Why does smoke 'billow'?

Question

Why does smoke 'billow'?

Answer 1

Gluotron motion.

Answer 2

The rising smoke is normally a little heavier than air at atmospheric temperature. As it's leaving a chimney or other hot location, its density is a little less than that of the air and it rises. It soon cools and becomes more dense than the air and begins to fall again but, it is met by more rising smoke  and moves around in a circulatory motion trying to find its way down to Earth.

This rising and falling and circulation of the smoke causes the billowing clouds.

(The same happens with steam from industrial sites and water vapour from the seas and oceans..etc, causing the Billowing clouds of water vapour in the sky).