Why does smoke rise?

Answer

Air is composed of several different types of molecules, which form a unified density. Smoke is composed of different molecules which together are less dense. Therefore the less dense molecules are pushed upwards by the "heavier" air molecules.

Underwater, if you released air and smoke in two seperate sets of bubbles, the smoke bubbles would rise to the surface faster. This is because while both air and smoke are thinner than water, the "heavier" air molecules would take longer to be pushed out.

As you probably know, smoke is made of a lot of tiny particles -- mostly solid, some liquid --
suspended in air. The particles are a lot heavier (more dense) than air, so why don't they sink out
and go down rather than going upward?

Here is an explanation of sorts:

A fire makes things hot. Above a fire, you get quite a lot of hot air. Hot air is less dense than cool
air, so it rises. Because there is quite a lot of hot air, it makes quite a big wind current as it rises,
and that drags the smoke particles upward with the air, in much the same way as wind can raise
dust. Some of the smoke particles are very small indeed, so they can be held up in the air for quite
a long time, and they do not settle out from the air until a long time after they leave the fire, and
usually some distance away from the fire.

In a large bushfire, all the hot air produced makes very strong currents indeed, and not only fine
particles, but also quite large burning twigs and leaves can be carried up in the smoke, and do not
fall out until they have travelled quite some distance. It is these pieces of burning vegetation --
called "embers" -- that often start secondary fires, and make fighting large bushfires extremely
difficult and dangerous.

Extracted from :- http://www.madsci.org/posts/archives/2004-03/1079497709.Ot.r.html

First answer by Surferpros. Last edit by Lokuhetty. Contributor trust: 13 [recommend contributor]. Question popularity: 30 [recommend question]