Airstream

The air current are concentrated zones of strong winds. They must mainly with differences of Pression S or of Température S. One divides them into running horizontal and vertical.

Horizontal currents

See also: Wind, Wind geostrophic, Jet-stream

A variation in the atmospheric pressure causes a air volume displacement and generates the wind. The Force of Coriolis deviates this movement of the air, towards the line in the northern Hemisphere and the left in that of the south, which makes the winds parallel with the Isobare S on a chart of pressure in altitude. It is what is called the wind geostrophic.

The differences in pressure depend, as for them, of the average temperature in the airstream. Like the Sun does not heat the uniform Ground in way, there is a difference in temperature between the poles and the equator, which creates masses of air having more or less homogeneous temperatures according to the latitude. The differences in atmospheric pressure are also at the origin of the atmospheric Circulation general whereas the masses of air are separated by ribbons where the temperature changes quickly. They are the face S. Along these zones, of the more important winds are formed in altitude. They are the jet-streams which can reach a speed of several hundreds of Kilomètre S per hour, and which can extend on thousands from kilometers length, but which can have only a few tens or hundreds of kilometers of width.

On the surface, the Friction had with the Relief and the other obstacles (building S, Tree S, etc) can contribute to a deceleration and/or a deviation of the wind. One obtains a turbulent wind thus more in the atmospheric Boundary layer. This wind could be channeled by contractings, such as for example of the valleys. This wind could also be raised along the slopes of the mountains to give local airstreams.

Vertical currents

The vertical currents are caused by two phenomena: rising by mechanical vertical movement of the mass of air, or movements of convection giving of the ascending currents and subsidents.

Mechanical forcing

See also: Ascent (meteorology), Subsidence, Effect of fœhn

In a mass of air in displacement, vertical movements occur when there is convergence between a level and Divergence with another. For example, when we are close to the jet-stream, the winds increase when its heart approaches an area and decrease when it moves away from there, with the result that one will have zones where the air accumulates and must go down, whereas for other zones, there are a loss and an in-draft, fact of the lower layers. These ascending or downward currents will be relatively diffuse.

In addition, the obstacles such as the mountains force the air to go up or go down, sometimes very quickly. As the obstacles are very localized, these currents will affect very limited zones, and thus will form corridors.

The atmosphere is filled of mechanical zones of movements, which one can recall with the atmospheric primitive equations, which explain the vertical currents.

Thermal forcing

See also: adiabatic Heat gradient, Convection

The air which pupil sees his pressure decreasing, and consequently its temperature. If the movement ascenscionnel is rather fast, one can consider that the volume of air which rises does not exchange heat with the surrounding air. In other words, it does not undergo any transformation Adiabatique. Under these conditions, and according to the law of Compression and adiabatic pressure drop, its temperature decrease. The rate of decrease varies according to whether the raised volume of air is saturated with steam (~0,6°/100m) or not (~0,98°/100m, according to altitude).

Thus a piece of air raised will undergo a Poussée of Archimedes if it becomes of a density different from that of the environment on its new level. That is translated in two ways:

  • If the vertical gradient of temperature of the mass of ambient air (temperature variation per vertical step size) is lower than that of the piece, any volume of air which starts to rise, thus finds in a mass of air hotter than him and less dense. The volume of air thus will go down again at its initial point and the mass to d'air is overall stable. No thermal current can be formed in its center.

  • If on the contrary the variation in temperature of the ambient air is higher than that of the piece, any volume of air which starts to rise is found, in spite of its fall of temperature, in a colder mass of air (thus denser) than him. It thus undergoes a push to the top and continues its rise until it reaches a level where the environment is colder. There is thus a mass to d'air instable in this case.

Least turbulence or warming in a mass of unstable air generates a thermal current. This can occur by a diurnal warming on the ground or in any time in altitude. One can arrive even there by a mechanical rising of the mass of air, in as much as the raised air becomes unstable compared to the environment. This ascending current, due to thermal instability, is responsible for the Nuage S convectifs like the Cumulus of good weathers and the Cumulonimbus of Orage S. In absence of moisture, these ascending currents are not visible but occur nevertheless and give place to thermics and diurnal turbulence.

On another side, in the storms, the dry air and more expenses coming from the mean levels can be engulfed in the cloud and be colder than the air of this last. It is thus put to go down and gives a current going down which can lead to downward gusts causing damage.

Importance

The study of the airstreams is the field Météorologie. It is important to locate them and to envisage them in all the fields which are sensitive to the wind. In particular for:

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