Does warm air rise or does cold air sink?

[somega]

I have a friend and he complained that at school they are teaching that warm air rises.
He said it's wrong. He said instead the cold air sinks.

Is this true?

 

[Dale]

What is the difference? What sort of experiment could be used to determine the difference between the two propositions

 

[Ibix]

Two ways of saying the same thing, really.

 

[gleem]

Place a mass of air in air that is cooler and it rises. That is why a hot air balloon rises. Warmer air is less dense than cooler air. Conversely a mass of cool air will sink if place in air that is warmer. This is easily experienced when you open a refrigerator and feel the cool air on your legs first.

 

[Chestermiller]

Both things happen at the same time. If you have some hot air rising, it is displacing cold air which has nowhere else to go but down. So hot air rising is always accompanied by cold air sinking.

 

[DrStupid]

It's the Rayleigh-Taylor Instability. Here is a visualization:



Edit: Just for the sake of completeness: A configuration of cold air on top of warm air can be stable if the temperature gradient doesn’t exceed the adiabatic lapse rate. The best example is the atmosphere.

 

[sophiecentaur]

Does the heavy person at one end of a see saw sink or does the light person at the other end rise?