Clouds continuously evolve and appear in an infinite variety of forms. However, there is a limited number of characteristic forms frequently observed all over the world, into which clouds can be broadly grouped in a classification scheme. The scheme uses genera, species and varieties. This is similar to the systems used in the classification of plants or animals, and similarly uses Latin names.
There are some intermediate or transitional forms of clouds that, although observed fairly frequently, are not described in the classification scheme. The transitional forms are of little interest; they are less stable and in appearance are not very different from the definitions of the characteristic forms.
There also two additional cloud classifications: Special clouds and Upper atmospheric clouds. These tend to be only rarely or occasionally observed and, in some cases, only in certain parts of the world.