The first attempt to explain the global atmospheric circulation was based on a simple convectional concept. In 1686 Halley associated the easterly trade winds with low-level convergence on the equatorial belt of greatest heating (i.e. the thermal equator). These flows are compensated at high levels by return flows aloft. Poleward of these convectional regions, the air cools and subsides to feed the northeasterly and southeasterly trades at the surface. This simple mechanism, however, presented two significant problems - what mechanism produced high-pressure in the subtropics and what was responsible for the belts of dominantly westerly winds poleward of this high pressure zone? It is interesting to note that not until 1883 did Teisserenc de Bort produce the first global mean sea-level map showing the main zones of anticyclones and cyclones (i.e. high and low pressure). The climatic significance of Halley's work rests also in his thermal convectional theory for the origin of the Asiatic monsoon which was based on the differential thermal behaviour of land and sea; i.e. the land reflects more and stores less of the incoming solar radiation and therefore heats and cools faster. This heating causes continental pressures to be generally lower than oceanic ones in summer and higher in winter, causing seasonal wind reversals. The role of seasonal movements of the thermal equator in monsoon systems was only recognized much later. Some of the difficulties faced by Halley's simplistic large-scale circulation theory began to be addressed by Hadley in 1735. He was particularly concerned with the deflection of winds on a rotating globe, to the right (left) in the northern (southern) hemisphere. Like Halley, he advocated a thermal circulatory mechanism, but was perplexed by the existence of the westerlies. Following the mathematical analysis of moving bodies on a rotating earth by Coriolis (1831), Ferrel (1856) developed the first three-cell model of hemispherical atmospheric circulation by suggesting a mechanism for the production of high pressure in the subtropics (i.e. 35°N and S latitude). The tendency for cold upper air to subside in the subtropics, together with the increase in the deflective force applied by terrestrial rotation to upper air moving poleward above the Trade Wind Belt, would cause a build-up of air (and therefore of pressure) in the sub-tropics. Equatorward of these subtropical highs the thermally direct Hadley cells dominate the Trade Wind Belt but poleward of them air tends to flow towards higher latitudes at the surface. This airflow, increasingly deflected with latitude, constitutes the westerly winds in both hemispheres. In the northern hemisphere, the highly variable northern margin of the westerlies is situated where the westerlies are undercut by polar air moving equatorward. This margin was compared with a battlefield front by Bergeron who, in 1922, termed it the Polar Front. Thus Ferrel's three cells consisted of two thermally direct Hadley cells (where warm air rises and cool air sinks), separated by a weak, indirect Ferrel cell in mid-latitudes. The relation between pressure distribution and wind speed and direction was demonstrated by Buys-Ballot in 1860.
During the nineteenth century it became possible to assemble a large body of global climatic data and to use it to make useful regional generalizations. In 1817 Alexander von Humboldt produced his valuable treatise on global temperatures containing a map of mean annual isotherms for the northern hemisphere but it was not until 1848 that Dove published the first world maps of monthly mean temperature. An early world map of precipitation was produced by Berghaus in 1845; in 1882 Loomis produced the first world map of precipitation employing mean annual isohyets; and in 1886 de Bort published the first world maps of annual and monthly cloudiness. These generalizations allowed, in the later decades of the century, attempts to be made to classify climates regionally. In the 1870s Wladimir Koeppen, a St Petersburg-trained biologist, began producing maps of climate based on plant geography, as did de Candolle (1875) and Drude (1887). In 1883 Hann's massive three-volume Handbook of Climatology appeared, which remained a standard until 1930-40 when the five-volume work of the same title by Koeppen and Geiger replaced it. At the end of the First World War Koeppen (1918) produced the first detailed classification of world climates based on terrestrial vegetation cover. This was followed by Thornthwaite's (1931-33) classification of climates employing evaporation and precipitation amounts, which he made more widely applicable in 1948 by the use of the theoretical concept of potential evapo-transpiration. The inter-war period was particularly notable for the appearance of a number of climatic ideas which were not brought to fruition until the 1950s. These included the use of frequencies of various weather types (Federov, 1921), the concepts of variability of temperature and rainfall (Gorczynski, 1942, 1945) and microclimatology (Geiger, 1927).
Despite the problems of obtaining detailed measurements over the large ocean areas, the later nineteenth century saw much climatic research which was concerned with pressure and wind distributions. In 1868 Buchan produced the first world maps of monthly mean pressure; eight years later Coffin composed the first world wind charts for land and sea areas, and in 1883 Teisserenc de Bort produced the first mean global pressure maps showing the cyclonic and anticyclonic 'centres of action' on which the general circulation is based. In 1887 de Bort began producing maps of upper-air pressure distributions and in 1889 his world map of January mean pressures in the lowest 4 km of the atmosphere was particularly effective in depicting the great belt of the westerlies between 30° and 50° north latitudes.
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