The wind is an atmospheric movement directional. It can appear on any Planet having an atmosphere. It is generated by the movements of masses of air, caused by their differential warming by the solar energy and the rotation of planet.

On Ground, several areas have characteristic winds to which the local populations gave particular names. It is energy source and was used through the centuries with various uses as the windmills, the sailing, the flight-with-veil or more simply drying.

Cause wind

The atmospheric pressure in a point is the result of the mass of the airstream above this point. The differences of Pression which one notes on the terrestrial sphere are due to a differential warming between these points. This difference is the force which moves the air.

If the Earth did not turn on its axis, circulation would be thus direct between the centers high and basic pressure. However, this rotation ducts the air in the direction perpendicular to displacement compared to an observer on the ground. In fact, it is the observer which moves but it is called nevertheless Force of Coriolis. It is proportional to the air velocity moved but towards the line in the Northern hemisphere and on the left in that of the south.

When the vectorial sum of these two forces became almost equal but opposed, the direction of the air volume displacement is stabilized to be perpendicular to the gradient of wind. The small difference which remains, plus the friction close to the ground, leaves an acceleration towards the lowest pressure, the direction of the wind thus remains directed a little more towards the low pressures with the result that the wind turns around the weather systems.

The force of Coriolis is exerted on long distances and varies the null one with the equator with maximum with the poles. In certain situations, the air volume displacement is not exerted at a sufficient distance so that this force has a notable influence. The wind is then caused only by the differential of pressure. Here three cases which occurs when the general circulation of the winds is zero or very low:

  • the denser cold air in top of a mountain creates a pressure stronger there than in the valley. The gradient of pressure then makes descend the slope with the air at an insufficient distance so that the force of Coriolis deviates it. That thus generates a wind known as catabatic . One generally meets this kind of effect the night. They are also very common to the face of a Glacier, for example, on the coast of the Greenland and the the Antarctic at any hour.

  • During the day, close to the coasts to a lake or sea, the sun heats the ground more quickly than water. The air thus takes more expansion on ground and rises creating a pressure lower than on the water level. Once again this difference in pressure is created at a very weak distance and cannot be counterbalanced by Coriolis. A breeze of sea (lake) is thus established. The same thing occurs the night but in opposite direction, the ground breeze, whereas it is the bank which becomes colder.
  • Under certain conditions of constraint, for example in very boxed valleys, the air can only follow one way. If the gradient of pressure becomes perpendicular to the valley, the wind will be generated exclusively by the difference in pressure.

In other case, the balance is exerted between the pressure and the Centrifugal force. It is the case of the Tornade S and of the swirls of dust where the rate of rotation is too large and the surface of the waterspout is too small so that the force of Coriolis has time to act.

Finally, in the case of clouds convectifs like the Storm S, it is not the difference in pressure but the instability of the air which gives the winds. Precipitation as well as the injection of cold air and dryness in the mean levels bring a Poussée of negative Archimedes (downwards) in the cloud. That gives downward winds which form localized faces of gusts.

Calculation of the wind

The wind thus depends on several factors. It is the resultant of the forces which are exerted on the piece of air: pressure, the force of Coriolis, the friction and the Centrifugal force. Complete calculation is made with the equations of the horizontal movement of the atmospheric primitive equations. In general, the centrifugal force is neglected because the number of revolutions around the depression is too slow and its value is thus very small compared to the other forces. However, in a fast circulation like that of a tornado, it is necessary to hold account of it. With these equations, the weather charts make it possible to estimate the wind by knowing the pressure, the latitude, the type of ground and the local effects even if there is not direct measurement.

  • In altitude, the friction is null and one can obtain for the Aviation one estimated of the wind by the equations of the Vent geostrophic.

  • Close to the ground, in the Boundary layer, the friction causes a reduction in the winds compared to estimated preceding according to what is called the Spirale of Ekman. In general, the wind is from 50 to 70% of the wind geostrophic on water and between 30 and 50% of this wind on the dry land. The more the wind is decreased by the friction, the more it turns towards the lowest pressure what gives a change towards the left in the Northern Hemisphere and towards the line in that of the South.
  • In the broken places where flow of air is channeled or in the situations where the wind is not due to a balance between pressure and force of Coriolis as previously mentioned, calculation is much more difficult. Among these cases one notes:
#le Wind antitriptic where one has a balance between the pressure and the friction;
#le Wind catabatic where the cold air goes down from the heights;
#le Wind anabatic where air is forced to the top of a slope.

Scale of fluctuation of the wind

For an altitude lower than approximately 1000 meters, where are the built works, the forces of friction due to the roughness of the ground and the thermal phenomena govern the flows of air mainly. These phenomena generate fluctuations the speed of the wind, in time and space, likely to excite the most flexible structures. This zone is called Boundary layer of atmospheric turbulence .

The spectral analysis the speed of the wind in the turbulent boundary layer makes it possible to highlight several temporal scales of fluctuation. Figure Ci against watch pace of a spectrum of density of power representative the horizontal speed of the wind to 100 meters with the top of the ground according to Van der Hoven.

The repeated and random requests of turbulences can request the clean modes certain works and lead to their ruin if that were not taken into account during dimensioning (such as for example the Pont of Tacoma in 1940).

