In general, because almost all of the heat arriving from the sun is absorbed by the ground, and the air in our atmosphere gets most of its heat from the ground, the air gets colder as you get further away from the Earth’s surface.
This means that the warmer, less-dense air near the ground can easily float upwards through the cooler, denser air above it. Being hot, the matter produced by a fire – a mix of gases and small solid particles – naturally moves upwards because the hot mixture is less dense than the cooler air it is floating through. The mixture, of course, is smoke and, as we know, smoke rises – usually.
