Table 1-1.—WMO Cloud Classification by Cloud Etnge, Form, Genus, Species, and Variety ETAGE FORM GENUS SPECIES VARIETY Cumulifrom cumulus humilis mediocris congestus fractus cumulonim capillatus bus calvus LOW-ETAGE Stratiform stratocumulus floccus opacus castellanus translucidus stratiformis undulatus lenticularis radiatus perlucidus stratus nebulosus opacus fractus translucidus undulatus nimbostratus altostratus opacus translucidus undulatus radiatus duplicatus MID-ETAGE Cumuliform altocumulus castellanus opacus floccus translucidus stratiformis undulatus lenticularis radiatus  perlucidus duplicatus lacunosus Cirriform cirrus uncinus radiatus spissatus duplicatus floccus intortus castellanus vertebratus HIGH-ETAGE Stratiform cirrostratus nebulosus duplicatus fibratus Cumuliform cirrocumulus stratiformis undulatus floccus duplicatus castellanus lacunosus lenticularis 6,500  to  25,000  feet  and  high-etage  from  20,000  to 60,000  feet). The low-etage cloud genera may be cumuliform, such as the cumulus or cumulonimbus (identified by their size and extent of development); stratiform, such as the stratus; or have mixed characteristics, such as the stratocumulus. The mid-etage cloud genera are mostly identified with the prefix alto. The mid-etage contains the cumuliform clouds, such as altocumulus, and the stratiform clouds, such as altostratus and nimbostratus. The high-etage cloud genera contain the prefix cirro. Cumuliform   clouds   in   this   etage   are   called cirrocumulus,  while  stratiform  clouds  are  called cirrostratus. Another form of cloud found only in the high-etage is the cirriform clouds that are the normally thin, wispy, or hairlike ice-crystal clouds that can be defined as neither cumuliform nor stratiform, but are simply called cirrus clouds. Cloud  Species Besides the identification of clouds by genera, most cloud  forms  may  be  further  identified  by  cloud species. The species identifies the size, shape, or form of the elements within a cloud layer. Table 1-1 lists the cloud 1-7