Paul A. Deck, assistant professor of chemistry at Virginia Polytechnic Institute and State University, gives a brief explanation:
"The earth's atmosphere--especially local parts of the atmosphere--is not at thermal equilibrium. The world turns, the sun rises and sets, and surface temperatures go up and down. There simply is not enough time to disperse evenly the effects of such huge thermal disturbances. The combination of all the disturbances (from temperature changes, ocean tides and so on) contributes to the variability that we observe over relatively short distances. The complexity of this problem underscores the pressing need for the National Weather Service to have the most powerful computational equipment and the finest minds in applied mathematics."
Steven A. Ackerman is an assistant professor of atmospheric and oceanic science at the University of Wisconsin-Madison. He offers a fuller answer:
"Most clouds form in rising air. As a volume, or parcel, of air rises, it expands and cools. In addition, the relative humidity of the rising air increases. As the parcel approaches the point of saturation, water vapor condenses to form tiny water droplets or ice particles, creating a cloud. Saturation occurs at a distinct altitude, which varies depending on the temperature and humidity structure of the atmosphere. Below this condensation level clouds do not form; this cutoff explains why cloud bases have a distinct appearance and are usually flat. Some clouds are more diffuse at their edges than others. How sharp the cloud edges appear depends on the type of cloud, the humidity of the surrounding clear air and how close you are to the cloud.
"In 1803, Luke Howard, who started his career as a pharmacist, developed the cloud classification system commonly used today. In this system, there are two fundamental cloud structures (layered or convective) and two basic compositions (water droplets or ice particles). The categories also include clouds that are precipitating. Stratus or layered clouds do have the appearance of fog. Cumulus clouds, the upper parts of which resemble cauliflower, are a type of convective cloud. These clouds form in rapidly rising air and are composed of many small water droplets. The small drops scatter light effectively and make the cloud appear bright. (Likewise, if you smash up glass and separate the small pieces from the large, the grouping of small pieces will appear brighter.)
"While the cumulus clouds form, there is little mixing between the cloudy air and the surrounding, cloud-free environment. As a result, the cloud's boundaries appear sharp. In contrast is the cirrus uncinus, a type of ice cloud that resembles a comma. These clouds appear diffuse near their bases, where the number of ice crystals decreases as they fall and evaporate into the dry air below. The cloudy air mixes with the clear air, giving the diffuse appearance. Thus, how sharp a cloud boundary appears is a function of how much the cloud air mixes with the clear air environment.
"If the environmental air has a high relative humidity, just a little mixing will cause the air to reach saturation, producing a relatively diffuse boundary. If the surrounding air has a low humidity, the water droplets or ice crystals that make up the cloud quickly evaporate as the cloudy air mixes with its surroundings; this results in the cloud maintaining a sharp boundary.
"Proximity also has a big effect on how well defined a cloud appears. Have you ever painted a line on a wall? Up close the edges of the line may look wavy and interrupted. Step away, and the line looks sharp and crisp. The next time you have an opportunity to fly, observe the clouds as you pass through them. You will notice that the edges of a cloud are often diffuse. Just try to stay out of the cumulonimbus cloud, whose distinctive boundaries could give your airplane a sharp bounce."
Harvey Wichman of Claremont McKenna College suggests this way to think about the question: