Why Warm Air Rises: Intro Physics Book Explained

[pivoxa15]

The intro physics book claims warm air raises. Why? Is it because we usually make our observations on ground level and warm air will go anywhere and there is more chance of it going up than down as there is more room at the top (from our point of view) .So we observe it going downwards.

Or is it because warm air is able to go against gravity more than cooler air. It doesn't all go up but it can go up so even though its motion is random, it has the ability to go up relative to cool air. We observe an overall shift upwards of those warm air so we say warm air raises.

The book then suggests that the fact that warm air rises is an explanation for why the freezing compartment of the fridge is at the top. I don't understand why this is the case. How are they related?

 

[pivoxa15]

Why is the freezing unit usually the top compartment?

Many fridges have the freezing unit on top and the normal cooling compartment down the bottom. Why?

The answers at the back of the book suggested it was because warm air rises. What has this got to do with this question?

 

[Gokul43201]

Warm air "goes up" because the cooler air above it "sinks" into it, displacing it upwards. This happens for the same reason that a stone will sink in water, displacing the water upwards, or that a piece of wood held under water will float upwards when released - buoyancy.

 

[pivoxa15]

Warm air "goes up" because the cooler air above it "sinks" into it, displacing it upwards. This happens for the same reason that a stone will sink in water, displacing the water upwards, or that a piece of wood held under water will float upwards when released - buoyancy.

That is interesting. What is the 'fundalmental' physics behind it? Does Archemides principle work with gases as well (the answer is probably 'why not?')? It seems completely different my first explanation. Could my second explanation relate to what you say?

 

[Hootenanny]

A good link explaining convection in terms of density changes: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heatra.html#c3

 

[EmilK]

Yes, Archimedes's Principle work with gases as well. The fundamental physical property behind it is density. As hot air contain atoms and molecules with higher kinetic energy than cold air. As a result it has a lower density, and the cold air goes downwards.

 

[billiards]

Under a constant pressure according to the gas law pV=nRT if you raise temperature T you raise volume V, i.e. a parcel of gas will expand as it gets hotter. When it expands its density decreases and it rises due to buoyancy as has already been said.

Also the particle motion might appear random but you do not need to consider the motion of individual particles, at the scales discussed here physicists consider a parcel.

 

[russ_watters]

That is interesting. What is the 'fundalmental' physics behind it? Does Archemides principle work with gases as well (the answer is probably 'why not?')? It seems completely different my first explanation. Could my second explanation relate to what you say?

Remember, when you heat a volume of gas, it expands, becoming less dense. So warm air is lighter than cold air.

 

[pivoxa15]

So gravity is the main reason behind this which basically is archemides principle applied to gases?

What about this question "Why is the freezing unit usually the top compartment?" in my second post?

 

[misnoma]

Many fridges have the freezing unit on top and the normal cooling compartment down the bottom. Why?

The answers at the back of the book suggested it was because warm air rises. What has this got to do with this question?

simply put: the cooling compartment must be at the top - when slightly warmer air in the fridge rises (from foods or open door etc), it will come into contact with the cooling fins at the top. Here it will loose energy, become more dense (for a given volume) and consequently sink in the warmer air around it. Thos process ensures a convection current and is able to maintain an even temp (ish) throughout the fridge compartment. If the cooling compartment was not at the top, the air would literally consist of trapped warmer air in the top half.

 

[DaveC426913]

Note that "warm air rises" is technically not true, and thus a bit misleading. Warm air falls under gravity like anything else. If you opened a can of warm air on the Moon, it would (on average, and eventually) fall.

This is obvious, yes, but now put it back in the context of an atmosphere. The warm air still wants to fall, it just can't. The correct way to view the phenomenon is that cold air sinks more than warm air does.

 

[pivoxa15]

simply put: the cooling compartment must be at the top - when slightly warmer air in the fridge rises (from foods or open door etc), it will come into contact with the cooling fins at the top. Here it will loose energy, become more dense (for a given volume) and consequently sink in the warmer air around it. Thos process ensures a convection current and is able to maintain an even temp (ish) throughout the fridge compartment. If the cooling compartment was not at the top, the air would literally consist of trapped warmer air in the top half.

Heat can be transferred by conduction through the layers connecting the two compartments? Note: I am assuming doors are closed.

Assume that is how heat is transferred. Then if the freezer was at the bottom, the fridge would have a heavy load of warm air in the upper porportion resulting in uneven temperture in the fridge - not good. As you pointed out.

If the freezer was at the top than warmer air in the fridge would touch the layer separating them and cool down again.

Although we still have the problem of the freezer having an uneven temperture distribution when at the top. It would have a more even temperture distibution being at the bottom. But heavier emphasis is put on the fridge having an even temperture distribution so that is why the freezer shoudl be at the top?

