Finally, there exists a group of clouds, rarely or occasionally observed, not included in the following classification. Some of these "special clouds" consist for the greater part or in their entirety of non-aqueous liquid or solid particles. The definition of a cloud given above is therefore not applicable to all special clouds. Included in the "special" clouds are the nacreous clouds which by day resemble pale cirrus, but after sunset, are characterized by brilliant colors. They occur at altitudes between 21 and 30 kilometers (70,000 and 100,000 feet). The physical constitution of nacreous clouds is still unknown. However, the simultaneous occurrence of various diffraction colors in more or less irregular patterns indicates the presence of minute particles, possibly spherical ice particles. Noctilucent clouds also resemble thin cirrus, but usually with a bluish or silvery, or sometimes orange to red color; they stand out against the dark sky. Measurements have shown that their altitude ranges from 75 to 90 kilometers (250,000 to 300,000 feet). Their physical composition is also unknown, but they are believed to be composed of fine, cosmic dust particles possibly with a thin, outer layer of ice. Noctilucent clouds become visible after sunset. They are at first greyish, then more brilliant and, as time advances, they appear bluish white like tarnished silver. Other special clouds include clouds from fires produced by the fine combustion products. These may appear as dark, cumulus or cumulonimbus clouds but usually are rapidly dispersed and carried great distances by the wind, spreading to resemble thin, stratiform veils. Clouds from volcanic eruptions, explosions, and industrial activities are also considered in the "special cloud" category.

Genera The classification of clouds is essentially based on ten main groups, called genera, which are mutually exclusive. These genera are classified as High Clouds, Low Clouds, Middle Clouds and Clouds of great vertical extent. Species. Most of the genera are subdivided into species. This subdivision is based on the shape of the clouds or their internal structure. A cloud, observed in the sky, belonging to a certain genus, may bear the name of one species only. Varieties. Clouds may exhibit special characteristics which determine their variety. These characteristics are related to the different arrangements of the macroscopic elements of the clouds and their greater or lesser degree of transparency. A given variety may be common to several genera. Furthermore, the same cloud may show characteristics pertaining to more than one variety. If this is the case, all the appropriate variety names are included in the name of the cloud.

TABLE OF CLASSIFICATION OF CLOUDS GENERA SPECIES VARIETY Cirrus fibratus, uncinus, spissatus castellanus, floccus intortus, radiatus, vertebratus duplicatus Cirrocumulus stratiformis, lenticularis castellanus, floccus undulatus lacunosus Cirrostratus fibratus nebulosus duplicatus undulatus Altocumulus stratiformis lenticularis castellanus floccus translucidus, perlucidus opacus, duplicatus undulatus, radiatus lacunosus Altostratus - - - - - - translucidus, opacus duplicatus, undulatus radiatus

The base of the following cloud genera is normally found in the étage indicated: Cirrus, Cirrocumulus and Cirrostratus for the high étage (high level clouds); Altocumulus and Altostratus for the middle étage (middle level clouds); Stratocumulus, Stratus and Nimbostratus, for the low étage (low level clouds). With regard to these cloud genera, (altostratus, nimbostratus, cumulus and cumulonimbus) the following remarks may be made: The base of Altostratus is usually found in the middle étage, but the cloud may extend into the high étage;

TABLE OF CLASSIFICATION OF CLOUDS, (continued) GENERA SPECIES VARIETY Nimbostratus - - - - - - - - - - - - Stratocumulus stratiformis lenticularis castellanus translucidus perlucidus opacus duplicatus undulatus radiatus lacunosus Stratus nebulosus fractus opacus translucidus undulatus Cumulus humulis mediocris congestus fractus radiatus Cumulonimbus calvus capillatus - - - - - -

The base of Nimbostratus is usually found in the low étage, but it almost invariably extends into at least the middle étage;

Cumulus and Cumulonimbus usually have their bases in the low étage, but their vertical extent is often so great that their tops may reach into the middle and high étages. They are thus classified as convective clouds of great vertical extent.

When the height of a particular cloud is known, the concept of étages may be of some help to the observer in identifying this cloud. Its genus can then be determined by making a choice from among the genera normally encountered in the étage corresponding to its height.

Definitions of the various species and varieties is found in the cloud glossary.