Why do clouds form in lumps?

What forces drive the water vapor to form the shapes every child draws? Why isn't water vapor forming an uniform mist layer consistently (which it some times does)?

I have two general ideas how this process can self-sustain itself - the shade of a forming cloud reduces sunlight to raising and condensing vapor - the shade of the forming cloud causes varying speeds for vapor raising from the shade and from outside the shade on the ground, causing varying rate of condensation .

Neither of these, however, can tell me how the formation of a lump happened in the first place

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Not all clouds form in lump. I recently watched a beautiful ted talk about beautiful clouds:Cloudy with a chance of joy – Ali Jul 18 '13 at 17:38
Related: Formation: how the air becomes saturated and Cohesion and dissolution and elsewhere on that page – Keep these mind Jul 18 '13 at 17:39
@Gugg I think the wikipedia article you mentioned is incredible. One should just copy and past the whole thing here $:)$ – Ali Jul 18 '13 at 17:43

Cumulous clouds form when the atmosphere gets unstable. For incompressible liquids, "unstable" means a high density liquid "water" is balancing precariously on a low density liquid "oil" (ignore surface tension). If a little bit of the lower density liquid rises, it will act like a "bubble", and will be pushed upward. The updraft will cause a positive feedback that pulls up more "oil". Eventually, large plumes of "oil" are rising as the "water" falls to take it's place. If you colored the "oil" white, you would see a cumulous cloud shape.

For air, the definition of "instability" is more complicated because air is compressible. "unstable" means that if you take a "parcel" of air and lift it up a little bit (Nature does this so quickly that little heat transfer happens), it will be less dense than the surrounding air. Thus it acts like the liquid case. Most instability is caused by solar radiation heating the ground and reducing the density of the low-level air.

As it rises, the cooling from the decompression (altitude) causes water to condense. The "background" air that the rising air is moving into is mainly displaced to the side and downward (to replace the air near ground level), so it remains clear. Thus, the billiowing cloud surface is the interface between the rising plume and the "background" air. The clear air in the space between clouds is in fact descending to replace the low-level air.

There is another reinforcing effect: As water condenses, it releases heat which slows down the cooling effect of decompression. This effect is stronger than the reduction of water volume as it changes from a gas to a liquid.

What about uniform mists? In this case, air rises and condenses uniformly. Something has to replace the air as it rises, and that something is a "front", a layer of air that is moving in horizontally to displace the air upward like a wedge.

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For readers desiring further info, this instability is known as the Rayleigh–Taylor instability. Other instabilities can also play a role, for instance the Kelvin–Helmholtz instability as seen in this picture. – Chris White Jul 22 '13 at 22:12

I would guess that you're thinking about cumulus clouds. These are your stereotypical woolly clouds that we all used to draw as children. Cumulus clouds are formed by water condensation from rising air currents, and because the rising air currents are turbulent they get their characteristic fluffy tumbled shape.

There are many different types of cloud, some of which do form uniform layers e.g. stratus clouds. If you're interested in finding out more about clouds I strongly recommend The Cloudspotter's Guide by Gavin Pretor-Pinney. This describes in detail how the different types of clouds form and what controls their shape, and it's written at a popular level and in an engaging style.

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