Real wind, speed, apparent

  • real Wind : the wind which one feels with the stop, related to a air volume displacement which surrounds us. It is for example the surface wind, whose force and direction are communicated to us by the weather bulletins . The concept of real wind is used to specify, on mobile machines, the rate of travel of the mass of air where they evolve/move, to differentiate it from the other winds like the apparent Vent or the wind due at the speed.

  • Wind speed or relative Wind : wind generated by the displacement of the mobile, equal in intensity, of the same direction, and opposed in direction, with the relative Speed of this one. It is for example the wind which one feels when one moves with bicycle, in the absence of any real wind.

  • apparent Wind (for the sea Transport): the wind such as it is felt since the mobile, summons vectorial of the two precedents, i.e. real wind and relative wind, or wind speed. The concept of apparent wind is especially used in veil or Sand yacht: indeed, the wind felt on the boat will depend not only on the real wind, but also on the speed of the boat, which results in having to adjust the adjustment of the veils. It is the wind which indeed a sail receives.

Measure wind

  • Scales:
    • the speed of the wind is measured by the sailors by using the scale of Beaufort, scale closed on 12 levels, if they do not have instruments to note it. This scale connects the effect of the wind on sea (height of the waves, production of spray, etc) at its speed. Otherwise, they use the nodes.
    • On the ground and in altitude, the wind is measured in km/h, meters/second or nodes.

  • the wind is raised:

    • On the ground and at sea, its speed is given by a Anémomètre and its direction is measured by a Girouette.
    • In altitude, one obtains it by Radio-sondage while following the movement of a Ballon-sonde.
    • Since space, thanks to the instruments of a Meteorological satellite , one can obtain the winds in all the atmosphere. These data are particularly useful for the uninhabited places like the Désert S and the Océan S.

Some famous winds

  • Trade wind : Regular wind of the North-East in the Northern hemisphere and south-east in the southern hemisphere.
  • North wind S : Bad winds heralding storm
  • white Southerly wind : Wind of dry and hot, blowing south-east in Top Languedoc.
  • black Southerly wind : Even wind that white Southerly wind, but in short. Is sometimes accompanied by rain.
  • Balaguère : Wind coming from Spain and sometimes from good further, breath in the valleys of the the Pyrenees a wind of the south which brings with him a perfume of Adventure.
  • North wind : Blowing cold wind of north or the North-East.
  • Breva : Blowing wind generally the day in the area of the Lake of Like.
  • Bora : Cold wind of the active North-East of Eastern Europe towards the Italy. Blow especially the winter on the Adriatique or the Black Sea.
  • Cers : Wind of west or south-west in bottom Languedoc, it is also indicated under the name of Narbonnese in the area of Narbonne. Cold in winter, heat in summer, always dry, it brings the good weather.
  • Chammal
  • Chamsin : Wind of the south, heat and dryness, blowing in Egypt in spring.
  • Chinook : Wind of outgoing west of the Rock , of origin identical to the wind of Foehn.
  • Etesien : Wind of the North-West coming from Greece
  • Fœhn : Wind of mountain dry and hot, Mediterranean towards the the Alps, blowing mainly Swiss spring in and with the the Tyrol.
  • Galerne : wind of the North-West, cold and wet, in the west of France.
  • Gregale : Wind of the North-East coming from Greece
  • Harmattan : Wind of the North-East, very dry, blowing in winter and in spring in Western Africa.
  • Hegoa : Wind of the south, heat and dryness, but followed rains to the Basque Country.
  • Joran : Thermics coming from North, blowing evenings of summer since the mountain of Chaumont to the Neuchâtel Lake (Swiss)
  • Khamsin : coming from is, known with the Middle-East, it is claimed over there that this wind makes insane, or that it causes migraines.
  • Ballasts : East wind, heat and dryness, blowing with Madeira.
  • Levanter : East wind which crosses the Straits of Gibraltar
  • Levêche : Wind of the south dry and choking in the south of the Spain and in Oranie.
  • Libeccio : Wind of west or south-west, violent in all seasons, which crosses the Italy and the Corsica .
  • Marine : Wind of hot and wet south blowing of the Mediterranean towards the Provence and the Languedoc.
  • Meltemi : Wind of north, cold, blowing in Turkey and Greece in the Aegean Sea, especially during the summer.
  • Mistral : Wind of north, blowing violently, in all season, in the valley of the the Rhone, in Provence and with the Balearic Islands.
  • Monsoon : Wind of Asia southernmost, blowing towards the sea in winter and the ground in summer.
  • Nordet : Cold wind of the North-East in the northern Hemisphere, heat in that of the south.
  • Pampero : Wind of west in Argentinian, blowing from July to September, mainly in the Rio of Plata.
  • Simoun : Wind of the south, dryness and heat in blowing North Africa in various directions.
  • Sirocco : Wind of the south, dryness and heat, the summer, in North Africa; wet and hot, the summer, in Italy of the South.
  • South-wester : Wind of the South-west which brings heat and good weathers.
  • Tramontana : Cold wind of the North-West and the north which blows in Languedoc and in the Roussillon.
  • Vaudaire : Wind of south-east in Switzerland.
  • Vendavel : Wind of west which crosses the Straits of Gibraltar
  • Vent of southerly wind : Wind of active south-east of the Mediterranean towards Toulouse
  • Williwan : Blowing wind along the Alaska
  • Zephyr : Wind soft, hot, coming from the East

Uses of the wind

by the plants: by the man:

Wind and mythology

Many religious legends personify the wind:

References

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