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Name: Ben Urquhart
Status: student
Age: 12
Location: N/A
Country: N/A
Date: 1999

Why does smoke rise?

What causes anything to rise (smoke, balloon, etc.) is what the density of the object is compared to the density of the environment that it is in. In the case of smoke, it's density is less than that of the surrounding air, and thus it rises. The same goes for a hot air balloon. The density of the hot air trapped inside a balloon is less than that of the surrounding cooler air. When this happens, the balloon will rise. Now the air has to be kept hot so that its density is always less than the surrounding air. The lower dense fluids (air is considered a fluid in engineering terms) will always "rise to the top" of a mixed fluid system. Another example of this is when you try to pour water and olive oil (or any kind of cooking oil) together. Pour about a half a cup of water into a measuring cup, and then pour a half a cup of liquid cooking oil into the same cup. Let it settle for about 2 min. and then look at it. You will be able to see the water on the bottom and the oil on top with there being a definite line where the two separate. This is because the oil is less dense than water so it rises to the top of the glass. Hope this helped explain it a little better.


SMOKE consists of solid microscopic particles formed as a byproduct of combustion, along with heat and light, all in varying amounts. Certain gaseous materials are also formed during combustion. Air and the gaseous materials absorb the heat of combustion, and become less dense than the surrounding air. The embedded microscopic particles, or "smoke" rise in the less dense air and gas. As they rise, they cool, and mix in clean air through turbulent mixing. When the heated air and gases cool to the temperature of the surrounding air, it no longer rises. But by that time the smoke particles have been diluted or dispersed enough to no longer be visible.

I hope this is not a too complicated answer to your question...!

Dale Bechtold, Meteorologist
Forecaster, National Weather Service
Weather Forecast Office, St. Louis, MO


Your observation should answer this for you. When one fulid (here, smoke) is dispersed into another fluid (here, air), the two fluids will orient themselves according to their effective weight. The smoke is apparently lighter in this case, so it rises.

You can note that warmer smoke would rise even faster and, if the smoke were cooler than the air, and thus heavier, it would not rise. You might have already seen subliming dry ice (solid carbon dioxide) spilling down like a waterfall....this happens because it is cool and heavier than air. Once the carbon dioxide warms it has less a tendency to fall and would instead mix with the air through the various wind currents etc..

Richard R. Rupnik
Internal Quality Auditor
Lucent Technologies
(610) 712-7152

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