flame expands outwards in all directions! So gravity is important. I also know that hot air balloons rise because hot gas is less dense than cold gas. The hot gases that are created when something is burning are actually the flame that you're seeing when you light a match. These gases rise because they're less dense than the surrounding air. The same thing happens with the smoke as well.
Jeff Herbstman, PhD
Postdoctoral Research Fellow
The University of Michigan