In Meteorology, the term precipitation indicates crystals of ice or water droplets which, having been subjected to processes of condensation and aggregation inside the Nuage S, became too heavy to remain in suspension in the atmosphere and fall on the ground.

Types

The frequency and the nature of precipitations in a given geographical area are important characteristics of sound Climat. They contribute in a way essential with the fertility and the habitability of the moderate or tropical zones; in the polar zones, they help with the maintenance of the icecaps. Precipitation can take the following forms (sometimes mixed):

Liquide:

  • Rain
  • Drizzle
  • Rain verglaçante/(Drizzle verglaçante)
Solide:

In the reports/ratios of meteorological observation, the type of precipitation is accompanied by an indication of intensity (light, moderate, or strong), as well as measurement of the visibility through precipitation. The reports/ratios of observations indicate also the temporal character of precipitation: if its intensity varies quickly and is accompanied by breaks, precipitation is called a downpour .

Mechanisms of formation of precipitations

Condensation

The drops start to be generally formed in air above the freezing point when the raised air becomes supersaturated compared to the surrounding temperature. For that one needs however cores of Condensation, dust or grain of salt, on which the steam settles. The chemical solution obtained lowers the surface stress necessary to form a drop.

There is initially formation of very fine drops which give the cloud. As these drops go up, they pass under the point of Congélation but will remain superfused if it there not presence of cores of congelation. The latter are much less available than the cores of condensation.

As they increase diameter, a second process must intervene, coalescence, in order to reach a sufficient diameter to form drops of rain. Indeed, the droplets formed by condensation reach only a few tens of microns in the time necessary usually to give rain.

Coalescence

The Coalescence is the amalgamation of two or several droplets by collision to form some larger. The droplets growing at different speeds, according to the steam concentration, they will move at a different speed which is connected to their diameter and the ascending current. Largest moving more slowly will capture smallest while going up then when they cannot be constant any more by the current, they will go down again and continue their growth in the same way.

Bergeron effect

The Effect Bergeron, of its discoverer Tor Bergeron, is most effective of the formation processes of the drops of rain or snow. When crystals of ice are formed finally by congelation of droplets, the latter have a saturated vapor pressure less than that of the surrounding droplets. The droplets thus evaporate and the steam will settle on the crystals.

These crystals will also end up falling and will enter in coalescence with others to form snowflakes. They will also capture by coalescence of the drops what them Givre ruffle if the temperature is under zero Centigrade. If the temperature of the atmosphere is everywhere under zero above the ground, there will be snow. On the other hand, if the level of congelation is not on the ground or if there are layers above the zero in altitude, there is a variety of the types of precipitations: rain, rain verglaçante, grésil, etc

Modes of formation of precipitations

In addition to their form, one distinguishes two types of precipitations according to their mechanism of formation: 1) Stratiform precipitations which come from slow rising and with large scales of the moisture which condense uniformly. Like example:

* the precipitations synoptic , caused by the depressions of the average latitudes.
* the coastal precipitations which take place near the littorals and have due the rising of the humid air coming from the ocean by the asperities of the continent.
* the orographical precipitations where the relief forces the masses of air to rise: the slopes with the wind are then very rainy, the slopes under the wind are drier. The Foehn is an illustration of this phenomenon.

2) The precipitations of Convection result from the abrupt rise in masses in air in charge of moisture, by the Poussée of Archimedes, because of the instability of the air. Like example:

* the Storm S and isolated or organized Downpour S.
* the precipitations of the zones of convergence where the storms develop because the unstable and wet air can concentrate and convecter with the diurnal warming. For example, one finds that in the Zone of intertropical convergence and in front of the cold faces.
* the cyclonic precipitations where generalized convectives precipitations are generated by the organization of the tropical cyclones.

These two types of precipitations are not mutually exclusive however. Indeed, there can be unstable zones in a mass of Pluie or stratiform Neige what will give places to stronger downpours in these sectors. In the same way, one can obtain by rising of the unstable conditions. For example, of the winds going up a slope can make exceed the free Niveau of convection to the piece of air raised and give a storm.

Sowing

See also: Sowing of the clouds

It is possible artificially to start precipitations while dispersing on a cloud of the dust of Iodure of money. This is equivalent to introduce cores of congelation what accelerates the crystal formation of ice and brings the effect Bergeron quoted previously. It is a means of limiting the size of also hail by creating more competition for the steam available.

The technique is very effective in laboratory, but in nature, its effectiveness is limited according to Jean-Louis Brenguier, chief of the group of experimental meteorology to Weather-France, unless one spends of the very important sums to follow the cloud during all his life. However, that does not prevent the Agence of atmospheric technologies Russian from using this technique to disperse clouds with the top of Moscow at the time of certain festivals and official visits or to limit the quantity of snow.

Deposition

Two types of depositions can be collected in a Pluviomètre but form seldom nothing any more but one trace of accumulation:

  • the Brouillard is a Nuage with the ground made of droplet S.
  • the Rosée and the Givre is condensation on objects of the excess of Steam of the atmosphere when the temperature goes down under the Dewpoint (saturation of more than 100%).

In these two cases one cannot speak about precipitations since the droplets are formed or settle on the ground or of the objects without falling.

Chemistry

In a more general way, in Chemistry and Metallurgy, the precipitation is the formation of a heterogeneous phase within an other phase, for example the formation of a solid crystal in a liquid, or the formation of a crystal of different composition in a Alliage.

See also: Precipitate

In chemistry, a difference is made in precipitation and crystallization. Crystallization is a controlled process consisting in making crystallize a product slowly, in order to purify it or to confer the desired properties to him (size of particles, morphism…). Precipitation is a fast process of crystallization (by addition of a solvent making the product insoluble, by modifying the pH,…). The product traps impurities or solvent in the crystals, those are small sizes.

Seek

2007. According to Xuebin Zhang of Climate Research Division (Toronto, Canada), the man of the developed countries is the direct person in charge of 50 to 85% of the increase in precipitations occurring with the moderate latitudes (40-70° NR). He thus analyzed the Pluviométrie by bands of Latitude between 1925 and 1999. The average increased by 62 mm in the average latitudes of the northern hemisphere (the United States, Northern Europe, Russia) against a reduction of 98 mm on average for the tropical regions of the northern hemisphere (the Sahel, Shara). The share of the man was confronted with various models (with and without gas emissions with greenhouse effect and sulphated grounds) to arrive at the conclusion quoted above. The most serious remainder which the evolution of precipitations evolved/moved more quickly than envisaged, just like the rise of the sea level. Current projections underestimate the climatic risks thus in the long run.

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