Q: Why do most clouds have defined edges (or at least appear to have)?
A: The clouds with the most clearly defined edges are the billowing white cumulus clouds rising into a clear sky. These are formed by the condensation of water vapour as air expands and cools. This does not occur in a homogeneous layer but in a discrete parcel or column of warmer and less dense air rising from below through colder air above.
Although the cloud is cooled by expansion as it ascends, it continues to rise as long as its temperature is higher than that of the air surrounding it. Only when the air forming the cloud reaches a level at which the surrounding temperature is the same does it mix with that air and become fuzzy. Until then there is a sharp boundary between the different air masses.
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A: Clouds may appear to be static objects. In fact they are dynamic and there is usually a convection current of air rising up into a cloud. As this air rises, it expands and cools. At a given altitude, condensation occurs, thus defining a sharp lower boundary to the cloud. The sharpness of the upper surface depends on how fast the air is rising and the extent to which turbulence mixes this damp air with the surrounding, drier air. When the convection current ceases, the cloud will tend to become diffuse and lose its well-defined edge.
Glider pilots use the appearance of the clouds above them in order to recognise where there are thermal currents which will enable their planes to gain altitude.