"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:
"A good analogy for cloud formation is the development of bubbles of steam on the bottom of a kettle. Some spots are slightly hotter than others; it is at these locations that the water is turned to vapor. When a bubble gets large enough, the water's surface tension can no longer hold it, and so it rises. Fluids having different densities behave quite independently. The bubble stays a bubble all the way to the top where it breaks free as steam.
"So, too, with clouds. A spot on the earth's surface gets hotter than the surrounding area. An example would be the black, flat tarred roof of a large building or a vacant parking lot. The air above it heats up and forms a bubble of hot air, which is less dense than the surrounding air. When the surface tension can no longer hold it, the bubble breaks free and rises. This is why soaring birds such as hawks and eagles are always circling--they sense an updraft and keep turning to stay inside the bubble of rising air. The hot air ascends until it reaches an altitude where the temperature is cool enough to condense the water vapor contained in the air bubble into visible droplets. The visible droplets become a cloud, and that altitude (temperature) at which it forms is called the condensation level.
"There can be some mixing of clouds on windy days, but in general the air mass at the cloud level is moving quite steadily. And again, fluids of slightly different densities do not mix well. This tendency not to mix accounts for one of the most familiar types of weather systems. When a cold front (a mass of cool, dense air) bumps into a warm air mass, it runs underneath the warm air mass and pushes all the warm air up. When that warm air reaches the condensation level, you get a solid cloud mass and rainy weather."
Jeffrey S. Peake is an assistant professor of geography at the University of Nebraska at Omaha. He adds:
"Clouds sometime form more or less randomly (especially on summer days), but most of the time they do appear to be organized either in clumps or in one part of the sky. The reason for this is that most clouds are associated with uplift, and the causes (fronts, mountains, upper air troughs, atmospheric waves) are themselves not randomly distributed; therefore, the clouds are going to appear organized. Also, on a summer day, when clouds result from convectional heating from below, the formation of one cloud and the resultant release of latent heat tend to lower the pressure of the air immediately around the cloud and encourages additional air to rise upward, expanding the cloud in one part of the sky.
"There are many different types of fogs; they are defined primarily in terms of their method of formation. All, with the exception of ice fog, are made up of the same material: small water droplets one to 20 microns in diameter. Most fogs tend to be found under conditions of light wind. At times of stronger winds, the air at the surface (which is saturated) tends to be mixed with drier air from aloft, and the fog tends to break up or evaporate."