My guess is that it's because smoke is hot. Hot air rises, just as cold air sinks. When a fire ensues, smoke (the product of carbon combustion) rises with the hot air.
Kurayami: Tar enters your lungs when you smoke and can build up.
Smoke goes up from chimneys, cigarettes, campfires, and assorted other locations of combustion.
Smoke A confused snowflake
smoke
Smoke
Smoke
Smoke from a chimney goes up then spreads out horizontally.
smokey
It does. It just pulls the colder air around it down harder, so the air gets pulled down furtehr than the smoke. Result: smoke goes up, air goes down, until the smoke cools enough to become air.
When smoke goes up through a tube to a container and into fire.
When smoke goes up through a tube to a container and into fire.
Being hot, it has expanded, and weighs less than the air it displaced.