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Clouds on a still summers day generally look flat at the bottom and fluffy on top. Why? I was asked the question a couple of days ago and he ventured that it had something to do with the density gradient of air. Is he right? I suspect he is, but wanted a more informed opinion.

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    $\begingroup$ The density and temperature drop together, as @benjimin says. $\endgroup$ – Mike Dunlavey Sep 1 '16 at 20:58
  • $\begingroup$ A simpler, interesting question to start with is whether moist air (at the same temperature as its surroundings) floats or sinks. $\endgroup$ – Emilio Pisanty Sep 1 '16 at 21:19
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As hot moist air rises, there is a specific height where the gaseous water vapour begins to condense into a mist of tiny suspended liquid droplets.

There is not a specific limit to how far this misty air can be carried upward by air convection (producing billowy cloud tops), but if it falls below that specific height the droplets will sharply start evaporating away into invisibility (since only the non-gaseous, droplet form scatters white light).

The boundary is termed the lifted condensation level or dew point. At greater heights there is less air pressure (because there is less air column weighing down from above). This weakening pressure lets ascending parcels of air push-out or expand, which results in an expenditure of temperature (eventually reaching the point where the water molecules on average no longer have enough kinetic energy left to overcome the intermolecular attraction force). The pressure gradient is also the reason low-density parcels are buoyed upwards. The cloud-forming parcels have low-density because they are warmer than surrounding air (having a comparative surplus of kinetic energy to push out with); these parcels are warmer because of the greenhouse effect (which directly heats the surface while cooling the upper atmosphere) and moister because of evaporation of water (which had eventually separated from the air and fallen to concentrate at the surface).

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    $\begingroup$ +1 I was looking for lapse rate, the drop of temperature with altitude, in your first paragraph. But you did finally get to it in the last. $\endgroup$ – Mike Dunlavey Sep 1 '16 at 20:56

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