Face (meteorology)

See also: Face

A weather face is a surface of wide Discontinuité, which separates two masses from air having Propriété S different physics (Ex: Temperature, moisture, pressure).

History of the analysis of the faces

See also: Meteorology

The history of the modern analysis of the charts of surface in Météorologie begins at the 19th century. The invention of the Télégraphe in 1845 made it possible to diffuse observations taken at places very distant from/to each other within a reasonable delay to be able to use them at end of forecast. Both in America and in Europe, of the networks of weather observation stations develops.

The Smithsonian Institution , under Joseph Henry, established the first network of observation stations starting from the Years 1840. As of 1849, the first weather charts of surface are traced by using the 150 stations already available. This network will cover the major part of the center and the east of the the United States in 1860. During this time, the November 14th 1854, a violent storm causes the shipwreck of 41 French ships in Black Sea, during the Crimean War. This storm had crossed all Western Europe, but nobody was able to announce, to even prevent danger. Vis-a-vis this report, Urbain the Glassmaker, director of the Observatory of Paris, decides to set up a vast network of weather stations covering the whole of Europe.

Robert FitzRoy also uses the telegraph for colliger the daily weather data coming from all the England and to trace its first charts synoptic S. By using the variation of these charts in time, it makes the first forecasts which it will start to publish in the newspaper The Times in 1860. The same movement spreads in all the British Empire. Thus, in 1839, a weather observatory is opened with Toronto and various observatories are open through the British colonies of the Canada by the enthusiastic ones or schools. In 1871, appropriations of 5.000 CAN are allocated with the Minister for the Navy and Fishings in order to set up a network of weather data acquisition of which the ultimate goal is to produce warnings of storms. In Australia, it is in 1877 that the first charts appeared in the newspapers. The Observatory meteorology of Tokyo, the ancestor of the weather service Japan board, makes the same thing starting from 1883. The charts as of these pioneers were generally produced once per day starting from the data colligées at one hour fixes the morning. For example, the American data were taken at 07:30, solar time, and were telegraphed with the Smithsonian . Only the data of pressure were pointed and the lines of equal pressure, or isobar S, was traced. One identified thus the depression S and the Anticyclone S. Like the data were snuff at the solar hour, one obtained a distortion of the owners of pressure. It been necessary to await the appearance of the concept of Time zone in 1879 and of the universal Time coordinated to cure this problem. However, the acceptance of these two concepts took time and it is only at the end of the 19th century that one finds them on the majority of the sphere.

As of 1841, Elias Loomis is the first to suggest the presence of faces to explain the weather but it is only after the First World War, in 1919, that the Norwegian school of meteorology imposes this concept. It is thus only as from this time that the faces, separating the various masses from air recovering the planet and with which one associates the time, are analyzed. Once again, this innovation took a certain time to be spread and it is only after the Second world war that the National Weather Service American started to indicate them on its charts.

The charts were analyzed manually until in the Années 1970 when the introduction of the computers made it possible to develop automatic programmes of tracings of the data of the stations and isobars. Starting from the Years 1990, the addition of the data of the satellite , weather Radars with the data of surface and altitude and the improvement of the computer programs made it possible to trace by computer a first jet of the faces, but generally the Meteorologist S must still refine the analysis.

Nomenclature

Masses of air and faces

In the Norwegian model, there are five masses of air in the average latitudes of planet. The zones where these masses of air meet carry the name of face and it there of thus four. These divisions come from the atmospheric Circulation general and of the position of various the Courant-jet S. In general, each one of these faces is identified with the coldest mass of air, almost always located on its northern facet:

• the Arctic face 2 : separate the Arctic continental air (cA) (beyond 60 degrees north and south) from the Arctic maritime air (mA)
• the Arctic face: separate the Arctic maritime air (mA) from the polar continental air (cP) (between 40 and 60 degrees of latitude).
• the maritime face : separate the polar continental air from the air (CP) of that maritime polar (mP)
• the polar face : separate the polar maritime air (mP) from the tropical maritime air (mT)

These faces vary in latitude according to the season. Thus the Arctic face 2 fact seldom its appearance in the south of the Arctic islands before the end of the autumn in the northern Hemisphere whereas the Sun illuminates the Scandinavian areas less and less and that the air cools gradually. The differences between masses of maritime and continental air of the same name is primarily due to the contents in moisture whereas the Température S are rather similar there.

Other more local faces also have names:

• the Saharan face : separate the Mediterranean air from the air tropical dryness;
• the equatorial face : separate the tropical air from the equatorial air;
• the face of the trade winds : separate the tropical air maritime from the air tropical dryness of the the Sahara.

Type of faces

See also: hot Face, cold Face, occluded Face and trowal

Because of the air circulation along these faces, one distinguishes various types of faces:

• the hot face is the zone where the air of the hottest mass of air is moved towards that colder by the winds

• the cold face is the place where the mass of cold air moves towards that hotter
• a stationary face is the limit between vast masses of hot air and cold which are in contact between them without producing relative movements of a great range because the winds in each mass of air are parallel to the face (for example the oceanic faces )
• a occluded face develops when a weather system intensify and that its cold face accelerates so that it catches up with the hot face. When the cold face reaches the hot face, the hot air becomes more and more gripped or wedged between the two faces in altitude
• a trowal is the hollow of hot air in altitude created by the occluded face. It is found slightly with the back of this one. Indeed, the position of the occluded face is that which the cold face wedged under the hot air would have. This concept is used in certain countries like Canada and the Great Britain.

One finds also certain lines of demarcation of Méso-scale with which one associates time and which is used as limits between very local masses of air:

• the barometric Creux S

• the faces of gusts outgoing of the storms
• the faces of dewpoint which separate from the zones in the same mass of air where one has a border of marked moisture
• the tropical waves.

Dynamics of the faces

See also: Cyclogénèse

The classical theory of the faces, of the École of meteorology of Bergen, known as that when two masses of air having various physical characteristics come into contact, the dense cold air, more , tends to slip under the hot air which rises while extending (ascent). Once the hot air cooled in the roadbases of the Troposphere, the moisture which it contains condenses by giving rise to Nuage S characteristics (Cirrostratus, Altostratus, Nimbostratus), which give Précipitations (Pluie, Neige, Grésil, Bruine, etc).

In the important depression S, the cold face is engulfed violently under the hot face by producing a fort running of ascent which gives rise to Nuage S convectif S: cumulus, Cumulus budding S and Cumulonimbus). The latter produce Averse S or even of the Orage S which can contain Grêle, to produce downward gusts violent and Tornade S.

In both cases, is restored more or less slowly a balance between the two masses of air (face occluded), and the phenomenon of slip ends. The passage of the cold or hot faces on an area causes the lowering or the rise in the local Température atmospheric.

In fact, les faces is only the result of the movements of the atmosphere and not them cause. The events described above are the result of the vertical movements of the atmosphere, a fluid in rotation, which one can explain by the Thermodynamique and the Mécanique of the fluids. The zones of contrasts which mark the presence of faces cause these movements and not the faces themselves. The softer air is not raised because it is pushed back upwards by the fresh air of surface but well parce the convergence of the winds in the airstream along the faces generates an ascending movement .

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