Why does air rise in the water cycle?

[erandall]

Hi there! I'm teaching about rain shadows to 6th graders, and I've reached a barrier in my content knowledge.

For the water cycle to work, warm, wet air near Earth's surface has to rise. In general, warm air is less dense than cold. However, due to air pressure, air near Earth's surface is less dense than air at the top of the atmosphere. Given that this is true, why does the warm air rise--isn't the air near Earth's surface more dense even though it is warm? It seems that both cannot be true at the same time!

 

[mjc123]

The warm air rises because it is less dense than the colder air immediately around it. As it rises, of course, it expands and decreases in density as the pressure around it drops. As long as it is warmer, it continues to be less dense than cooler air around it at the same pressure.

 

[Rive]

air near Earth's surface is less dense than air at the top of the atmosphere.

I think you messed that up

Apart from what @mjc123 said as the (warm) air rising, its density will drop with height (given the pressure change). This also means a temperature drop - however, as long as the temperature is still higher than the temperature of the surrounding air, it will still rise.

If it is about weather, then worth noting that with the temperature drop the moisture in the air will condense, and that also means a lot of heat (to fuel further rise). This can become pretty violent.

 

[jim hardy]

Another point for them
Moist air rises too because it is less dense than dry air at same temperature&pressure.
Look at the molecular weights(MW) of the molecules in it --
air being mostly N2 and O2, its average MW is about 29
water vapor being H2&O is only 18.

Try the old mirror above a coffeepot demonstration
it shows two things -
that steam rises
and it has amazing heat content ( be careful to not let little hands linger in the rising steam.)

6th graders haven't had any chemistry i suppose
but making them aware of the concepts and demonstrating them sensorially is i think good for their thinking

old jim

 

[klimatos]

Warm air does not automatically rise. Nothing moves against the force of gravity except in response to a greater force. Warm air will often simply form a "bubble" on the surface much like the vapor bubbles on the bottom of a pan of warming water. It does not go up (rise) unless pushed up by cooler/dryer air underneath. Balloonists sometimes use this knowledge by dropping sandbags on hot parking lots to break loose a bubble of hot air for increased "lift".

It should be noted that when the cooler/dryer air stops pushing, the warmer air stops rising--regardless of the surrounding air temperatures.

 

[CWatters]

That doesn't sound correct. Hot air balloons are more buoyant in cold air than hot so wouldn't they want the hot air to stay in contact with the ground rather than have it rise and surround their balloon?

 

[jim hardy]

Which physics principle to use in one's mental model is a personal choice,

Archimedes tells us buoyant force equals weight of fluid displaced
Newton tells us if buoyant force on a floating object exceeds its weight it will accelerate upward.
and that's about as automatic as anything i can imagine.

A great demonstration of those two principles is to watch your grandkids' helium balloon toys behavior in the car.
When going around a corner the balloons lean inward, into the curve , not outward like you'd first expect.
Ask them "why?". It's a great opportunity to get them started thinking about physics.

old jim

 

[sophiecentaur]

The thermal Energy in a volume of warm air with significant water content determines how high it will rise. It's yet another example of a Heat Engine, with work being done on the air to make it rise. As it rises, it expands (ambient pressure is less) but eventually, the Heat Energy contained can no longer expand the air and it stops rising ( no more 'fuel'). It is interesting to see that most of the fair weather cumulus clouds you see on a 'nice' day have all stopped rising at about the same height because they have all formed over similar warm ground and start with about the same Energy per unit Mass.
@jim hardy your comment about 'wet air' being less dense than 'dry air' is well and and points to a common misconception. As also does the idea of Air acting like a Sponge, containing a certain amount of water which will be 'squeezed out when it cools. Air is just a mixture of gases and water happens to be very near its boiling point on Earth so its vapour pressure varies significantly around the temperatures we find in the air. The proportion of water in the air depends on the vapour pressure and the ambient atmospheric pressure and equilibrium will be reached over water (or damp land).
Once the temperature drops and water starts to condense, it's the vertical airflow that keeps the tiny droplets suspended. As they coalesce, they will reach a size where the flow of air can no longer support them and - whoops, it's raining. The higher the flow speed, the bigger the drops. They can form hail when the temperature is low enough and big stones happen with high flow rates

 

[jim hardy]

It is interesting to see that most of the fair weather cumulus clouds you see on a 'nice' day have all stopped rising at about the same height because they have all formed over similar warm ground and start with about the same Energy per unit Mass.

I grew up in South Florida, definitely Cumulus territory.

The height of the clouds tells a weatherman a lot.
When there's good mixing as in summertime, the height of their bottoms tells him the humidity.
(to be more accurate with my wording - it tells him the difference between ground air temperature and dewpoint, which is closely related to humidity).


Rising air cools about 2 degrees C per thousand feet.
When it cools to dewpoint the water begins to condense,
and when a seasoned forecaster was just starting out as a lowly observer tracking weather balloons he became good at estimating cloud heights

(image courtesy NWS)

so if those cloud bottoms are say 2000 feet, it means dewpoint is about 4degC (7F) lower than surface temperature .
90F with dewpoint of 83 is a muggy Miami day.