 

[pivoxa15]

I checked my own fridge and saw that the evaporator was installed near the top of the fridge. This makes sense since it would mean warm air rises and gets cooled again so a stable convection mechanism can happen. However, I checked my freezer and didn't see the evaporator anywhere. Where could it be? What would it look like? In the fridge, it was a rectangular board/plate with the evaoparation probably at the back of that board.

 

[daniel_i_l]

As i was reading this i thought of the following question:
Lets say that you start with hot air on the bottom and cold air on top. after a while they change places - my question is - is the hot air which is now on top the same air [= same particles] that were previously on the bottom or are the particles in basically the same places just with different velocities? in other words, do the particles themselves move or does the "heat" move via colliding particles?
Thanks.

 

[Gokul43201]

in other words, do the particles themselves move or does the "heat" move via colliding particles?

Both, but often in fluids, it's more of the former - the molecules themselves moving (this is called convection, as opposed to the latter case, which describes conduction).

 

[pivoxa15]

since daniel_i_l was talking about air hence gas that are moving than convection is even more prevalent and conduction is negligible.

The physics behind warm air and fluids (that are of the same chemical element) rising compared to their colder counterpart is due to Archemides principle. The key to this principle is that the particles are immiscible and particles can move through oneanother. The latter is satisfied by air and fluid while the former is satisfied because of the temperture differences in the particles.

 

[DaveC426913]

Yes. There's more convection than conduction. This why we have weather - masses of moving air.

It is possible, and often happens in the summer, to have an "temperature inversion", where a layer of warm air gets trapped under a layer of cold air. The warm air is unable to rise. (Think of how hard it is to get ketchup out of the bottle if you hold it directly upsidedown)

 

[russ_watters]

So gravity is the main reason behind this which basically is archemides principle applied to gases?

Yes! And this is related to another recent thread (not sure if it was started by you) about buoyancy in nonmixing fluids. Much of the convection in the atmosphere invovles descrete "bubbles" of relatively uniform temperature/density regions that percolate up through the atmosphere in a way that if you could see it would look a lot like a lava lamp.

Sometimes, though, you can see it - the white puffy cumulous clouds you see developing in spring and early summer are updrafts - bubbles of warm air - that have risen, expanded, started to cool, and thus condensed out the water vapor in them. So now you can explain rain... If the air mass is even warmer and even more moist, that creates a cumulonimbus cloud - a thunderstorm. Next time you see one, take a closer look (with binoculars, even). If it is still well-defined (puffy, not wispy), it is growing. You can actually see it grow, gain altitude, and expand, not too unlike a bubble in a lava lamp.

 

[billiards]

Yes. There's more convection than conduction. This why we have weather - masses of moving air.

It is possible, and often happens in the summer, to have an "temperature inversion", where a layer of warm air gets trapped under a layer of cold air. The warm air is unable to rise. (Think of how hard it is to get ketchup out of the bottle if you hold it directly upsidedown)

Why is the warm air unable to rise?

 

[DaveC426913]

Why is the warm air unable to rise?

Because there's a cold layer of air pressing down on top of it. The interface is so flat that the warmer air has no chance to "bubble up" through the cool air layer. That's why I compared it to ketchup coming out of the bottle.

 

[billiards]

Contrary to what you say, according to my meteorology book an inversion occurs when temperature increases with height. Also I don't believe it is very likely that "the interface" would be perfectly flat - not that there is a sharp interface anyway.

The situation you talk about is unstable and pretty standard in the atmosphere, convection cells will form with local heating at particularly warm spots on the surface, this air expands rises, expands some more cools and sinks.

 

[pivoxa15]

Because there's a cold layer of air pressing down on top of it. The interface is so flat that the warmer air has no chance to "bubble up" through the cool air layer. That's why I compared it to ketchup coming out of the bottle.

How does ketchup coming out of the bottle compare with your example?

 

[billiards]

Sorry don't mean to ruffle any feathers but it's a bad analogy. Primarily because although Dave was correct that a temp inversion is unstable, he got it the wrong way round , A TEMP INVERSION IS WHEN HOT AIR OVERLIES COLD AIR!

 

[DaveC426913]

OK.

1] Not sure if Hoot is joking or not. The ketchup bottle analogy is not about tempertaure it's merely about the "glug". Even though ketchup wants to fall out of the bottle, and air wants to rise into the bottle, they can't both do their thing at the same time (in this case, becasue the opening of the bottle is too small), so neither do.

The key to getting ketchup out of the bottle is to hold it sideways so that the ketchup can fall while the air can rise at the same time - without the two getting in the way of each other.

You see, the cold air in the atmo can't fall because there's warm air under pushing up, and the warm air can't rise because there's cold air pushing down.

