The only driver of atmospheric circulation is sunlight. Under the constraints of gravity, Archimedes’ thrust and Coriolis’ force due to the Earth’s rotation, temperature differences between the equator and the poles cause air to circulate all around the Earth. This global circulation, driven by trade winds in tropical regions, has a well-defined organisation in each hemisphere: three convective cells in the meridian planes are associated with five winds along parallels, the weak eastern equatorial current in the vicinity of the equator and at low altitude, and in each hemisphere two westerly winds, the polar jet stream at a latitude near ±60° and at high altitude, as well as the subtropical jet stream, slower than the first, located at latitudes near ±30° and, also at high altitude. In their motion, these air masses transport and redistribute both the heat transmitted by the continents and the moisture produced by evaporation over the oceans.
1. First loop: Hadley’s cell
In the overheated equatorial region, which sees the Sun at its zenith, the air is lighter. Like smoke from a chimney, it rises upwards from the troposphere (see The Atmosphere and the Earth’s Gas Envelope) and draws in the air around it, creating winds that converge towards the equator. Since the Earth rotates, they are affected by the Coriolis force. Air from the north is diverted to the right, air from the south is diverted to the left. The convergence of these trade winds near the ground or the sea generates the equatorial easterly current, a steady wind, relatively slow since its speed is around 20 km/h, but which was sufficient to push Christopher Columbus’ schooners from Spain towards the West Indies and Venezuela. More details on this inter-tropical convergence zone can be found in the article The key role of the trade winds.
In each hemisphere, below the tropopause (at an altitude of about 8 to 10 km above mean sea level), on either side of Ferrel’s cells and at high altitude, westerly winds appear which circulate all around the planet oscillating around a medium latitude. A major fraction of the atmospheric circulation, these winds are often referred to as the jet streams (Figure 4). They were discovered by the Japanese meteorologist Oishi Wasaburo in 1920 and described in a report [4]written in Esperanto so that it would be accessible to a large number of readers.
This article presents only the average state of atmospheric circulation, emphasizing its remarkable organization. Due to the seasonal movements of the zenith, the alternation between the oceans, which are sources of intense evaporation, and the drier continents, but also to its own instabilities, this atmospheric circulation is also subject to strong fluctuations, the description of which is the subject of complementary articles: The key role of the trade winds and jet streams.
4. Messages to remember
The air in tropical regions, heated by solar radiation and thus lightened, rises to the top of the troposphere. In doing so, it sucks in the trade winds that converge from the north and south towards the equator, creating the equatorial easterly flow and an updraft that leads to the formation of the Hadley cell in the meridian planes.
The rotation of the Earth, modelled by the Coriolis force, limits the extent of the Hadley cell to latitudes around 30° to the north and south. In the highest part of the troposphere, the air is diverted eastward, which generates a relatively slow subtropical jet stream (less than 100 km/h) in each hemisphere.
Between the poles and the ± 60° parallels, temperature differences impose a circulation similar to that of the Hadley cell, which leads to the formation of polar cells where the winds near the ground are oriented from the pole towards the temperate regions.
Between each Hadley cell and the polar cell of the same hemisphere, the Ferrel cell appears by continuity where the dominant wind at ground level is oriented towards the nearest pole.
At the top of the troposphere, the transition between the Ferrel cell and the polar cell is much more influenced by the Coriolis force than that between the Hadley cell and the Ferrel cell. This gives rise to polar jet streams, which are also oriented eastward. These jet streams are fast (100 to 300 km/h) and their trajectories are quite unstable.
References and notes
[1] George Hadley, Concerning the cause of the general trade winds, Philosophical Transactions of the Royal Society, 1735, vol. 39, p. 58-62
[2] Gaspard Gustave Coriolis, Mathematical Theory of the Effects of Billiards, Carilian-Goeury, 1835
[3] William Ferrel, An essay on the winds and the currents of the oceans, Nashville Journal of Medicine and Surgery, No. 4, 1856
[4] Oishi Wasaburo, Raporto de la Aerologia Observatorio de Tateno, Aerological Observatory Report 1, Central Meteorological Observatory, Japan, 1926 (in Esperanto)
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The Earth is surrounded by a gaseous domain, commonly referred to as the atmosphere, although,…
René MOREAU, Professor emeritus at Grenoble-INP,Laboratoire SIMaP (Science et Ingénierie des Matériaux et des Procédés), member of the Académie des sciences and the Académie des technologies