Why Does Smoke Sometimes Rise and Sometimes Fall?
To answer this question, we must first understand what smoke is. You know that smoke usually results when a substance such as coal, wood or tobacco burns. Smoke is a mixture of gases, mostly water vapor, in which tiny solid particles are held. It is because of these solid particles that we can see smoke. As it leaves a burning log or a cigarette, it is of course quite hot, and this makes it lighter than the surrounding air. So it drifts up, as warm air will rise above cool air.
Eventually the gases become mixed with the air and the solid particles fall as dust or soot. On a damp day, however, when the air already contains as much water vapor as it can hold, the water vapor in the smoke is precipitated, that is, it forms into tiny drops of water and falls just as rain does. As it falls it carries with it the solid particles, and so we see the smoke fall though the amount of water present is too small for us to notice it. We see only the dark particles.
