# Convection currents

Hi, please could someone explain how a convection current occurs. There are a few different explanations I have come across which in my view are not consistent. Warm air rises, becomes less dense and due to its buoyancy will rise. What mechanism causes the cooler air to replace this air? Some sources say it is 'sucked in' due to pressure differences whilst others say the cooler air from above sinks. I have looked at another thread (https://www.physicsforums.com/showthread.php?t=676317) but its a bit too complex for me but seems to suggest it has nothing to do with pressure differences?

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NascentOxygen
Staff Emeritus
Probably heat (or something) causing a change in density. I guess you'll need a gravitational field, real or artificial, so that the material with altered density is acted on differently.

Oh, and I'm sure you need it to be a fluid, or at least behave like a fluid, so convection currents can exist.

Simon Bridge
Homework Helper
Hi, please could someone explain how a convection current occurs. There are a few different explanations I have come across which in my view are not consistent.
You certainly seem confused!

You may be better off picking a description you have found and asking about that.
It will also help to have some idea of your education level.

Warm air rises, becomes less dense and due to its buoyancy will rise. What mechanism causes the cooler air to replace this air? Some sources say it is 'sucked in' due to pressure differences whilst others say the cooler air from above sinks.
It is both at the same time.

Cool air is more dense than the hot air so it sinks in, the rising hot air also sucks in surrounding air.
When convection starts up it is very messy but quickly settles into a "cell" form where hot air rises in a rough column with cool air sinking around it. However, there are other factors contributing - i.e. a chimney effect near tall cliffs.

One way you can study convection is in water - put a glass pan on the stove on a low heat (you don't want the water to boil at all) and inject a little bit of ink (some crystals are good for this) towards one edge and watch the way the color moves about.

But the basic principle is what you have read: hot air rises as a low-density bubble in the cooler air and the cold air flows around it.

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russ_watters
Mentor
Hi, please could someone explain how a convection current occurs. There are a few different explanations I have come across which in my view are not consistent. Warm air rises, becomes less dense and due to its buoyancy will rise. What mechanism causes the cooler air to replace this air? Some sources say it is 'sucked in' due to pressure differences whilst others say the cooler air from above sinks. I have looked at another thread (https://www.physicsforums.com/showthread.php?t=676317) but its a bit too complex for me but seems to suggest it has nothing to do with pressure differences?
Place a volume of warm air on a balance scale next to an equal volume of cold air. Does the cold air push up the warm air or does the warm air pull down the cold air?

Lift an object off a table. Did it leave an empty vacuum on the table where it used to be? Why or why not?

1 person