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Why do clouds have flat bottoms?

  1. Jul 20, 2007 #1

    DaveC426913

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    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)
     

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  3. Jul 21, 2007 #2
    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
     
  4. Jul 21, 2007 #3

    FredGarvin

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    I think the winds aloft have a larger effect on the situation which dictates that separation.
     
  5. Jul 25, 2007 #4

    DaveC426913

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    Last edited: Jul 25, 2007
  6. Jul 25, 2007 #5

    DaveC426913

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    Please elaborate. We're talking about cumulus clouds - not in any particularly windy situation.
     
  7. Jul 25, 2007 #6

    FredGarvin

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    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).
     
    Last edited: Jul 25, 2007
  8. Jul 26, 2007 #7
    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 :wink: (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 any one portion of the cloud.

    Is my rationalization for this at least :smile:
     
    Last edited: Jul 26, 2007
  9. Jul 26, 2007 #8
    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.
     
  10. Jul 26, 2007 #9

    DaveC426913

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    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.
     
  11. Jul 26, 2007 #10
    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).

    -------------------------------

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

    -----------------------------

    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.
     
    Last edited: Jul 26, 2007
  12. Jul 26, 2007 #11

    berkeman

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    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.)
     
  13. Jul 26, 2007 #12

    turbo

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    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.
     
    Last edited: Jul 26, 2007
  14. Jul 29, 2007 #13
    I was thinking the difference between a visible condensation and not visible may be only a couple of degrees difference.
     
  15. Jul 29, 2007 #14

    DaveC426913

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    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.
     
  16. Jul 29, 2007 #15

    DaveC426913

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    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.
     
    Last edited: Jul 29, 2007
  17. Jul 29, 2007 #16

    turbo

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    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.
     
  18. Jul 29, 2007 #17
    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.
     
  19. Jul 29, 2007 #18
    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.
     
  20. Jul 29, 2007 #19
    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.
    ------------------------------------------------------------------------------------------------

    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 occuring 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 occuring 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.
     
  21. Jul 29, 2007 #20
    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'?

    -------------------

    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).
     
    Last edited: Jul 29, 2007
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