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Clouds and their form

  1. Sep 22, 2006 #1
    It's a partly cloudy day here and I was checking out the clouds as they flew by. Noticing the shapes of the clouds, I was wondering how they held their shape instead of being just a haze of water vapor when blown. When a pot of water boils the steam keeps its billowy cloud shape for a small distance and then just turns into a shapeless nothing. Is it that they are dense enough to resist being torn apart by the wind and pushed along en masse?

    ...the answer my friend, is...
  2. jcsd
  3. Sep 22, 2006 #2
    Last edited by a moderator: Apr 22, 2017
  4. Sep 22, 2006 #3


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    Staff: Mentor

    Look closer next time you get a chance. Perhaps even with binoculars. Some clouds are well-defined, but some are wispy. The ones that are well-defined are often well-defined because they are growing.

    And clouds can get pulled apart by shear, but only pulled apart: Shear won't cause clouds to dissipate because the clouds ride with the wnd.
  5. Sep 24, 2006 #4
    Interesting question. You don't find the same cohesiveness in smoke. As smoke rises it dissipates rapidly. I am not a physicist, but it is my understanding that because water molecules are dipolar, water droplets are slightly polarized and act like small magnets, attracting each other. However, droplets are small (commonly only a few tens of microns in diameter, a property largely controlled by the size of cloud condensation nuclei, which is another complex topic) and commonly too far from each other to coalesce due to that attraction. They are small enough that despite being heavier than still air, even the slightest breeze will keep them aloft. In fact, there may need to be no breeze at all, with cloud droplets suspended in Brownian motion. (This also applies to ice crystals in clouds.) It is the proximity of water droplets to each other that allows you to see clouds. As light impinges upon a cloud, it is refracted and reflected in all directions, rendering what is mostly composed of air, opaque (water droplets comprise only about a millionth of the volume of the cloud). As clouds are constantly changing shape due to wind shear, the cohesiveness is largely maintained due to the resistance of this shearing by the dipolar property of water.

    As far as the pot of water is concerned, turbulent flow is created by the upwelling of steam from the pot. The steam condenses rapidly a few millimeters from the pot forming water droplets. Water droplets rapidly move upward due to the turbulent updraft. However, what seems like dissipation, may be due more to the fact that there are too few water droplets to cause opaqueness. Maybe there is just enough dissipation to cause transparency, but maybe less dissipation from the pot of boiling water than you think.

    A few useful websites:
    http://www.sundog.clara.co.uk/droplets/clouds.htm [Broken]
    http://www.espere.net/Unitedkingdom/water/uk_cloudform.htm [Broken]
    http://en.wikipedia.org/wiki/Water_(molecule [Broken])
    Last edited by a moderator: May 2, 2017
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