Supplied with energy by sunlight, subject to the laws of fluid mechanics, air movements are less random than they may appear. In addition, the Earth rotating around its axis, the large fluid masses like the air of the atmosphere are subjected to a force a little strange because insensitive on the human scale: the force of Coriolis. These constraints impose the very particular organization of large-scale air circulation around our planet, of which this text attempts to outline.
Trade winds, the equatorial easterly current and Hadley’s cell
In overheated tropical regions the air is lighter than elsewhere. Like smoke from a chimney, it rises upwards in the atmosphere and sucks up whatever is around it, creating winds that converge towards the equator and are influenced by the Coriolis force. These winds are called the trade winds. Air from the north is deflected to the right, air from the south is deflected to the left. Their convergence in the vicinity of the ground or the sea (see Figure 1, left) generates the current is equatorial, steady wind, relatively slow since its speed is around 20 km / h, but which was sufficient to push the schooners of Christopher Columbus towards the West Indies and Venezuela. This regularity of the Trade winds brings a few days of safe and efficient sailing to the sailors of the Vendée Globe, before they face the South Seas and their stormy winds.
In its fight against gravity, the updraft does not manage to exceed the altitude of the tropopause, around 15 km in tropical regions, but its flow must be preserved. This is only possible if its trajectory curves in the form of horizontal winds oriented, either north or south, depending on the hemisphere. Long before they reach the poles, the Earth rotating around its axis giving way to the west, these winds are systematically diverted to the east, in the northern hemisphere as in the southern hemisphere. These mechanisms thus impose on the atmospheric circulation in the tropics the helical structure illustrated on the right part of Figure 1, known under the name of Hadley cell.
Polar cells and Ferrel cells
In the polar regions a convective circulation similar to the Hadley cell is imposed by the fall of cold, dry and heavy air, which arrives from the top of the troposphere, thinner at this latitude (about 7 to 8 km) than in tropical latitudes (about 15 km). However, the Coriolis force is maximum near the poles, so that its influence is clearly greater than on the Hadley cell. This is why the air recirculation in the vicinity of the poles remains contained between the poles and the parallels at ± 60 °. Thus, between the extreme latitudes of the Hadley cell (± 30 °) and of the polar cell (± 60 °), driven by their respective movements, appears the Ferrel cell (Figure 2), discovered by the American meteorologist whose name it bears.
Jet streams
In each hemisphere, at the borders between the three convective cells described above, at an altitude of the order of 10 km above mean sea level, two westerly winds appear which circulate all around the planet while undulating in the vicinity of their middle latitude. These winds are often referred to by their English name, the jet streams. They were discovered by Japanese meteorologist Oishi Wasaburo in 1920 and described in a report written in esperanto so that it is accessible to a large number of readers. Unlike the slow equatorial east current, the jet stream polar is very fast (speed between 100 and 300 km / h) and very turbulent. These instabilities are responsible for the influx of cold air from the polar regions into the temperate regions of Europe. On the contrary, the jet stream subtropical is slow (50 to 100 km / h) and much more stable.
Depressions and cyclones
The instabilities and turbulence of the atmosphere are systematically accompanied by pressure variations which can be significant. At sea level the normal atmospheric pressure is 1013 hPa. The highest values can reach 1040 hPa, they appear in anticyclones which constitute real obstacles bypassed by the winds. The pressure can also drop to 900 hPa (see the Saffir-Thompson scale on the attached table) in depressions and cyclones where winds can be strong and where large clouds are present since the low pressure causes a condensation of water vapor.
Even if the place and the moment when a depression is formed are random, the depression structure has a very characteristic organization. Since the pressure is higher outside than inside, at ground or sea level the air is pushed inward. In the northern hemisphere the Coriolis force adds a systematic deviation to the right. By combining, these two effects deflect the trajectories that enter this depression to the right, and the combination of these two effects generates a rotation of the entire depression, thus forming a gigantic vortex, which turns in the same direction as the Earth, meaning cyclonic. Thus, at horizontal scales of the order of 200 to 1000 km, an over-rotation may appear which is added to the rotation of the planet. The deeper the depression, the more violent the winds. The highest level is that of a category 5 cyclone, where the wind speed exceeds 240 km / h, which causes real catastrophes by tearing up trees and destroying all light constructions.
In any depression, the air coming from the north opposes that coming from the south, the air coming from the west opposes that coming from the east, and all these movements constrict the central region, where a rise local pressure appears with two consequences. First of all, it pushes upwards the air which arrives from all horizontal directions and which has no other outlet. Moreover, like any compression of a gas, it is necessarily accompanied by a release of heat which lightens the air located near the axis. These two new effects form a kind of vertical chimney with a significant draft. Such a movement, ascending in the center and of course descending in the periphery with the precipitation, is added to the rotation due to the Coriolis force. The air circulation in large depressions is therefore made up of two large circuits, on the one hand the cyclonic rotation in horizontal planes, on the other hand the ascent in the center and the descent at the periphery.
When such a low hits a warm sea, receiving even more heat the air becomes lighter and lighter and the chimney draft even more efficient. Since the temperature and pressure are higher than the surrounding area, droplets from clouds at the periphery can evaporate. When this evaporation is total, the opaque air coming from the clouds becomes locally transparent again. This is how eye of the storm in the vicinity of the axis. This eye is limited by a fairly sharp border called the wall of the eye, in the vicinity of which is localized the ascent of the air in a helical trajectory.
If the image of Figure 4 clearly shows the structured organization of a cyclone, that of Figure 5 suggests fear in the face of the destructive power of such a phenomenon. Since this power is linked to the temperature of the sea, which has continued to increase since the 20the century, we can fear that these catastrophes will become more and more serious.
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This text is taken from several articles published in encyclopedie-environnement.org, mainly the following four:
- Atmospheric circulation: its organization and Jet streams by René Moreau, Emeritus Professor at Grenoble-INP, SIMaP Laboratory (Science and Engineering of Materials and Processes), member of the Academy of Sciences and of the Academy of Technologies
- Tropical cyclones: development and organization and Tropical cyclones: impacts and risks by Frank Roux, Professor at Paul Sabatier University, Aerology Laboratory, Toulouse
This work was carried out with the financial support ofUGA Editions as part of the “Investissement d’avenir” program, and the Auvergne Rhône-Alpes region.
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