Why do clouds have flat bottoms?

  1. I know clouds are just condensed water vapor forming tiny droplets. Why do they have flat bottoms and all at the same plane? Is their a air density threshold that clouds form at? Thanks simply curious.
     
  2. jcsd
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  3. davenn

    davenn 3,585
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    because that's the condensation level.
    Different days, with other humidity and temperature levels and that base can be lower or higher

    Dewpoint is commonly used to calculate the height at which the cloudbase will form

    cheers
    Dave
     
  4. Evo

    Staff: Mentor

    Not all clouds have flat bottoms, this only applies to some clouds in some situations.
     
  5. Ok we'll for those that do why?
     
  6. I think I know what you mean by condensation level but what causes it to be there? Is it because the air density is low enough there for water to condense? I can't see the temperature dramatically dropping off there so it must be density of medium
     
  7. jim mcnamara

    jim mcnamara 1,491
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  8. davenn

    davenn 3,585
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    read up on dewpoint, it will answer most of your questions

    jim also has a good set of links there as well :)

    Dave
     
  9. edward

    edward 1,003
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  10. davenn

    davenn 3,585
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    [​IMG]

    yup mammatus ... very nice pic, have seen some good ones myself during my years of stormchasing. But there's a really good reason why they form like that during thunderstorms.

    I wonder if you know the reason ? :)

    You will normally see them on the underside of the anvil ahead of the main updraft and occasionally to the rear if its a fast moving storm. But you will still notice that the base of the stormcell is relatively flat. These are much higher in altitude ( 10 - 25,000 ft) compared to the base that may be only a couple of 1000 ft.

    Sometimes there can be strong updraft from almost ground level feeding into that base.

    If I get a chance when I get home tonite I will post a pic of a storm like that that I photo'ed a couple of years ago

    Dave
     

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  11. russ_watters

    Staff: Mentor

    Due to adiabatic expansion (google that), any homogeneous volume of gas will cool as it rises or if it is large enough have a temperature gradient. So as you increase altitude and keep absolute humidity (humidity ratio) constant, relative humidity rises. Reach a certain altitude and relative humidity will equal 100%. Above that, clouds form.
     
  12. Thank you all I will read the resources when I am able to.
     
  13. jim hardy

    jim hardy 4,673
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    What Russ said.......

    An old timey weatherman will estimate the dewpoint and relative humidity by observing the height of those cloud bottoms. It's a skill they picked up from watching weather balloons in the old days.
    Dad was a weather forecaster ~1940-1970 and he explained the uniform cloud bottoms to me this way:

    In daytime air is heated by the ground so is warmest at surface. Also its pressure is highest on the ground, that's why your ears pop when going up or down a high hill in an automobile.
    Air that's been warmed at the ground rises by convection. Air is a gas that cools as its pressure is lowered so it cools as it rises. It cools by ~3.5 degrees F per thousand feet and that's called the "lapse rate".. http://en.wikipedia.org/wiki/Lapse_rate

    When the moist rising air has cooled to the point the water vapor in it condenses, you get a visible cloud . When the cloud bottoms are all flat and at same level it tells you the vertical mixing of air is pretty good that day.

    Knowing the lapse rate and the approximate height of the cloud bottoms, old weathermen can tell you by eye how much below ambient temperature it is to the dewpoint. They also have in their memory banks the tables of relative humidity vs dewpoint for various temperatures...

    If you pay attention, this time of year you'll notice cloud bottoms are way more regular than in wintertime. I watch the clouds in movies made in S Florida- after growing up there one can tell what time of year a scene was shot by the clouds . They tend to work there in winter.

    Maybe there's a genuine meteorologist in the house?

    old jim
     
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