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Why do clouds stay up in the air?

  1. Apr 29, 2006 #1
    i got a question from my physics lecturer and would like to see your opinions.

    "Why do clouds stay up in the air? Isn't gravity acting on it?"
  2. jcsd
  3. Apr 29, 2006 #2


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    What do you think? Why does a log float on water?
  4. Apr 29, 2006 #3
    density. but clouds? Why doesn't it sink right down to earth nor float away to space? It is within Earth's atmosphere but seems unaffected by gravity.
  5. Apr 29, 2006 #4

    Physics Monkey

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    Hi whiteholes,

    Wood floats because it is less dense that water. You can check this assertion yourself. A higher density kind of wood will float lower in the water than a lower density kind of wood. A piece of wood will also float lower as it becomes saturated with water.

    However, clouds are not held up in this fashion. This is obvious since clouds are composed primarily of liquid water and ice which are both more dense than air.

    Since this sounds like a hw question, you should tell us what your ideas on the matter are before we can help you further. I have two suggestions. 1) Go out and look at some clouds, you can make several interesting observations. 2) Do some research.
  6. Apr 29, 2006 #5


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    While its true that density of the log allows it to float the word buoyancy was key and is also key to why clouds stay afloat. The problem is working out what makes a cloud bouyant. (should have used a different water floating example apologies for that if i misled you)
  7. Apr 29, 2006 #6
    From: http://www.sciam.com/askexpert_question.cfm?articleID=00057690-D38E-1C71-9EB7809EC588F2D7

    "Clouds are composed primarily of small water droplets and, if it's cold enough, ice crystals. The vast majority of clouds you see contain droplets and/or crystals that are too small to have any appreciable fall velocity. So the particles continue to float with the surrounding air. For an analogy closer to the ground, think of tiny dust particles that, when viewed against a shaft of sunlight, appear to float in the air.
    Indeed, the distance from the center of a typical water droplet to its edge--its radius--ranges from a few microns (thousandths of a millimeter) to a few tens of microns (ice crystals are often a bit larger). And the speed with which any object falls is related to its mass and surface area--which is why a feather falls more slowly than a pebble of the same weight. For particles that are roughly spherical, mass is proportional to the radius cubed (r3); the downward-facing surface area of such a particle is proportional to the radius squared (r2). Thus, as a tiny water droplet grows, its mass becomes more important than its shape and the droplet falls faster. Even a large droplet having a radius of 100 microns has a fall velocity of only about 27 centimeters per second (cm/s). And because ice crystals have more irregular shapes, their fall velocities are relatively smaller.

    Upward vertical motions, or updrafts, in the atmosphere also contribute to the floating appearance of clouds by offsetting the small fall velocities of their constituent particles..."
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