 

[davenn]

Balloonists sometimes use this knowledge by dropping sandbags on hot parking lots to break loose a bubble of hot air for increased "lift".

It should be noted that when the cooler/dryer air stops pushing, the warmer air stops rising--regardless of the surrounding air temperatures.

That doesn't sound correct.

Agreed

@klimatos ... 99.99% of hot air ballooning is done on cold autumn/winter mornings ... the air at ground level is very cool
I have never seen ballooning done in hot weather where the ground or air above it is hot
there is just to little lift in hot weatherDave

 

[sophiecentaur]

@klimatos ... 99.99% of hot air ballooning is done on cold autumn/winter mornings ... the air at ground level is very cool

I can appreciate your argument here but I have very often seen hot air balloons flying in the late afternoon in Sussex UK. I guess there are several other factors, such as the wind speed, which conveniently drops towards evening.
The simple argument that cold air is better is actually questionable because the energy needed for a temperature rise is the same, whatever the start temperature happens to be. I would suspect that the variation of temperature of the atmosphere with height would make a difference. Warm air near the ground and cool air at height would (i think) be an advantage.
It's obviously not simple.

 

[davenn]

I can appreciate your argument here but I have very often seen hot air balloons flying in the late afternoon in Sussex UK.

But I would be extremely surprised/ seriously doubtful if that was a hot summer day !

and if you read what I wrote ... that was the whole point

 

[sophiecentaur]

But I would be extremely surprised/ seriously doubtful if that was a hot summer day !

and if you read what I wrote ... that was the whole point

My version of a hot summer day is based on UK weather so that could make a difference but when you consider that ballooning has to be in clear weather then that's another factor when you count the number of really sunny days in the UK.
But perhaps you'd explain to me why the lift doesn't work on temperature rise produced by the burners. There may be something I've missed as it's easy to do the wrong thing with thermodynamic calculations. I would imagine that there is at least as much contribution of the local temperature variation with height for 'good' ballooning conditions.
I found this link about the best weather for ballooning and the opinion there is that cold air is better but not the only factor. They include the temperature of the gas bottles in the calculation but what about the gas needed to heat the air? They don't seem to consider the temperature rise that's achievable with the fuel. I guess I could find a better informed source.

 

[FactChecker]

Another point for them
Moist air rises too because it is less dense than dry air at same temperature&pressure.
Look at the molecular weights(MW) of the molecules in it --
air being mostly N2 and O2, its average MW is about 29
water vapor being H2&O is only 18.

This is very important in some places like North Texas. "Dry fronts" come in from the West and create thunderstorms even though the temperature does not change.

 

[Steelwolf]

Getting the extra lift by popping an atmospheric thermocline and creating your own thermal lift zone sounds like a nifty trick, one that may not always work, but sounds like a method designed to get one closer to the designated landing zone by use of a handy man-affected (not exactly man-made) thermal upwelling.

But watching steam rise off from even a cup of cocoa or tea on a hot day can show the kids that yes, even though the air is warm to us, at 90F or so, at 130F it will still rise and carry visible water vapor with it, at 90 it is still rising vs 60 degree air, and the moisture it contains is just going to be carried with the air until it cools to that condensation point.

We used to get muggy warm mornings on Guam, then a brief wind and sudden, 20 minute downpour, and then the sun would be out and turning a good deal of that water back to steam, and it got to then be hot, muggy and already drenched. Such was monsoon season. Severe version of that was the seasonal Typhoon, same as hurricane here in US. In fact, my first time to sea was in a typhoon because you DO NOT want to sit next to a dock or pier during one as it would destroy or ground the ship. But it was certainly an experience!

And all because warmer air carries that humidity up, and that draws in outside air. Lot of heat in the water is going to mean a lot of air to cool it, thus increasing the size of Hurricanes,Typhoons and Cyclones of all sorts.

 

[sophiecentaur]

visible water vapor with it,

Water vapour is not visible. What you see is tiny drops of liquid water. They only rise because the air plus vapour is also rising. Because the initial size of the droplets is so low, they will either fall very slowly or be lifted up with only a very low upward air velocity. The volume of a floating mass of air plus water vapour will decrease when the water starts to condense.

 

[Steelwolf]

True, I should have said visible condensation, but usually the H2O is evaporating back into the background humidity and 'disappearing' again. For that short time, above the cup, the density of the water condensation droplets makes it appear as 'smoke'. But that is basic steam for you.

 

[sophiecentaur]

It's a matter of scale though. Clouds will hang around for hours. But there again, if you look at some clouds, they are continually forming and dispersing as you watch them. Those castles in the air can change into elephants in a matter of minutes.

 

[FactChecker]

Those castles in the air can change into elephants in a matter of minutes.

Good point. To appreciate how fast (instantaneously?) the clouds can form, see sonic boom cloud. The cloud is not being dragged along by the plane, it is instantly(?) forming as the plane goes by and changing the pressures.

PS. The plane does not have to be going at Mach speed. Here is an example with a slower plane slower plane