Air

Different shapes

The first scientific classification of clouds was made by an English chemist,  L. Howard, in 1803, and the classification system that he proposed is, with some modifications, still used today.  It is based on two principal groups, divided according to the development (vertical and horizontal) and three types: cirrus, cumulus and stratus clouds. The different combinations of groups and types lead to the different cloud formations.
Different shapes
Stratus  (St) clouds, are widely extended horizontal sheets in the lower elevations, and are generally grey. They seem to cover the sky in a uniform manner at various levels. The thickness of the stratus cloud can vary from 15 to  800 km. Their regular shape and surfaces are due to the fact that within these clouds there are no internal convective phenomena, which give clouds  bizarre shapes. Also fog banks belong to this category. These clouds can bring  drizzles or light snow.
Cumulus (Cu) clouds, are dense clouds with a marked outline. They have a typical flat base that starts to form at altitudes around 1,800 m, that indicates the elevation of the level of condensation (vedi Immagine: nubicumuli01). The upper part that can reach considerable heights instead is  swollen and irregular due to the internal convectional movement. The base is generally dark, while the higher parts of the cloud are white and brilliant. Cumulus clouds are generally a sign of good weather conditions.
Cirrus (Ci) clouds are the typical high altitude clouds, they are formed by ice crystals  which characterize their evanescent fibrous  shape. Often these clouds provoke precipitations, however, since they are situated at very high altitudes, the precipitation generally evaporates before reaching the ground. Due to the effect of refraction of light in the ice crystals, cirrus clouds  form iridescent halos around the Sun or the Moon. Often these are indication of a warm front or a storm system. Nimbus (NB) clouds are dense clouds with a strong vertical development, with towering forms. The lower part of these clouds is dark and jagged, the upper part is generally swollen and continuously evolving. These clouds are dark and gloomy, they always bring precipitation and often they move at great speeds along the bad weather front.
The main types of clouds can be combined together thus obtaining more complex forms:
cumulonimbus (Cb) clouds that are dense and  have a strong vertical development. These clouds bring precipitation and showers in the form of storms. When the upper part reaches the limit with the stratosphere, the top of the cumulonimbus clouds becomes flattened, thus taking on the typical anvil shape;

• stratocumulus (Sc) clouds, in banks, formed by rounded grey coloured masses, with darker parts, generally produce light rain;
• nimbostratus (Ns) clouds, are grey, dark, not clearly defined shapes, due to the precipitation that accompanies them;
• altocumulus (Ac) clouds consist of white rounded masses that are often arranged in a regular , tidy manner, known as “cielo a pecorelle”  sheep-shaped clouds in the sky;
• altostratus (As) clouds are grey or bluish extended clouds, with a fibrous, striped appearance, and are formed by drops of water and ice crystals that can give rise to continuous precipitation;
• cirrocumulus (Cc) clouds appear in thin transparent banks and are sometimes arranged in striped bands;
• cirrostratus (Cs) clouds are shaped like whitish fibrous veils.