Why do clouds have flat bottoms?

[DaveC426913]

I know that the water vapour condenses once it hits the colder air above. What I don't understand is why the warm-colder boundary is always parallel to the ground even while the masses of air are pushing through it. Why wouldn't the rising mass of moist air also determine warm-cold boundary? i.e. the mass of rising moist air would act as a unit and carry the warmer air with it. (see pic)

 

[MaWM]

Well.. the warm air cools as it rises. I would expect that, at the large scales of clouds, the air rises and cools at a fairly uniform rate. thus it reaches the dew point at about the same height. Of course the bottoms are not exactly flat, but they may appear so from thousands of feet away, so deviations have to be large to be seen. again, clouds are large scale objects

 

[FredGarvin]

I think the winds aloft have a larger effect on the situation which dictates that separation.

 

[DaveC426913]

Well.. the warm air cools as it rises. I would expect that, at the large scales of clouds, the air rises and cools at a fairly uniform rate. thus it reaches the dew point at about the same height.
But the whole thing about a cloud is that it is a column of warm, moist air rising, yet the dry, clear air right next to it is not. Your suggestion of uniform rate of cooling is analagous the steam cloud rising from a pot of boiling water having a flat top as it all cools at a fairly uniform rate.

Of course the bottoms are not exactly flat, but they may appear so from thousands of feet away, so deviations have to be large to be seen. again, clouds are large scale objects

Cumulus are the lowest clouds - typically < 5000 feet and you can often see this effect across scores of miles of cloudscape. While they're large-scale objects, so is the scale by which we measure the flatness. They're flat within a few percent of the cloud size.

 

[DaveC426913]

I think the winds aloft have a larger effect on the situation which dictates that separation.

Please elaborate. We're talking about http://www.dkimages.com/discover/previews/930/50171989.JPG- not in any particularly windy situation.

 

[FredGarvin]

Winds aloft are the wind velocities experienced at altitudes. If you have ever seen a pilot's winds aloft report from a weather station, you would see a report of altitudes with wind directions and speeds. The winds aloft can reach very high speeds. Even though there is no wind on the ground you still will have them at altitude. However, since I missed that you were talking about cumulus clouds, I am pretty sure that the wind speeds will usually not be that great at low altitudes. So that can't be it.

Perhaps the situation you picture is simply not possible due to the mass difference between the surrounding atmosphere and the cloud. The cloud simply does not have enough effect on the local temperature to cause the warm/cold line to bulge like that. I will keep looking to see if I can find anything (which I am sure you are doing as well).

 

[Renge Ishyo]

The confusion might be about the idea of a cloud being in a "static" location? My understanding is that water vapor moves (spreads out) while it cools as it rises away from the heat source (much in the way that warm currents rise in the center of a boiling pot of liquid and then cool and sink back into the liquid as they spread out towards the sides of the pot, far away from the center where the heat source is). Condensation as we know it can occur at a location much farther away from where the water originally evaporated in this manner (in this respect the clouds would be forming over cooler air having moved away from the portion of the sky where the hot water vapor was originally rising).

For the proof that clouds are moving and not static...a simple day of cloud gazing will get that for you (I remember doing this on a break a few years back; the clouds on that day, which might have looked static at a glance, where actually moving VERY rapidly once I stared at them for awhile and compared them to the speeds of planes flying by, etc.)

As for why the clouds tend to a flat shape as they condense, this could be related to minimizing potential energy due to gravitation attraction. To show you what I mean, if you take a bucket of liquid water while standing on a flat concrete floor, toss the water up in the air in a vertical column, and stand back and watch the shape the water takes as it hits the ground you will notice that the water will spread out as flat as possible in all directions across the ground surface (the explanation for this is that the flat shape minimizes the graviational potential energy for the water molecules in the liquid). Using imagination then, imagine a vertical updrift of warm water vapor being tossed up into the sky and then spreading out and cooling as it settles onto the cooler more dense gas below it. If the condensed gas/cloud behaved like the liquid water it will eventually form when it cools down enough, it will want to be in as flat a shape as possible so as to minimize the gravitational potential energy on anyone portion of the cloud.

Is my rationalization for this at least

 

[rewebster]

I think 'part' of the reason is the same as why cigarette/smoke layers in a calm room.---and I would guess that the bottoms aren't really flat--they just appear that way from a distance.

 

[DaveC426913]

Huh. when first asking the question, I felt almost foolish, assuming it was meteorology 101. But judging by the number of educated guesses going on, it seems it wasn't such a dumb question after all.

 

[rewebster]

This is my thinking (in a little more detail):

When you centrifuge a 'mixed' fluid, it doesn't take much (in the way of a density difference) for it to 'layer'. Clouds form according to many things, including humidity, air density, temperature, etc. If the temperatures are 'right' for condensation, the 'condensate' will 'stay' at that altitude layer, unless 'mixed' (billowy tops). I'm thinking that these are generated from specific spots (air 'tunnel' ports on the bottoms of the 'flats' of the clouds that aren't readily visible from the ground). The creations of the flat 'bottoms' are that specific 'layer' (sort of like in a centrifuge) where the layer above is a 'condensate' and the layer below (slight, very slight, but enough to 'show' the difference) is not an observable condensate (the cumulative mist that forms a cloud).

