Steven A. Ackerman is an assistant professor of atmospheric and oceanic science at the University of Wisconsin-Madison. 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.
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. 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.