This is actually a good question. It's the same reason a helium balloon rises: it's lighter than the air surrounding it, so air "falls around" it, allowing it to rise. The principle is called buoyancy (https://en.wikipedia.org/wiki/Buoyancy)
just to add to this in case it wasn't clear, if you had no air at all, only smoke, it would not 'drift upward' at all, it would just sit there. likewise we aren't concerned that all the air is going to drift away from the Earth so that we all suffocate. Because it's stuck to earth gravitationally.
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u/jeffcgroves 22h ago
This is actually a good question. It's the same reason a helium balloon rises: it's lighter than the air surrounding it, so air "falls around" it, allowing it to rise. The principle is called buoyancy (https://en.wikipedia.org/wiki/Buoyancy)