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oh, yeah, and the 'smoke layering' analogy has to do with the bouyancy of the system (density of air/temperature).

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Something else to go along with this---there are different thoughts that 'condensation' forms around particulates--dust, smoke particles, ash, even microbes---wars of old with a lot of expelled particulates led to the idea of 'rain making' and I think they mix some microbe with the water when 'making snow' for skiing.

 

[berkeman]

Wow, I just wiki'd clouds, and got more than I bargained for!

http://en.wikipedia.org/wiki/Clouds

When I looked at the pictures of the cumulus clouds, though, they didn't have flat bottoms. Can you point to a picture of the flat-bottomed clouds that you're referring to? (I've seen them too, but don't know what their names are.)

 

[turbo]

There is water vapor in the air all the time. Usually (except for the odd inversion) the temperature of the air decreases with increasing altitude and at some level the water vapor in the air condenses to form clouds. If the temperature transition is fairly abrupt and the air is well-stratified, the altitude a which the dew point is reached forms the lower boundary for clouds, giving cumulus clouds flattened bottoms.

 

[rewebster]

I was thinking the difference between a visible condensation and not visible may be only a couple of degrees difference.

 

[DaveC426913]

...If the temperature transition is fairly abrupt and the air is well-stratified,...

This is what I'm getting at.

How can the air be well-stratified when the nature of a cloud is that it is moist, warm, rising air. By definition, I would tihnk that clouds disrupt stratification.

 

[DaveC426913]

Can you point to a picture of the flat-bottomed clouds that you're referring to? (I've seen them too, but don't know what their names are.)

I did. See post 5.

Best I could find, despite being a poor example. I've seen cloudscapes where it's almost a cliche of fluffy tops with perfectly flat bottoms all the way to the horizon.

 

[turbo]

Clouds do not dictate weather, though some spectacular updrafts can cause a lot of excitement. Weather occurs along differences in temperature and air pressure. Air can be (and often is) easily stratified in temperature by atmospheric conditions.

 

[arunma]

Interesting thread. Last Wednesday when I was flying back home, the plane passed through a region with just a few scattered clouds. I noticed that the bottoms were flat, and began wondering about this. And given that I was flying back from a trip that my astrophysics department had sent me on, I (much like Dave) felt pretty foolish being a physicist who can't answer such a seemingly simple question. I guess it's not a stupid question after all.

Anyway, I'll be watching this thread carefully to look for a good answer. So far the one about minimizing gravitational potential seems pretty convincing.

 

[cesiumfrog]

How can the air be well-stratified when the nature of a cloud is that it is moist, warm, rising air. By definition, I would tihnk that clouds disrupt stratification.

Looking at your first picture, of a bubble of (warm) rising air (mixed, fairly uniformly, with water vapour - which is what gives the lowered density).. As this bubble rises it also expands, the temperature continually decreases and the pressure continually decreases. Doesn't it stand to reason that temperature and pressure would depend mainly on height (and time, but not depend strongly on position over the ground), so the point at which water begins to condense (determined purely by temperature and pressure) should be at a roughly constant height (at any given point in time)? As the bubble rises further, the "surface of first condensation" (the bottom of the cloud) would remain fairly flat whilst progressing downward through the bubble.

Meanwhile, above that boundary (inside of the cloud), the condensation of water vapour is liberating heat into the cloud. Parts of the cloud with high water content to be warmed further, continuing to rise up (causing the asymmetric fluffiness, since while the cloud-bottom is bounded by a pressure level, the cloud top continues however far as convection "randomly" takes it). In fact, larger bubbles of water vapour would take longer to cool than smaller bubbles, therefore individually rising to different heights, giving the cotton-wool-balls appearance.

Since the top of the cloud (determined by convection which is characteristically chaotic, combined with the unstable positive feedback process from heat of condensation) is so irregular, the bottom only needs to be roughly flat to give the observed cloud appearance. No prior stratification is necessary, because we know that pressure is always going to be a uniform and suitably smoothly decreasing function of height, and that water vapour will not condense (to become visible cloud) until it reaches a particular pressure level.

 

[Renge Ishyo]

How can the air be well-stratified when the nature of a cloud is that it is moist, warm, rising air. By definition, I would tihnk that clouds disrupt stratification.
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You can see clouds form in one of two situations:

1. As water is boiled and intially converted to water vapor (if the conversion is fast enough the vapor is still dense enough in water molecules at this point to be seen as white steam as is typical from the stove or industrial plant).

2. As water vapor cools to such an extend that it begins the journey of condensation back into liquid water.

In an "ideal" situation you would see these occur in nature in two different places. The vertical updrift occurring at the point of the intense heat source (as at the plant with the steam exiting the circular pipes at an industrial plant), and the soft flat condensation occurring after the water vapor has cooled and is settling back down to the surface (ultimately deposited as rain).

