Cloud

See also: Cloud (homonymy)

A cloud is made of a whole of water droplets (or crystals of ice) suspended in the air. The aspect of the cloud depends on the light which it receives, of nature, dimension, the number and the distribution of the particles which constitute it. The water droplets of a cloud come from the condensation of the steam contained in the air. The maximum quantity of steam (invisible gas) is function of the temperature of the air. The hotter the air is, the more it can contain steam

One also speaks about clouds of Fumée S and clouds of Poussière, by analogy with the forms which take the atmospheric clouds (cluster, filaments, volutes).

Always by analogy, one speaks about clouds of Sauterelle S (displacement of great numbers of Criquet S) and of groups of dots (regrouping of points on mathematical diagrams)).

Formation of the clouds

The formation of clouds results from the cooling of a volume of air until the Condensation of part of its steam. If the process of cooling occurs on the ground (by contact with a cold surface, for example), one attends the formation of Brouillard. In the free atmosphere, cooling generally occurs by rising, under the terms of the behavior of the Perfect gas S in an atmosphere Hydrostatique, according to which a gas cools spontaneously when the pressure drops. Conversely, the dissipation of the clouds occurs when a warming allows the droplets or the crystals of ice to evaporate. The clouds can also lose part of their mass in the form of Précipitation S, for example in the form of rain, hail or snow.

The Condensation of the steam, out of liquid water or ice, occurs initially around certain types of solid matter microparticles (Aérosol S), which one calls of the cores of condensation or congelation. It should be noted that the spontaneous congelation of liquid water in ice, in a very pure atmosphere, does not occur above -40 °C. Between 0 and -40 °C, the water drops remain in a metastable state (superfusion), which ceases as soon as they return in contact with a core of condensation (dust, crystal of ice, obstacle). When this phenomenon occurs on the ground, one witnesses freezing fogs.

Just after condensation or congelation, the particles are still very small. For particles of this size, the collisions and aggregation cannot be the principal factors of growth. It rather occurs a phenomenon known under the name of Effet Bergeron . This mechanism rests on the fact that the Pression partial of Saturation of the ice is lower than that of liquid water. This means that, in medium where coexist of the crystals of ice and the water droplets superfused, the ambient steam will condense in ice on the already existing crystals of ice, and which the water droplets will evaporate of as much. One sees thus that rising is doubly important in the formation of clouds and precipitation: initially like mechanism of cooling, and then as carrying liquid water droplets up to the level where they become superfused.

Rising can be due to the Convection, with the presence of mountainous grounds making obstacle with the air flow or factors of atmospheric dynamics, like the waves baroclines (also called frontal waves ).

Types of clouds

There existed at the 19th century a complicated classification of the clouds according to their appearance which made use of terms in Latin. This classification is not used any more in modern meteorology, although there remain remainders about it.

The modern nomenclature divides the clouds into two great types: convectifs and stratiform.

  • convectif rising: often vigorous and with abrupt release, produced clouds characterized by a high vertical extension, but a limited horizontal extension. These clouds are indicated génériquement by the term cumulus .

  • rising known as synoptic is the result of the processes of dynamics in stable atmosphere, in a flow laminated. This rising is gradual, producing cloud systems of a uniform texture, being able to cover thousands of square kilometers. These clouds are indicated génériquement by the term stratus . It happens sometimes that this gradual rising destabilizes the atmospheric layer, giving place to clouds convectifs overlapping in the stratiform cloud.

Classification

The two types of clouds (cumulus and stratus) are divided into four groups according to the height of their base (and not the altitude of the summit). One qualifies the high cloud of cirrus and the cloud of average altitude of altus . There does not exist prefix for the low clouds. This system was proposed in 1802 by Luke Howard.

High clouds (A) Family

It are formed with the top of 5000 meters in the cold area of troposphere. They are classified by using the prefix cirro- or Cirrus. At this altitude, water almost always freezes: the clouds are thus composed of crystals of ice.

The clouds in the family has are:

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