Finally, there exists a group of clouds, rarely or occasionally observed, not included in the following classification. The definition of a cloud given above is therefore not applicable to all special clouds. 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. 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. 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 .