Perhaps the key to understanding this is that large masses of air of differing temps tend not to mix.

However, Billiards is correct. I had a simplistic understanding of temperatiure differences in the atmo and a misunderstadning of temp. inversions. Oops.

 

[billiards]

That makes slightly more sense but to be honest I think the ketchup thing has more to do with the internal surface area of the bottle and the remaining volume of ketchup in the bottle. I mean it's pretty easy to get the ketchup out of a full bottle right?

I stumbled across a paradox whilst thinking about this hot air rising thing. Okay, first warm air rises because it is buoyant; as it rises it expands which makes it cool down some, now that it's cooler it wants to sink but why??
Presumably if it cooled by expanding it would be less dense than it was before, so what makes it more dense again? I have a funny feeling it's got something to do with condensation...?

 

[DaveC426913]

Okay, first warm air rises because it is buoyant; as it rises it expands which makes it cool down some, now that it's cooler it wants to sink but why??

Well, it rises and cools until its at equilibrium.

 

[pivoxa15]

That makes slightly more sense but to be honest I think the ketchup thing has more to do with the internal surface area of the bottle and the remaining volume of ketchup in the bottle. I mean it's pretty easy to get the ketchup out of a full bottle right?

A full bottle is easier to get the ketchup out because the volume hence weight of the ketchup is larger so the force downwards is greater than the force of the rising air.

For a bottle without much ketchup, if you tilt it upside down I do observe that its often very difficult to get the ketchup out. The reason is that
1. Now the force down is much less than a full bottle because of decreased volume.
2. Surface tension between the ketchup and glass is now not neglible as the ketchup is attracted to the ionicly bonded glass.
3. The warmer air that was rising is pushing up into the bottle

Point 3 is because usually I store my ketchup in a fridge and so air in the bottle is cooler than outside air so whenever the bottle is tilted upside down and opened, warmer air in the room rushes into the bottle due to archemides principle.

I stumbled across a paradox whilst thinking about this hot air rising thing. Okay, first warm air rises because it is buoyant; as it rises it expands which makes it cool down some, now that it's cooler it wants to sink but why??

When it's cooler it dosen't want to sink but the warmer air at the bottom wants to rise and overtake the cooler air.

Presumably if it cooled by expanding it would be less dense than it was before, so what makes it more dense again? I have a funny feeling it's got something to do with condensation...?

Yes it's less dense than before but it's now cooled so has stopped expanding (it doesn't have to get more dense) whereas the warmer air below it is still expanding so getting less and less denser until eventually overrides the cooled air because it's now less denser than the cooled air which has stopped expanding for some time.

 

[pivoxa15]

You see, the cold air in the atmo can't fall because there's warm air under pushing up, and the warm air can't rise because there's cold air pushing down.

Perhaps the key to understanding this is that large masses of air of differing temps tend not to mix.

However, Billiards is correct. I had a simplistic understanding of temperatiure differences in the atmo and a misunderstadning of temp. inversions. Oops.

As Billiards pointed out corrected temperture inversion is when cool air is trapped by warm air on top. Normally its the other way round, near the Earth's surface it's wamer due to accepting radiation from the sun. The warm air rises and as it cools, gets overtaken by warmer air below it. Hence a convection cycle is established. However, if the upper level of the atomsphere contain warmer air than the lower level, than no convection cycle occurs which is called a temperture inversion.

http://en.wikipedia.org/wiki/Temperature_inversionSo Dave, you seem to be talking about a different phenomena where theoretically the convection cycle (whereas in temperture inversion the convenction cycle cannot even theoretically occur) should be running but not because the cooler air cannot stably fall and warmer air rise. What is this phenomena called?

 

[billiards]

Well, it rises and cools until its at equilibrium.

Well yes but the question is why does it sink again? There must be a mechanism.

When it's cooler it dosen't want to sink but the warmer air at the bottom wants to rise and overtake the cooler air.

Although painfully obvious (if not slightly confused) I think you might be on to something, the warm air keeps rising and cooling until you reach the stratosphere where there's a big temp inversion. The air can rise no further so it advects, there must be a conservation of mass because there's no less air today than there was last year, so for every bit that rises another bit somewhere else goes down.

Yes it's less dense than before but it's now cooled so has stopped expanding (it doesn't have to get more dense) whereas the warmer air below it is still expanding so getting less and less denser until eventually overrides the cooled air because it's now less denser than the cooled air which has stopped expanding for some time.

I'm not sure if that's the mechanism though. I'm too lazy to read my meteorology book but I do know that air comes back down in areas of high pressure (at the surface), basically air converges in the upper atmosphere starts to spin anti-cyclonically and is pumped down to the surface where it diverges.