The confusion seems to be of considering situation #1, but not #2. Clouds certainly can exist in moist, cold, sinking air as well (as anybody that has gone to work in morning fog can attest). It is likely that any flatness will occur on the cooling of water vapor from the upper atmosphere and NOT on the intial vaporization. The two processes do indeed seem to produce contrasting shape tendencies.

 

[rewebster]

This is what I'm getting at.

How can the air be well-stratified when the nature of a cloud is that it is moist, warm, rising air. By definition, I would tihnk that clouds disrupt stratification.

I am guessing you don't like my explanation---

now, it seems like your asking

what forms a cloud?

what makes the flat bottom on clouds (sometimes)?

and what keeps a cloud from being 'disrupted'?

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have you ever driven through fog?

fog is just a low laying 'cloud'--does the temperature drop 20, 30, 50 degrees in a fog bank? It's like frost, 1 or 2 degrees difference and frost forms sometimes in the lower areas (like lower areas in slight differences in air strata.

clouds aren't static, ever---they are constantly in the process of condensing and evaporating (like the steam above the pot).

 

[DaveC426913]

Clouds do not dictate weather, though some spectacular updrafts can cause a lot of excitement. Weather occurs along differences in temperature and air pressure. Air can be (and often is) easily stratified in temperature by atmospheric conditions.

Sorry, is that what you thought I was saying? No.

Cumulus clouds in particular form when warm, moist air rises in a column where the ground has been heated by the sun.

What I don't understand is how, if the column of air is rising, how it can not disrupt the stratified layers.

 

[Renge Ishyo]

That seems to be an incomplete definition of how cumulous clouds form. For example, the following site gives that definition (as do many other sites), BUT also mentions that the actual formation of the clouds appears only upon cooling as it spreads (as it cools down the clouds are on the way back down):

http://schoolscience.rice.edu/duker/weatypeclouds.html

The problem with the original question is that it assumes that clouds will be condensing in the exact same area where evaporation originally took place. This is unlikely...if the temperature were hot enough to evaporate liquid water at a given spot it would surely evaporate any clouds that were starting to form at that spot before any new water vapor was created from a liquid source (it takes far less energy to evaporate a partially condensed body of water than a completely condensed form that requires all that extra energy to induce the phase change). You'd probably just see a clear sky in that situation and be oblivious to all the evaporating columns of water vapor. The clouds on the other hand would appear elsewhere after traveling to a colder location where they can condense more readily.

Compare this process to a volcano for instance. During an eruption the temperature at the peak of the volcano is so hot that you don't see rock solidifying at the peak (as it tries to solidify the extra heat would turn it back into lava in a heartbeat). It only forms rock after diffusing away from the heat source far enough that temperatures are low enough for condensation (the formation of the rock) to begin to take place. I assume that this sort of process occurs with cloud formation as well.

 

[rewebster]

Sorry, is that what you thought I was saying? No.

Cumulus clouds in particular form when warm, moist air rises in a column where the ground has been heated by the sun.

What I don't understand is how, if the column of air is rising, how it can not disrupt the stratified layers.

I get the feeling once in a while that you're looking at a 'cloud' as a 'thing'. It's a dynamic system. Think of that image of the atomic/hydrogen bomb with the 'mushroom' cloud ---there is only one shaft of air rising to create the effect (heat rising/lower density/vacuum pulling). In a 'cloud' the shaft/hole where the air is rising may not be 'visible' as a 'hole'. The lower pressure causes condensation (like the clouds that form on the 'back' of mountains), so the 'hole' (very small area compared with the 'size' of the cloud) may be filled with condensate. As the air (and condensate) falls (cools) back to the strata where the temperature (maybe just a couple of degrees as with the fog)/pressure(air density-maybe just a couple of millibars)/humidity level (maybe just a percentage point or two) is at that critical level for maintaining the humidity level as a condensate (over a much, much broader area than the air shaft---like the smoke layering in a still room), the condensate (cloud) is either maintained with a possible 'flat' bottom (below that level, the condensation droplets evaporate), or is a totally 'puffy' cloud. If you think about it, those 'flat' bottom clouds are usually formed only on calmer, less windy days---when the 'air' can form 'strata'--like laminar flow. Think of some of those Navier-Stokes fluid dynamic photos.

 

[turbo]

Good explanation, rewebster. I was remiss in not always carrying a camera while climbing mountains, because there were times when I was just about at the level where the clouds bottomed out and the effect was impressive.

On one such climb, we were met at the summit of Mt. Katahdin by a ranger who was warning people to get to lower elevations. It was mid-day, and the sun had warmed the ground enough to start some updrafts. Some clouds were boiling pretty vigorously, and it looked like we'd get a thunderstorm. Since we were experienced and promised not to dawdle on the peak, he let us cross the Knife Edge and descend by our planned route instead of heading back the way we had ascended.

 

[rewebster]

thanks--I've had a couple days to think about it, to now, since the thread began to get it into better 'wording'.

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funny thing about cameras, isn't it----they're like the 'written' word

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or a good painting (on 'that' different level)